Resources

Application of GFRP Rebar in Slabs on Grade and CRCP 

Steven E. Williams, PE 
SE Williams Consulting LLC, 1038 Wren Ave SE, Grand Rapids, Michigan 49506 
Board Member, Neuvokas Corporation, Ahmeek, Michigan  
March 5, 2021 

Abstract: 

Steel reinforcement of concrete has been used for over 150 years to create lighter, more cost effective concrete structures.  While engineers are familiar with the design of structures using steel reinforcement from its many years of usage and extensive research into the composite action of steel and concrete, it is far from a perfect solution.  Steel’s primary shortcomings are that it is easily corroded, is heavy, and its use in concrete has a relatively high, negative environmental impact.  Recently, there has been a significant amount of interest in using non-corroding, lightweight GFRP rebar to overcome the issue of corrosion and thus lengthen the life of structures using it, which also reduces the environmental impact. 

Most civil engineers are with the properties of steel reinforced concrete, but not with the properties of GFRP reinforcement.  One of the main differences in properties between steel and GFRP is the relatively low modulus of elasticity of GFRP rebar.  This difference is often misunderstood as a detrimental property. 

This paper addresses the issue of whether the lower modulus of elasticity of GFRP rebar is a benefit as it relates to its use in slabs on grade, such as floors, parking lots, roads and highways.  Other research is reviewed and synthesized to conclude that the lower modulus of elasticity of GFRP is positive as it relates to its use in slabs on grade. 

Design guidance from published documents is summarized for both slabs on grade and CRCP. 

Keywords: Modulus of elasticity, reinforced concrete, FRP, GFRP, BFRP, CRCP, slabs on grade, temperature and shrinkage reinforcement. 

OVERVIEW 

The use of fiber reinforced polymer (FRP) rebar, particularly those using glass and basalt fiber, is rapidly becoming more common.  These rebar are cost effective, corrosion resistant, lightweight, very strong and environmentally beneficial, as well as having other positive attributes.   

This paper is a general discussion of the benefits of the use of glass or basalt fiber reinforced polymer (GFRP or BFRP) rebar in non-structural slabs on grade such as roadways, parking lots, and floors.  Since most practicing civil engineers do not have experience with the use of FRP rebar, and its use is rapidly increasing, this paper also addresses the design considerations for their use in these applications.  A particular emphasis is placed on the benefits of their having a lower modulus of elasticity than steel, which to many may seem counterintuitive. 

Slabs on grade can take many forms such as plain concrete slabs with temperature and shrinkage reinforcement, jointed reinforced concrete pavement, continuously reinforced concrete pavement, and structurally reinforced concrete slabs. 

FRP rebar consists of bundles of continuous, longitudinal fibers with high tensile strength bonded together by a resin into a bar shape analogous to a steel reinforcing bar.  The fibers are typically glass, aramid, basalt or carbon.  Most commonly glass fiber is used, to form GFRP rebar.  The most common resins are epoxy and vinyl ester.  Dicyclopentadiene (DCPD) is a less commonly used, but is a high performing, resin.  Polyester has also been used; but due to its lower resistance to chemical attack, lower bond strength and hazardous styrene off-gassing (a problem it shares with vinyl ester), it is not commonly used.  GFRP is a standardized product for which ASTM 7957 and ACI 440.6-08 provide specifications defining minimum acceptable physical and chemical characteristics for use in structural and non-structural applications.  This paper primarily addresses the use of GFRP rebar meeting these standards. 

The use of fiber reinforced polymer (FRP) rebar has been widely studied over the last 40 years.  The impetus for this has been the expensive, frequent repairs needed for steel reinforced concrete structures, particularly those exposed to corrosion and cyclic loadings.  Steel corrosion products, mainly iron oxides, occupy up to seven times the volume of the steel, expanding within the concrete and resulting in tensile failure of the concrete.  Also the effective cross section may be significantly reduced by corrosion.  Both of these conditions will lead to the need to rehabilitate or replace the structure.  The studies of non-corrodable FRP rebar, mainly of GFRP rebar, have shown that their use can increase structure life by up to four times in aggressive environments and reduce maintenance (Feldman et al, (2008)). This increase in structure life leads to reduced life-cycle costs.  Eamon, etal, (2012) reported that for bridge superstructures, even the more expensive carbon fiber reinforced polymer bar (CFRP), is usually lower cost than steel on a life cycle basis. 

The primary reasons for its limited use have been the historically higher initial cost and the lack of engineering design standards.  A Michigan company, Neuvokas Corporation, has made advances in high speed manufacturing allowing GFRP rebar to be made at costs competitive with plain steel and at a lower cost than epoxy coated or corrosion resistant steel.  Industry standard setting organizations, such as the American Concrete Institute (ACI), the American Association of State Highway and Transportation Officials (AASHTO), the Canadian Standards Association (CSA), have all developed design guidelines for the use of FRP rebar.   

With this reduction in price and the availability of design standards, the industry is at an inflection point regarding wider adoption of this beneficial product. 

BENEFITS 

Comparing GFRP to steel the following attributes are noted. 

1. Competitive with steel on first cost basis
2. Lightweight, approximately 30% of steel, resulting in increased worker safety, lower transportation cost
3. Lower installation cost, with contractors reporting 20% less labor and equipment cost due to lower weight
4. Corrosion resistant, resulting in increased structure life (2 to 4 times)
5. Higher tensile strength, approximately 2.5 X steel
6. Electromagnetic transparency, MRI rooms, toll plazas, substation foundations, cattle barns, (stray currents)
7. Low overall embodied energy of concrete reinforced with GFRP, (approximately 25% to 50% of embodied energy in steel reinforced concrete exposed to corrosive elements) (Ozcoban (2017))
8. Lower CO2 emissions, approximately 50% (Ozcoban (2017), Inmana (2016))
9. Lower life cycle cost for structures exposed to corrosive elements
10. Lower modulus of elasticity, reducing internal stress on concrete
11. Coefficient of thermal expansion nearly the same as concrete, reducing internal stress on concrete. 

REVIEW, ANALYSIS, AND SYNTHESIS OF RESEARCH AND DESIGN GUIDANCE BY OTHERS 

GFRP rebar has unique physical and mechanical properties that must be taken into consideration during design.  It is not a direct, one for one, replacement for steel reinforcement.  The American Concrete Institute Committee 440 has developed and published design guidelines for the use of GFRP reinforcement that reflects GFRP’s unique properties (ACI 440.1R (2015)).  AASHTO has done the same for bridges (AASHTO (2018)). 

Since GFRP is anisotropic, designs incorporating its use must consider its properties in both the longitudinal and transverse directions.  It also does not yield like steel, but is elastic to failure.  From a longitudinal tensile strength perspective GFRP rebar is much stronger than steel at approximately 2.5 times that of 60 ksi steel.  However, its modulus of elasticity (Young’s modulus) is much lower (6,000 ksi v. 29,000 ksi), as is its transverse shear strength (22 ksi v. 45 ksi) than steel. 

Slabs on grade are generally reinforced for serviceability reasons and sometimes for structural reasons. 

The primary serviceability concern for slabs on grade is controlling the size and spacing of cracks.  As concrete cures it shrinks.  Cracks result when the tensile stress from drying exceeds the tensile strength of the concrete.  Tensile stresses may also occur as a result of temperature changes.  In both cases when the negligible tensile strength of the concrete is exceeded, the tensile stress is then transferred to the reinforcement and cracking occurs.  The frequency/spacing of cracks, the internal concrete stresses (thermal and contraction) and the modulus of elasticity of the reinforcement determine the width of those cracks.  Maintaining those cracks at a small enough width to allow aggregate interlock to be preserved, and thus mechanical load to be transferred across the crack, as well as to minimize water intrusion and spalling, is the primary design consideration for slabs on grade.  Crack spacing is important to prevent punchout failure resulting from closely spaced cracks in slabs subjected to loads capable of causing this type of failure.   

As an example, the AASHTO guideline for crack width in continuously reinforced concrete pavements is less than 0.04 inches (1 mm) and the minimum crack spacing is 3.5 feet (1.07 m).  The maximum crack width is largely established to minimize water intrusion and spalling. 

Vetter (1933) recognized the importance of internal stresses in concrete resulting from volume changes, quantified them, and discussed the amount of steel reinforcement needed to resist them to minimize or prevent cracking.  The amount of steel required to prevent cracking was “relatively high, and in many cases, …prohibitive.”  

Choi and Chen (2005) evaluated the concrete stresses resulting from temperature and shrinkage in continuously reinforced concrete pavements using GFRP reinforcement.  They found that the lower Young’s modulus of GFRP when compared to steel led to lower stress levels in the concrete and greater crack spacing. Chen, Choi, GangaRao and Kopac (2008) reported on a CRCP test in West Virginia where a section of highway was paved with steel reinforcement at mid-depth of a 10 inch roadway and a section was paved with GFRP also at middepth.  The result was that the GFRP reinforced section had significantly greater spacing between cracks, theorized to be the result of the low modulus of elasticity of the reinforcement.  The crack widths were found to be somewhat greater in the GFRP section than the steel section, but the crack widths were within the AASHTO guidelines.  The longitudinal reinforcement ratios were 0.7 percent for steel and 1.12 percent for GFRP.  Thebeau, Eisa and Benmokrane (2008) conducted a similar study in Quebec and found that crack widths for GFRP sections reinforced with the same 0.77 percent used for steel had crack widths within AASHTO guidelines.  

CRACK CONTROL 

Cracks can be allowed to develop naturally/passively or actively controlled.  In CRCP applications, it has been found that if allowed to propagate passively the crack pattern is commonly one in which there are widely spaced clusters of tightly spaced cracks (Benmokrane et al (2020), and Ren et al (2014)).  These tightly spaced clusters as well as random, non-uniform cracks can lead to failures.   

In order to actively control cracking in CRCP, active measures may be taken.  In jointed plain concrete pavement as well as jointed reinforced concrete pavement active crack control has been considered good practice for many years (McCullough and Dossey (1999)).  Kohler and Roesler (2004), discussed active crack control for CRCP applications and demonstrated the ability to reduce clustered cracks as well as non-uniform cracks using active crack control.  Active crack control has taken the form of transverse, partial depth, vertical tape, or metal strip, insertion to create a stress concentration or partial depth sawcutting.  However, these methods make construction more difficult, reduce the load transfer efficiency by decreasing the contact area, and reduce ride quality through spalling.   

Ren et al (2014) noted these shortcomings and observed that CRCP sections without active crack control developed controllable cracks at points of increased stress induced by adjacent pavement.  From this observation, it was deduced that a small sawcut at the pavement edge could achieve controlled cracking.  A portion of a CRCP highway in Belgium was constructed in 2012 with 400 mm (15 ¾ inch) long, transverse, sawcuts at the pavement edge with some being 30 mm (approximately 1 1/8 inch) deep and others 60 mm (approximately 2 ¼ inch) deep.  They were spaced at 1.2 meters (4 feet).  The conclusion was that the deeper cuts, made early in the curing of the pavement, resulted in significantly reduced crack clusters and non-uniform cracking and substantially increased the occurrence of uniformly spaced, straight cracks.  The resulting cracks were also smaller than those in a passively cracked section. 

Cyclic fatigue is not often a consideration in well supported slabs on grade, but it is worth noting that the use of GFRP can also be a benefit in this regard.  Mufti and Neale (2007) looked at a bridge deck slab reinforced with GFRP and found that it could withstand over 20 times the cyclic fatigue of steel as a result of its lower modulus of elasticity. 

Katz (2004) found that an additional benefit of FRP rebar usage is its much lower environmental impact load.  This is a result of the lower environmental impact in the reinforcement manufacturing and transportation process, reduced maintenance activities, and the reduced impact of disposal.  The reduced maintenance is in part the result of increased life because of the lower modulus of elasticity that increases the number of cycles resulting in cyclic fatigue failure, as well as the elimination of corrosion.  Hammond and Jones (2011) quantified the embodied energy and CO2 of common construction materials. 

APPLICATIONS 

SLABS ON GRADE 

Many jurisdictions have empirical standards for reinforcement of jointed concrete pavement or slabs used for parking, etc. that are simple prescriptions of a certain size rebar placed at a prescribed depth at a certain spacing in a pavement of a given thickness.   

As an example, Harris County, Texas (Houston area) requires #4 bars at 18 inches on center for a 7 inch concrete pavement section. This is approximately equivalent to 0.13 square inches of reinforcement per foot of concrete or 0.156%.  The reinforcement is to be placed at mid-depth and contraction joints installed at 20 foot spacing with expansion joints at 80 foot spacing.  Thus, the reinforcement provides negligible structural strength, and the pavement is designed more like a plain jointed concrete pavement with larger than “normal” contraction joint spacing (“normal” is less than twenty four times the thickness of the concrete or 14 feet in this example).  The reinforcement’s apparent purpose is to restrain cracking and to maintain slab integrity and is considerably less than the commonly used minimum 0.6 percent ratio for CRCP. 

When considering what amount of GFRP reinforcement would yield a similar result to the empirically prescribed amount of steel in the above example, the question to be answered is what is the general purpose of the reinforcement and how much GFRP can result in a similar or acceptable design.  In this case it is maintaining sufficiently tight cracks that load transfer across the crack is not affected by the lack of aggregate interlock and that corrosive deicing or saltwater does not wash through the slab, corroding the steel.  AASHTO standards, based on field studies, indicate that a 0.04 inch crack is acceptable for these purposes. 

The Portland Cement Association uses the drag equation to estimate the area of steel in slabs on grade based on the allowable stress in the reinforcement as follows: 

𝐴𝑠 = 𝜇𝐿𝑤𝑠𝑙𝑎𝑏 
        2𝑓𝑠,𝑎𝑙𝑙𝑜𝑤 

where 𝐴𝑠 is the cross-sectional area of steel per linear foot in in.2; fs,allow is the allowable stress in steel reinforcement in psi, commonly taken as two thirds to three-fourths of fy; μ is the coefficient of subgrade friction (1.5 is recommended for slabs on ground (Portland Cement Association (1990)); L is the distance between joints in feet; and w is the dead weight of the slab in lb/ft2, usually assumed to be 12.5 lb/ft2 per in. of slab thickness.  The stress in the steel in psi, fs, is therefore inversely proportional to the area of steel. 

Rearranging this equation to solve for the stress in the reinforcement: 

𝑓𝑠 = 𝜇 𝐿 𝑤
       2 𝐴𝑠 

Solving for the stress in the steel using the area of steel in the Harris County design, the stress in the steel would be 10,100 psi, resulting in a strain of 0.00035. 

If the steel is replaced with #4 FRP, with a modulus of elasticity of 6,000 ksi, the strain in the FRP would be 0.0017 at this same stress.  Even using a #3 FRP bar in place of the steel would only increase the stress to 17,700 psi and the strain to 0.003. 

ACI 440.1R equation 7.3.1a calculates the maximum spacing of GFRP reinforcement resulting in a target maximum allowable crack width: 

𝑠𝑚𝑎𝑥 = 1.15 𝐸𝑓 𝑤  −2.5 𝑐𝑐 ≤ 0.92 𝐸𝑓 𝑤 
                     𝑓𝑓𝑠𝑘𝑏                        𝑓𝑓𝑠 𝑘𝑏 

Where smax is bar spacing in inches, Ef is modulus in psi, w is maximum allowable crack width in inches, kb is the bond dependent coefficient (1 for steel and 0.8 for Neuvokas GatorBar Glass), cc is the clear cover of the reinforcement in inches, and ffs is the service level stress in the reinforcement in psi.  Rearranging this formula to solve for crack width yields the following:

𝑤 =(𝑠𝑚𝑎𝑥 +2.5 𝑐𝑐)(𝑓𝑓𝑠𝑘𝑏) 
1.15 𝐸𝑓      

Using this formula the crack width may be estimated from the calculated stress in the slab.  For this example it is assumed that kb is equal to 1.  This results in a crack width of just under the AASHTO guideline of 0.04 inches of crack width using #4 GFRP.  The tension on the bar is approximately 2,000 pounds, and the bond stress at failure of 2,500 psi requires just over ½ inch of embedment. 

For #3 bar the crack width is approximately 0.065 inches.  Checking this stress, 17,700 psi, against the ultimate bond stress of #3 GFRP bar, the tension on the #3 bar would be just less than 2,000 pounds and the bar has a bond stress at failure of approximately 2,500 psi, or nearly 3,000 pounds per inch of embedment length. 

ACI 440.1R-15 Appendix A – SLABS-ON-GROUND states, “Because of the lower modulus of the FRP reinforcement, the governing equation should be based on the strain rather than the stress level when designing shrinkage and temperature FRP reinforcement.  At the allowable stress, the strain in steel reinforcement is approximately 0.0012; implementing the same strain for FRP will result in a stress of 0.0012 Ef, and” the above equation “can be written as  

And “…can also be used to determine joint spacing L for a set amount of reinforcement.  No experimental data have been reported on FRP slab-on-ground applications; research is required to validate this approach.” 

Using the allowable stress in GFRP as 0.0012 Ef, the allowable stress is 7,200 psi.  In the above example this would reduce the #4 bar spacing from 18 inches to approximately 12 inches with all other factors constant.  Contraction joints more closely spaced could also offset the reduction in spacing, as could changing the coefficient of subgrade friction. 

In general it may be said that slabs-on-grade using GFRP will require modestly more reinforcement cross sectional area than steel to achieve the same crack width, but that they will have a longer life as a result of corrosion being eliminated. 

CONTINUOUSLY REINFORCED CONCRETE PAVEMENT 

CRCP is commonly used for high traffic, heavily loaded highways.  It is continuous rigid pavement with longitudinal reinforcement to control cracking and is constructed without expansion or contraction joints.  It provides a smooth ride quality over a longer period of time (20-30 years) than other rigid pavement types.  This extends the life of the pavement, which reduces the life cycle cost and environmental impacts. 

Steel reinforcement in CRCP has traditionally been used, with the first use in about 1920.  Today CRCP is typically designed in accordance with AASHTO guidelines (AASHTO 1993).  Nearly 70 years of experience with CRCP and extensive research and testing has been used to arrive at these guidelines. However, steel corrosion, particularly in areas using deicing salts or subject to saltwater, diminishes the longevity of CRCP and is a major problem for steel reinforced CRCP (Kim et al (2000(, Choi and Chen (2005)). 

The use of FRP reinforcement eliminates the corrosion issue.  Walton and Bradberry (2005) using finite element analysis theorized that the use of GFRP could result in better performance than steel.  This study and others led to the construction of full scale test sections of GFRP reinforced CRCP in 2007 in West Virginia (Choi and Chen 2008) and in 2013 in the Province of Quebec (Benmokrane et al (2020)). The Quebec section was designed after several years of experimentation with smaller sections (Benmokrane (2008)) and the application of steel CRCP design criteria modified for the unique physical characteristics of GFRP (AASHTO (1993), USDOT (1996), Choi and Chen (2003)) to guide the selection of reinforcement ratios and placement in the test sections. 

The theoretical longitudinal reinforcement ratio was calculated by both teams (Quebec and West Virginia) to be approximately 1%.  Benmokrane (2008) tested short (22 meter) sections with reinforcement ratios varying from 0.77% to 1.57% with varying bar sizes, spacing and layers before deciding to construct the full scale test section with a reinforcement ratio of 0.93%, using #8 bars in a single layer spaced at approximately 6 ¾ inches at mid-depth in a 12 ½ inch slab.  In the West Virginia section a 1% reinforcement ratio was achieved in a 10 inch slab using #7 bars spaced at 6 inches at mid-depth. 

Both the West Virginia and Quebec test sections were constructed alongside conventionally designed steel reinforced sections with the same subbase, subgrade and concrete. 

After 6 years of use, Benmokrane et al (2020) reported on the longer term results of the Quebec test section.  They found that both the steel and GFRP sections had similar crack widths and in both cases less than the AASHTO standard.  The crack spacing consisted of widely spaced groups of cracks.  The test section was deemed to be a successful demonstration of GFRP CRCP.  The West Virginia test section yielded similar good results with the crack spacing in the GFRP section approximately twice those of the steel reinforced section.  In both sections the average crack widths were less than AASHTO standard.  The reduced internal stresses related to the lower modulus of elasticity of GFRP were believed to result in reduced spalling and susceptibility to punchout failure.  

Strain gauge test results in the Quebec section showed that the GFRP and concrete had similar strain responses to temperature changes. 

Benmokrane et al (2020), using Huang’s (2003) design formulae for CRCP, based on the AASHTO (1993) method of calculating reinforcement, modified those formulae to reflect the lower modulus of GFRP.  A coefficient of (200/EGFRP)0.15 was proposed to address the difference in modulus based on regression analysis of test section results (200 gPa being the modulus of steel).  The maximum bar stress was limited to 35% of guaranteed maximum tensile stress to avoid large crack widths.  

Where P is the longitudinal reinforcement ratio in %, ft is concrete tensile strength in mPa, α is the coefficient of thermal expansion °C-1, φ is the bar diameter in mm, σw is the wheel load stress in mPa, X is mean crack spacing in m, DTD is design temperature drop in °C, CW is crack width in mm, σ is the allowable stress in the reinforcement and Z is concrete shrinkage in 28 days in mm/mm.  The allowable stress in GFRP rebar was set at 35% of the guaranteed maximum tensile strength to maintain crack width at less than the AASHTO standard of 1 mm (based on field test observations) and the crack width in the formulae was set at 1 mm to reflect this.  Using these calculations the reinforcement ratio should be set above the highest minimum ratio calculated and below the maximum. 

SUMMARY 

The benefits of using GFRP rebar in concrete slabs on grade are significant, and are many, including: 

• Lower internal stresses in the concrete during shrinkage and temperature change
• No corrosion, increasing life of structure and no bleed through
• Increase in crack width spacing
• Reduced life cycle costs and in many cases reduced installed cost
• Significantly reduced environmental impacts
• Lower weight, reducing installation cost and increasing worker safety
• Electromagnetic transparency 

These benefits combined with the recently lowered cost of GFRP rebar, and the availability of design information based on field testing is resulting in wider adoption of its use. 

For engineers to use this product in slabs on grade, they must understand the differences in its properties from the steel that has traditionally been used.  For slabs on grade the primary difference is GFRP’s lower modulus of elasticity.  Properly designed, the lower modulus of elasticity is an advantage over steel, since it is closer to the modulus of concrete.  Through field experience and research, engineering design tools have been developed to allow the design of slabs on grade to meet varying conditions using GFRP rebar.  This paper provides an introduction to some of these design tools. 

It is also important to note that very large environmental benefits can be achieved by wider use of GFRP rebar.  The World Business Council for Sustainable Development (2002) estimated that 8% of world CO2 emissions were the result of cement production. Doubling the life of reinforced concrete structures could significantly reduce worldwide CO2 emissions. 

REFERENCES 

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Benmokrane, Brahim, A.Bakouregui, H. Mohamed, D. Thebeau and O. Abdelkarim, (2020), “Design, Construction, and Performance of Continuously Reinforced Concrete Pavement Reinforced with GFRP Bars: Case Study”, Journal of Composite Construction, American Society of Civil Engineers, doi: 10.1061/(ASCE)CC.19435614.0001064 

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FRP Rebar in Slabs on Grade Benefit from Low Modulus of Elasticity 

Steven E. Williams, PE 
Williams & Works, Inc., 549 Ottawa Ave. NW, Grand Rapids, Michigan 49503 
April 10, 2015 

Abstract: 
Steel reinforcement of concrete has been used for many years.  Its primary shortcoming is that it is easily corroded, and corrosion results in structural failure.  Recently, there has been a significant amount of interest in using non-corroding FRP rebar to overcome the issue of corrosion. 

While the properties of steel reinforcing bars are well known to most structural engineers, the same cannot be said for FRP rebar. One of the main differences in properties between steel and FRP is the relatively low modulus of elasticity of FRP rebar. 
This paper addresses the issue of whether the lower modulus of elasticity of FRP rebar is a benefit as it relates to its use in slabs on grade.  Other research is reviewed and synthesized to conclude that the lower modulus of elasticity of FRP is positive as it relates to its use in slabs on grade. 

Keywords: Modulus of elasticity, reinforced concrete, FRP, GFRP, BFRP, temperature and shrinkage reinforcement. 

OVERVIEW 

This white paper is a general discussion of the benefits of the use of non-prestressed, fiber reinforced polymer (FRP) rebar in non-structural slabs on grade such as roadways, parking lots, and floors.  These slabs on grade can take many forms such as plain concrete slabs with temperature and shrinkage reinforcement, jointed reinforced concrete pavement, continuously reinforced concrete pavement, and structurally reinforced concrete slabs. 

The use of FRP rebar has been widely studied over the last 40 years.  The impetus for this has been the expensive, frequent repairs needed for steel reinforced concrete structures, particularly those exposed to corrosion and cyclic loadings.  These studies, mainly of glass fiber reinforced polymer (GFRP) rebar, have shown that their use can increase structure life by up to four times and typically reduces maintenance.  When life cycle costs are considered, FRP rebar, even the more expensive carbon fiber reinforced polymer bar (CFRP), is usually lower cost than steel (Eamon, etal (2012)) for bridge superstructures. 

The primary resistance to its more common use has been its higher initial cost.  A Michigan company, Neuvokas Corporation, has made advances in high speed manufacturing allowing FRP rebar to be made at costs competitive with steel and at a lower cost than epoxy coated or corrosion resistant steel (Neuvokas Corporation’s primary product is basalt fiber reinforced polymer (BFRP) rebar.  Neuvokas has chosen basalt because of basalt’s superior alkali resistance to glass fiber and higher tensile strength). With this reduction in price, the industry may be approaching an inflection point regarding wider adoption of FRP rebar. 

REVIEW, ANALYSIS, AND SYNTHESIS OF RESEARCH BY OTHERS 

FRP rebar has unique physical and mechanical properties that must be taken into consideration during design.  It is not a direct, one for one, replacement for steel reinforcement.  The American Concrete Institute Committee 440 has developed and published design guidelines for the use of FRP reinforcement that reflects FRP’s unique properties. 

From a longitudinal tensile strength perspective GFRP rebar is much greater than steel.  However, its modulus of elasticity (Young’s modulus) is much lower (6,000 ksi v. 29,000 ksi), as is its transverse shear strength (22 ksi v. 45 ksi).  BFRP rebar has nearly three times the tensile strength of 60 ksi steel and a similar Young’s modulus to GFRP.  The lower Young’s modulus of FRP rebar is the primary difference with steel, affecting slab on grade performance. 

As concrete cures it shrinks, and cracks will result when the tensile stress from drying exceeds the tensile strength of the concrete.  The tensile stress is then transferred to the reinforcement.  The frequency/spacing of cracks, the internal concrete stresses (thermal and contraction) and the modulus of elasticity of the reinforcement determine the width of those cracks. 

Choi and Chen (2005) evaluated the concrete stresses resulting from temperature and shrinkage in continuously reinforced concrete pavements using GFRP reinforcement.  They found that the lower Young’s modulus of GFRP when compared to steel led to lower stress levels in the concrete and greater crack spacing.  Mufti and Neale (2007) looked at a bridge deck slab reinforced with GFRP and found that it could withstand over 20 times the cyclic fatigue of steel as a result of its lower modulus of elasticity. 

Controlling the location and width of cracks in slabs on grade is a major purpose of reinforcement and tooled or saw cut contraction joints.  The width of cracks is important since they must be minimized to retain the aggregate, on either side of the crack, in contact with each other to engage the shearing strength of the concrete across the crack.  An additional consideration, where rapid corrosion of the reinforcement is likely, is that the reinforcement will be exposed at the cracks and corrosion resistant reinforcement is preferred to extend the life of the slab.  As an example, the American Association of State Highway Transportation Officials (AASHTO) guideline for crack width in continuously reinforced concrete pavements is less than 0.04 inches.

Chen, Choi, GangaRao and Kopac (2008) reported on a continuously reinforced concrete pavement in West Virginia where a section was paved with steel reinforcement at mid-depth of a 10 inch roadway and a section was paved with GFRP also at mid-depth.  The result was that the GFRP reinforced section had significantly greater spacing between cracks, theorized to be the result of the low modulus of elasticity of the reinforcement.  The crack widths were found to be somewhat greater in the GFRP section than the steel section, but the crack widths were within the AASHTO guidelines.  The longitudinal reinforcement ratios were 0.7 percent for steel and 1.12 percent for GFRP.  Thebeau, Eisa and Benmokrane (2008) conducted a similar study in Quebec and found that crack widths for GFRP sections reinforced with the same 0.77 percent used for steel had crack widths within AASHTO guidelines.  While more study is needed, it may be said that FRP rebar is a preferable reinforcement to steel in many respects and its lower modulus of elasticity appears to have positive benefits. 

Katz (2004) found that an additional benefit of FRP rebar usage is its much lower environmental impact load.  This is a result of the lower environmental impact in the reinforcement manufacturing and transportation process, reduced maintenance activities, and the reduced impact of disposal.  The reduced maintenance is in part the result of increased life because of the lower modulus of elasticity that increases the number of cycles resulting in cyclic fatigue failure, as well as the elimination of corrosion. 

AN EXAMPLE 

Many jurisdictions have standards for reinforcement of jointed concrete pavement that are simple prescriptions of a certain size rebar placed at a prescribed depth at a certain spacing in a pavement of a given thickness.  As an example, Harris County, Texas (Houston area) requires #4 bars at 18 inches on center for a 7 inch concrete pavement section.  This is approximately equivalent to 0.13 square inches of reinforcement per foot of concrete.  The reinforcement is to be placed at mid-depth and contraction joints installed at 20 foot spacing with expansion joints at 80 foot spacing.  Thus, the reinforcement provides negligible structural strength, and the pavement is designed more like a plain jointed concrete pavement with larger than “normal” contraction joint spacing (“normal” is less than twenty four times the thickness of the concrete).  The reinforcement’s apparent purpose is to restrain cracking and to maintain slab integrity and is considerably less than the commonly used 0.6 percent ratio for continuously reinforced concrete pavement.   

Solving for the stress in the steel using the area of steel in the Harris County design, the stress in the steel would be 11,500 psi.  If the steel is replaced with FRP, with a modulus of elasticity of 6,000 ksi, the strain in the FRP would be 0.002 at this same stress.  Even using a #3 FRP bar in place of the steel would only increase the stress to 20,400 psi and the strain to 0.0034.  This small strain will result in less than the AASHTO guideline of 0.04 inches of crack width.  Checking this stress, 20,400 psi, against the ultimate bond stress of Neuvokas BFRP bar, the tension on the #3 bar would be just over 2,250 pounds and the bar has a bond stress at failure of approximately 2,500 psi, or nearly 3,000 pounds per inch of embedment length.   

Substitution of FRP for steel would likely be beneficial, since it would also eliminate pavement deterioration from corrosion of the reinforcement. 

OTHER BENEFITS 

FRP rebar has other benefits that are also worth considering in this application.  Principally, they are corrosion resistance and a coefficient of thermal expansion similar to concrete. 

Both ACI 440 and AASHTO identify the corrosion resistant properties of FRP rebar as significant.  This is particularly important as the failure of epoxy coated steel rebar, which had previously been thought to provide corrosion protection, to adequately protect against corrosion has been experienced in the field.  Thus, if substantial corrosion resistance is desired only stainless steel, galvanized steel, corrosion resistant specialty steels, or FRP are the remaining alternatives.  All of which have been dramatically more expensive than epoxy coated steel.  Although, recent developments in FRP manufacturing have brought the cost down to the point of being competitive with epoxy coated steel and in some instances competitive with black steel. 

The higher coefficient of thermal expansion of steel than concrete has resulted in concrete pops and spalling in pavements exposed to higher temperatures.  FRP rebar has a coefficient of thermal expansion similar to concrete and thus little or no differential in expansion occurs.  In pavements where either high ambient temperatures or large differential ambient temperatures are experienced, this is an important consideration in reducing maintenance costs and poor ride quality. 

SUMMARY 

In summary, the lower modulus of elasticity of FRP rebar must be incorporated into the design of slabs on grade.  And if it is, the lower modulus of elasticity is an advantage over steel, since it is closer to the modulus of concrete.  The benefits are primarily:

•  Lower internal stresses in the concrete during shrinkage and temperature
• Greater flexibility
• Increase in crack width spacing
• Reduced life cycle co

Corrosion resistance and a coefficient of thermal expansion closer to that of concrete are additional advantages of FRP rebar for their use in slabs on grade. 

REFERENCES 

Eamon, C. D., Jensen, E. A., Grace, N. F., and Shi, X. (2012). "Life-Cycle Cost Analysis of Alternative Reinforcement Materials for Bridge Superstructures Considering Cost and Maintenance Uncertainties" Journal of Materials in Civil Engineering, 24(4), 373-380, doi : 10.1061/( ASCE )MT .1943-5533.0000398

Chen, Roger H. L., J.-H. Choi, H V. GangaRao, and P. A. Kopac (2008), “Steel Versus GFRP Rebars?”, FHWA-HRT-08-006 

Thebeau, D., M. Eisa, and B. Benmokrane. 2008. “Use of Glass FRP Reinforcing Bars instead of Steel Bars in CRCP in Quebec.” Proceedings, 9th International Conference on Concrete Pavements [CD], San Francisco, CA, August 17-21. International Society of Concrete Pavements 

Choi, Jeong-Hoon and R. H. L. Chen, Ph.D.(2005) ”Design of Continuously Reinforced Concrete Pavements Using Glass Fiber Reinforced Polymer Rebars”, FHWA-HRT-05-081 

Katz, A. (2004). ”Environmental Impact of Steel and Fiber–Reinforced Polymer Reinforced Pavements.” J. Compos. Constr., 8(6), 481–488

Advantages of Bundling FRP Rebar 

Steven E. Williams, PE 
SE Williams Consulting LLC, 1038 Wren Ave SE, East Grand Rapids, Michigan 49506 
December 24, 2017

Abstract: 

Steel reinforcement of concrete has been used for just over two centuries.  Iron, the primary element in steel, is the most commonly used metal and one of the least expensive.  It has fairly high tensile strength, is relatively stiff, and is isotropic.  Its primary shortcomings are its weight and that it is easily corroded, and corrosion of reinforcing steel often results in structural failure.  Recently, there has been a significant amount of interest in using non-corroding FRP rebar to overcome the issues of corrosion and weight.  Because of its differing mechanical properties from steel, design and construction methods have been developed.  ACI 440 contains design guidance for the use of FRP in reinforced concrete design.   

Both FRP rebar and steel rebar may be bundled in accordance with ACI guidelines.  This paper looks at the advantages of bundling FRP rebar, particularly basalt FRP (BFRP) rebar (GatorBar®) as manufactured by Neuvokas Corp., as a further way of reducing weight, reducing installation labor, overcoming the lower guaranteed tensile strength (f*fu) of larger bar sizes, and providing other advantages. This paper presents the potential advantages of bundling BFRP rebar and concludes that this may be a preferred method of design and construction in many instances.  

Keywords: Reinforced concrete, bundling, modulus of elasticity, FRP, BFRP, basalt, tensile reinforcement. 

OVERVIEW 

The use of FRP rebar is becoming more common and many in the concrete industry believe it is the wave of the future because of its superior chemical and mechanical properties.  A harbinger of this belief is the recent purchase by Owens-Corning of the Aslan brand of FRP rebar from Hughes Brothers, indicating their desire to play a major role in this trend. 

A further factor in FRP rebar gaining attention in the construction industry is that its environmental footprint is much less than steel, with up to 50% less emissions in precast panels.i  Consideration of LEED and Institute for Sustainable Infrastructure standards in project design and planning will create even further interest in FRP rebar. 

For decades, FRP rebar has been almost exclusively used in specialized situations, such as where: 

• extreme chloride corrosion protection (exposure to deicing chemicals or marine environments) is required, such as bridge decks and seawalls,
• other corrosive environments (chemical or wastewater containment) are encountered,
  interference with electromagnetic fields or radio frequency is problematic,  such as MRI rooms, toll plazas, and radio transmission facilities,
• high voltages or stray currents are a problem, such as substations, duct banks, and dairy barns
• rust staining is an aesthetic concern, such as architectural panels,
• future penetration of cast in place concrete must occur as in  tunneling to create “soft eyes”, and
• rock bolting must occur in corrosive environments

For corrosion protection, epoxy coated steel (ECR) has been most commonly used, since it is lower cost than FRP rebar or other corrosion resistant reinforcements.  However, it has been found to not provide the degree of corrosion protection needed in many applications, leaving stainless steel, corrosion resistant steels, and FRP as the preferred alternatives to epoxy coated steeliiiii.  And in some jurisdictions, epoxy coated steel is not allowed, forcing the use of these alternative reinforcements.iv  All of these alternatives can be more expensive than epoxy coated steel.  Although on a life-cycle cost basis, they can be lower in cost where corrosion is a concern.

For  applications requiring electromagnetic or radio frequency transparency, FRP has been almost exclusively used. 
Recent technological advances, however, have reduced the cost of FRP rebar, and one manufacturer, Neuvokas Corp., is using a patented high-speed process to produce basalt FRP (BFRP) at a cost competitive with black steel.  This “sea change” potentially takes FRP out of the niche markets that it has inhabited and makes it a worthy competitor in nearly all reinforced concrete construction. 

With FRP rebar use likely to become more common-place, this paper looks at its unique properties and suggests that bundling, as a design consideration, will result in superior structures.  With FRP approaching a cost competitive with steel and with less labor, the intrinsic advantages of FRP rebar can be realized.  For general reinforced concrete structures these advantages are that FRP rebar is much lighter than steel creating advantages in lightening completed structure weight, reducing labor and machinery costs, and because of its corrosion resistant properties the potential of reducing concrete cover and increasing structure life. 

BUNDLING DESIGN 

Bundling of steel rebar has long been permitted by the major code agencies, but has not been commonly used.  Primarily, this is because of the additional labor required to field tie and handle bundles, without any significant improvement in structural properties.  Bundling, however, is sometimes used in heavily reinforced structures where uniform spacing would present difficulties in getting concrete thoroughly consolidated and distributed. 

It has been found that bundling up to four bars results in them functioning as an equivalent area single bar, with certain differences.vi  The ACI Code allows bundling of rebars in up to four-bar bundles, but does increase the development length based on the number of bars in a bundlevii.  This is true whether the reinforcement is steel or FRP (ACI 318 and ACI 440).  Further, ACI 318 7.6 allows bundling bars no larger than #11 bars, and requires enclosing stirrups or ties. 

FRP rebar while it has higher tensile strength than steel its guaranteed tensile strength, f*fu, decreases with size.  One manufacturer publishes a decline from 120 ksi for a #3 bar to 80 ksi for a #10 bar.  Using smaller bundled FRP bars will increase the tensile strength of a given reinforcement ratio when compared to a larger single bar, decreasing the amount of bar required.  This is the primary reason for considering bundling of FRP rebar. 

Also, bundling of rebar has advantages in terms of reducing bond stress, potentially increasing  rebar spacing in congested members, or allowing narrower, more aesthetically pleasing structural members.viii  The bond dependent coefficient describes the bond created between tension reinforcement and concrete relative to a crack width in a prescribed flexural beam.

From this equation it may be seen that for a given allowable crack width in a particular beam, the tension reinforcement will experience a certain average strain.  It also may be seen that the lower the bond dependent coefficient the higher the average strain to achieve a given crack width and that they are inversely proportional. 

Recent testing of Neuvokas basalt fiber rebar with two different coatings (#3 size) by the University of Nebraska has shown the kb to be 0.67 for a sand coating and 0.8 for uncoated bar.ix  For steel bar kb is approximately 1.0.  The conclusion being that BFRP in this size, used as tensile reinforcement in a flexural beam, can be subjected to 1.25 to 1.5 times the average strain as steel at the same crack width.  This is consistent with previous testing that has shown that the bond of FRP rebar to concrete is superior to steel, measured using direct pullout tests and flexural beam testsx to measure bond strength.  

Surface area for purposes of calculating bond strength is relatively easily determined for a single bar, but for bundled bars the entire surface area of each bar, with some of the surface area being interior to the bundle, cannot be bonded to the concrete to the same extent as the exterior.  A Texas DOT studyxi  found that the surface on the exterior of the bundle could be used to calculate bond strength of a bundle, if the bond strength of a single bar is known. 

ACI 318 R 12.4.2 states that, “Development length of bundled bars increased by 20% for three bar bundles and 33% for four bar bundles.  The cover and spacing criteria are derived from the equivalent bar diameter of a single bar.” 

So for reasons of gaining higher tensile strength and greater bond, bundling of FRP rebar is a recommended design consideration.

BUNDLING CAN REDUCE INVENTORY 

With just #3, #4 and #5 bars all bar sizes between #3 and #10 can be replicated with very little excess cross sectional area, using bundling. In many cases, this can be done with very little excess material as shown in Table 1 following: 

Table 1 - Bundled Bars for Equivalent Single Bar 

OTHER BENEFITS 

FRP rebar also has a longitudinal coefficient of thermal expansion that is similar to concrete.  The higher coefficient of thermal expansion of steel than concrete has resulted in concrete pops and spalling in pavements exposed to higher temperature differentials.  FRP rebar has a coefficient of thermal expansion nearly identical to concrete and thus little or no differential in expansion can occur.  In pavements where either high ambient temperatures or large differential ambient temperatures are experienced, this is an important consideration in reducing maintenance costs and poor ride quality. 

SUMMARY 

• FRP rebar is likely to become more commonly used as its cost decreases and the industry becomes more familiar with design of reinforced concrete structures using FRP rebar.
• FRP rebar is environmentally less impactful than steel and can assist in meeting higher LEED and ISI Envision ratings.
• FRP rebar has some unique properties that make bundling more attractive as a design consideration than for steel rebar.
• Because of declining FRP rebar tensile strength with bar diameter, bundling can lead to higher overall reinforcement tensile strength.
• Bundling results in greater surface area for bonding.
• Bundling can reduce inventory requirements.

For these reasons, it is recommended that bundling be considered during the design of reinforced concrete structures using FRP rebar.

ENDNOTES 

i Inman, M., E.R, Thorhallsson, K. Azrague, “A mechanical and environmental assessment and comparison of basalt fibre reinforced polymer (BFRP) rebar and steel rebar in concrete beams”,8th International Conference on Sustainability in Energy and Buildings, SEB-16, Turin, Italy, 11-13 September 2016 

ii Hartt, W.A.,”A Critical Review of Corrosion Performance for Epoxy-coated and Select Corrosion Resistant Reinforcements in Concrete Exposed to Chlorides” March 17, 2012 

iii  Clear, K.C., W.A. Hartt, J. McIntyre, and S.K. Lee, “Performance of Epoxy-Coated Reinforcing Steel in Highway Bridges,” Report No. 370, National Cooperative Highway Research Program, Washington, DC, 1995 

iv Ibid 

v Eamon, C. D., Jensen, E. A., Grace, N. F., and Shi, X. (2012). "Life-Cycle Cost Analysis of Alternative Reinforcement Materials for Bridge Superstructures Considering Cost and Maintenance Uncertainties" Journal of Materials in Civil Engineering, 24(4), 373-380, doi : 10.1061/( ASCE )MT .1943-5533.0000398  

vi ACI Code, 2015 

vii ibid 

viii.Jirsa, J.O., W. Chen, D.B. Grant and R.Elizondo, “Development of Bundled Reinforcing Steel”, Center for Transportation Research, Bureau of Engineering Research, The University of Texas at Austin, FHWA/TXDOT 96/1363-2F, December 1995

ix Morcous,G., Ph. D., P.E., “Determining the Bond-Dependent Coefficient of Basalt Fiber-Reinforced Polymer (BFRP) Bars”, University of Nebraska-Lincoln, Omaha, NE, December 2016 

x Morcous,G., Ph. D., P.E., Tawadrous, R.,  “Pullout Bond Strength of GatorBar (BFRP) Bars”, University of Nebraska-Lincoln, Omaha, NE, March 2015ibid, e 

xi Ibid, Jirsa 

EVALUATION OF GATORBAR FOR USE IN REINFORCED CONCRETE APPLICATIONS IN HAWAII

for Aloha Marketing 
Honolulu, Hawaii 
By Ian N. Robertson, Ph.D., S.E. 

April 27, 2017 

Introduction 

A review of literature related to the use of Basalt Fiber Reinforced Polymer (BFRP) bars as reinforcement in structural concrete was performed. In particular, this review focused on a number of test programs that evaluate the performance of BFRP bars fabricated by Neuvokas and referred to as “GatorBar”. This report presents the results of this assessment. 

In the past decade, Fiber Reinforced Polymer (FRP) reinforcing bars have been used extensively in structural concrete applications. The bars consist of continuous fibers embedded in an epoxy or vinylester polymer matrix. To date, the most commonly used fibers were glass, carbon and aramid, referred to as GFRP, CFRP and AFRP, respectively.  These reinforcing bars have gained in popularity primarily because of their immunity to corrosion. This leads to far superior durability of FRP reinforced concrete members when compared with concrete reinforced with traditional steel reinforcing bars, particularly in a marine or coastal environment common to the Hawaiian Islands. One major disadvantage of FRP bars has been the cost comparison with traditional steel reinforcing. However, increased production and use of FRP’s has led to decreases in the cost of these new materials. 

More recently interest has grown in the use of Basalt fibers to create BFRP reinforcing bars (Fiore, et al., 2015). Basalt is a natural inorganic material found in volcanic rocks originating from molten lava. Basalt has a melting temperature between 1500 oC and 1700 oC.  Once molten, the basalt can be extruded through small nozzles to produce continuous filaments of basalt fiber with diameters ranging from 13 to 20 m (Patnaik, 2009). BFRP bars have the same advantage of immunity to corrosion as do the other FRP bars. They are also poor conductors of heat and electric current, similar to glass fiber FRP bars. In addition, BFRP can be produced at a cost that is competitive with steel reinforcing bars. This makes them very attractive for use in concrete cast in marine or coastal exposure conditions. 

This literature review is intended to provide background to the performance of BFRP when used as reinforcing bars in concrete members. The review covers general aspects of BFRP reinforcing bar performance, and then focuses on the specific BFRP bars produced by Neuvokas Corporation and distributed under the trade name “GatorBar”. 

General Characteristics of Basalt Fiber Reinforced Polymer (BFRP) 

Basalt Fiber Reinforced Polymer (BFRP) reinforcing bars have high tensile strength while being significantly lighter than steel reinforcing bars. The tensile behavior of BFRP bars is linearly elastic with no yield point. Traditional reinforced concrete member design using steel reinforcement cannot be used to design members with BFRP reinforcement. Design of reinforced concrete members with BFRP reinforcing bars should follow the same procedures as outlined in ACI 440.1R design guidelines for glass (GFRP) and carbon (CFRP) reinforcing bars (ACI, 2006). 

Compared with reinforcing bars produced with traditional glass (GFRP) and carbon (CFRP) fibers, BFRP reinforcing bars have strength values greater than that of GFRP, but less than CFRP, while the modulus of elasticity is similar to that of GFRP (Elgabbas, et al., 2016). Bending tests performed on concrete beams reinforced with BFRP flexural reinforcement show that the bond between the bars and concrete is adequate to produce a well-distributed flexural crack pattern similar to what is observed in traditional reinforced concrete beams. Because of the relatively low modulus of elasticity of the BFRP bars, the bending stiffness reduces significantly after concrete cracking (Elgabbas, et al., 2016). The performance of BFRP reinforced concrete beams is similar to that observed for GFRP reinforced beams.  If BFRP is to be used in beam construction, this low post cracking stiffness must be considered when estimating the beam deflections (Lapko and Urbanski, 2015). 

As with other fiber-reinforced polymers, BFRP is a composite of fibers embedded in a polymer matrix. Although elevated temperatures have little effect on the basalt fibers, the polymer will lose strength and stiffness. Lu, et al. (2016) found that at 390oF (200oC), the tensile strength and elastic modulus of BFRP reduced by 37.5% and 31%, respectively, compared with room temperature. When embedded in concrete, the thermal insulation of the concrete cover will help to protect the BFRP, but this strength and stiffness reduction must be considered when designing structural load-bearing members using BFRP. 

Wei et al., (2011) performed durability tests on the performance of BFRP and GFRP composites when exposed to a seawater environment. They found that “the chemical stability of BFRP and GFRP in seawater is nearly the same.” Altalmas et al., (2015) performed bond tests on BFRP and GFRP reinforcing bars after exposure to various environments, including seawater. They conclude that sand-coated bars showed higher bond strength and higher adhesion to concrete than ribbed bars for both fiber types, with no change evident due to seawater exposure. 

Performance of GatorBar Reinforcing Bars 

GatorBar reinforcing bars are produced by Neuvokas Corporation, Ahmeek, Michigan. Neuvokas has sponsored a number of laboratory tests of the GatorBar physical and mechanical properties and their performance in concrete beams.  The technical reports from these tests were reviewed and are summarized in the following sections. 

Physical Properties of GatorBar Reinforcing Bars 

Two test series were performed under the direction of Prof. Brahim Benmokrane at the University of Sherbrooke in Canada to determine the material properties of GatorBar specimens. These tests were performed during the development phase of GatorBar, and show significant improvement in the material properties from the first series (Cousin and Benmokrane, 2014a) to the second series of tests (Cousin and Benmokrane, 2014b). 

Chemical resistance of the basalt fibers sourced from China and Russia was tested through immersion in deionized water, saline solution (10% NaCl), alkaline solution (pH: 12.9) and acidic solution (10% HCl). The report concludes that the “basalt fibers performed very well and no significant degradation was observed in the four chemical solutions” (Cousin and Benmokrane, 2014b). 

Both the basalt fibers and the polymer matrix are not electrically conductive.  In addition, they will not corrode in the presence of chlorides or other corrosive environments. They are therefore well suited to marine and coastal applications where corrosion of reinforcing steel presents a significant durability and maintenance concern. 

Physical properties were assessed during both test series. This included tests of fiber content, transverse coefficient of thermal expansion, water/moisture absorption, cure ratio, glass transition temperature (Tg), wicking, and microscopy analysis. The test results were compared with the ACI 440.6M-08 “Specification for Carbon and Glass Fiber-reinforced Polymer Bar Materials for Concrete Reinforcement” (ACI, 2008).  

In the first study, it was found that the test specimens satisfied the ACI requirements for fiber content and cure ratio, but some specimens did not meet the specified values for thermal expansion, absorption and glass transition temperature (Cousin and Benmokrane, 2014a). In addition, optical and scanning electron microscopy identified voids and other defects in the bars. 

After further product development, the second study was performed on new GatorBar specimens (Cousin and Benmokrane, 2014b). All specimens tested in the second study met or exceeded the ACI specifications. Fiber content was an average of 75.5% compared with the ACI required minimum of 70%. The average absorption for the test specimens was 0.70%, which satisfies the ACI maximum limit of 1%. The average glass transition temperature was 232oF (111oC) which exceeds the ACI required minimum of 212oF (100oC). The optical and scanning electron microscopy performed in the second study shows a reduction in voids and other defects in the bars. The study concludes that the tested GatorBars satisfy the ACI requirements for glass fiber content, water absorption and glass transition temperature (Cousin and Benmokrane, 2014b). 

The final phase of testing showed that immersing the GatorBars in an alkaline solution at 140oF (60oC) for 21 days did not affect the interlaminar shear strength or the bar microstructure (Cousin and Benmokrane, 2014b). 

Mechanical Properties of GatorBar Reinforcing Bars 

Mechanical tests of 3/8” diameter BFRP bars produced by Neuvokas Corp. were performed at Michigan Technological University (Fraley, 2016). The average results for the five specimens tested in this study are listed in Table 1. These tests were performed as part of a successful trial application of GatorBar reinforcement in curb and gutter sections on the Quincy Street reconstruction project in Hancock, Michigan (Jansson, 2017). Jansson reports that “Performance of the curb and gutter sections utilizing GatorBar were similar to those sections with conventional steel reinforcement, with no cracks noted.” 

Table 1: Average Tensile and Shear Test Results for Neuvokas Corp. GatorBar BFRP bars and Aslan GFRP and CFRP bars, for comparison. 

1 Hughes Brothers, Inc., 2011a; 2 Hughes Brothers, Inc., 2011b 

Also shown in Table 1 are typical mechanical properties for GFRP and CFRP reinforcing bars produced by Hughes Brother, Inc. under the trade name “Aslan FRP”. The basalt bars have 30% greater tensile strength than GFRP with a similar modulus of elasticity. 

Bond Strength for GatorBar in concrete 

In order for FRP bars to work effectively as reinforcement for concrete members, it is critical that sufficient bond strength be developed along the interface between the bar and the surrounding concrete. Glass and basalt fiber reinforced bars use a similar bond enhancement by means of a sand coating epoxied to the exterior of the bar. 

Bond tests were performed at the University of Nebraska, Lincoln, on behalf of Neuvokas Corp. Three bond test programs were performed under the direction of Prof. George Morcous. The first two studies investigated the pullout bond strength of GatorBar bars (Morcous and Tawadrous, 2015a and b), while the third study involved flexural beam tests to determine the bond-dependent coefficient of GatorBar bars (Morcous, 2016). The conclusion of all of these bond test programs was that the GatorBar BFRP bars with the primary exterior coating provide better bond with the concrete than deformed steel reinforcing bars of the same size. They also meet the bond requirements of the ACI 440K subcommittee for use in reinforced concrete applications. 

Recommended use of GatorBar BFRP bars 

Based on the literature review on basalt fiber-reinforced polymer bars, and the test results of GatorBar BFRP bars produced by Neuvokas, Inc., the following recommendations are made regarding the use of GatorBar reinforcement in concrete applications in Hawaii. 

1. GatorBar reinforcing bars can be used in all reinforced concrete applications where GFRP or CFRP reinforcing bars are currently used.
2. If GatorBar reinforcing bars are to be used as flexural reinforcement in concrete beams, walls or columns, careful attention must be paid to the anticipated deflection of the member under load.
   GatorBar reinforcing bars are particularly suited for use in concrete exposed to a marine or coastal environment because they will not corrode.
3. The use of GatorBar reinforcing bars in slab-on-grade applications will work effectively to limit crack size and distribute cracking in the same way as deformed steel reinforcing bars.
4. As with GFRP and CFRP reinforcing bars, Gatorbar cannot be bent in the field, but must be fabricated with the necessary bends in place. Therefore, applications that require only straight bars present a particular advantage for Gatorbar reinforcing.

R eferences 

ACI, 2006. ACI440.1R-06: Guide for the Design and Construction of Structural Concrete Reinforced with FRP Bars. ACI Committee 440, American Concrete Institute, Farmington Hills, MI, USA, 2006, pp. 44. 

ACI, 2008. ACI 440.6M-08: Specification for Carbon and Glass Fiber-reinforced Polymer Bar Materials for Concrete Reinforcement, American Concrete Institute, Farmington Hills, Michigan. 

Altalmas, A., El Refai, A., and Abed, F., (2015). Bond degradation of basalt fiber-reinforced polymer (BFRP) bars exposed to accelerated aging conditions, construction and Building Materials, 81, Elsevier Ltd. 

Cousin, P. and Benmokrane, B., 2014a. Preliminary Testing of Neuvokas Re-Bar Basalt Fiber-Reinforced Polymer (BFRP) Reinforcing Bars: Physical Properties, Technical Report No. 1, University of Sherbrooke, Canada, September 10, 2014. 

Cousin, P. and Benmokrane, B., 2014b. Phase I: Chemical Resistance of Three Types of Basalt Fibers; Phase II: Physical Properties and SEM Analysis of G1 Neuvokas Basalt Fiber-Reinforced Polymer (BFRP) ReBar; Phase III: Mechanical Characterization and SEM Analysis of Fractured Specimens, Technical Report No. 2, University of Sherbrooke, Canada, December, 2014. 

Elgabbas, F., Vincent, P., Ahmed, E.A., and Benmokrane, B., 2016. Experimental testing of basalt-fiber-reinforced polymer bars in concrete beams, Composites Part B, 91, Elsevier Ltd. 

Fiore, V., Scalici, T., Di Bella, G., and Valenza, A., 2015. A review on basalt fibre and its composites, Composites Part B, 74, Elsevier Ltd.  

Fraley, P., 2016. Test Results for Neuvokas Corp., Letter Report, Michigan Technological University, Houghton, Michigan, June 7, 2016. 

Hughes Brothers, Inc., 2011a. Glass Fiber Reinforced Polymer (GFRP) Rebar – Aslan 100 series; Fiberglass Rebar, Seward, Nebraska, http://www.aslanfrp.com/Media/Aslan100.pdf 

Hughes Brothers, Inc., 2011b. Carbon Fiber Reinforced Polymer (CFRP)Bar – Aslan 200 series, Seward, Nebraska, http://www.aslanfrp.com/Media/Aslan200.pdf 

Jansson, P.O., 2017. Letter from Michigan Department of Transportation, April 19, 2017. 

Lapko, A., and Urbanski, M., 2015. Experimental and theoretical analysis of deflectiosn of concrete beams reinforced with basalt rebar, Archives ScienceDirect, Elsevier Ltd. 

Lu, Z., Xian, G., and Li, H., 2016. Effects of elevated temperatures on the mechanical properties of basalt fibers and BFRP plates, Construction and Building Materials, 127, Elsevier Ltd. 

Morcous, G. and Tawadrous, R., 2015a. Pullout Bond Strength of GatorBar (BFRP) Bars, Final Report, University of Nebraska-Lincoln, Omaha, Nebraska, March 26, 2015. 

Morcous, G. and Tawadrous, R., 2015b. Pullout Bond Strength of GatorBar (BFRP) Bars, Final Report, University of Nebraska-Lincoln, Omaha, Nebraska, July 16, 2015. 

Morcous, G., 2016. Determining the Bond-Dependent Coefficient of Basalt Fiber-Reinforced Polymer (BFRP) Bars, Final Report, University of Nebraska-Lincoln, Omaha, Nebraska, December, 2016. 

Patnaik, A., 2009. Applications of basalt fiber reinforced polymer (BFRP) reinforcement for transportation infrastructure. Dev Res Agenda Transport Infrastruct – TRB 2009, 5 p. 
 

Modeling and Simulation of Subbase Improvements to Existing Clay for Rigid Pavement 

Matt Kero, VP of Engineering, Neuvokas Corp. 
Zhen Liu, Assistant Professor Civil Engineering, Michigan Technological University 

Challenge 

Customer has an over-budget situation for paving a laydown yard due to an unanticipated specification.  Customer needs to create the equivalent stiffness of the current specification using a more economical approach.  This should be accomplished with minimal changes to standard operating procedures for all involved if possible. 

Current Specification 

Concrete is to be 3,500 psi concrete (at 28 days).  Rebar is specified as #5 rebar at 24” OCBW centered in the concrete thickness.  This concrete is specified to be placed on a subbase scarified to 6”, stabilized with 36 pounds per square yard of lime and compacted to 95% maximum dry density.  Below the treated subbase there is 10’ of FAT clay and some FAT clay with sand. 

The slab will be exposed to fully loaded COMBiLift, C26000 type, forklifts.  These lifts do 60 passes per day per design lane.  The actual tire pressure loading can be seen in the Figure 2 below.  

Solution 

This paper analyzes three proposed methods of using base and subbase improvements and/or a higher than specified compressive strength concrete mix to achieve the same “composite” stiffness as current specification. This paper proposes incorporating corrosion resistant basalt fiber reinforced polymer rebar (BFRP – GatorBar by Neuvokas Corp) to improve the concrete crack development and virtually eliminate the life shortening effects of steel rebar corrosion.   These combinations of material properties were used to form a multilayered composite, slab-on-grade structure that could offer promising and economical alternatives to the proposed 10” concrete slab on 6” of lime-treated base.  Each of the proposed methods can be seen together in Figure 3. 

This paper will also analyze the deflection and maximum stress assuming the COMBiLift loading that is described above in the current specification. 

Neuvokas has prepared a multiphysics model with Michigan Technological University (MTU) that analyzes the current specification compared to the proposals listed below.  This model uses inputs such as subbase, base, and concrete stiffness and loading from the COMBilift that will be used in this steel laydown yard.  The model then outputs a “composite” stiffness, slab deflection, and slab ultimate stress.  The “composite” stiffness can be understood as the completed stiffness of the support system from concrete slab to subbase materials. 

A budgetary cost of each potential solution is also analyzed. 

Proposal 1 

1. 6” of 5,000 psi concrete reinforced with #3 GatorBar placed at 1” above the center of the slab thickness at 18” OCBW.    Contraction saw cuts should be 1.5 inches deep. This concrete will result in an almost 20% improvement in concrete elastic modulus and modulus of rupture
2. 6” of cement treated (CTB) base.  See Table 1 for material properties.

It is understood that to maximize the effect of cement in a soil it is necessary to have greater than 50% of the soil retained over 200 mesh (75 micron).   Cleaned sand would meet this standard and potentially provide an economical aggregate when mixed at a 50/50 ratio with the site clay soils (Little, 2009).  In this approach Neuvokas proposed preparing the base through removing 3” of soil below the subbase level, tilling 3” below the cut line and then adding 3” of cleaned sand (>200 mesh).  6% by weight of Portland cement would be added.  This mixture would then be thoroughly mixed and wetted, after which it would be pulverized and compacted in place.   

Proposal 2 

1. 6” of 5,000 psi concrete reinforced with #3 GatorBar placed at 1” above the center of the slab thickness at 18” OCBW
2. 6” of cement treated (CTB) base.  See Table 1 for material properties.
3. 6” of lime stabilized subgrade

In this proposal Neuvokas looked at a three layer composite created with a 6” subbase treated with lime as specified in the current plans covered with a 6” base of imported CTB (made from site spoils and imported sand as in proposal one) after which it would be covered with a 6” top slab.  The resulting composite slab would be 18” thick.  

Proposal 3 

1. 6” of 5,000 psi concrete reinforced with #3 GatorBar placed at 1” above the center of the slab thickness at 18” OCBW.
2. 6” of cement treated aggregate base (CTAB).  See Table 1 for material properties.

This proposal requires creating a six-inch CTAB base.   CTAB is defined as a mixture of aggregate material and a measured amount of Portland cement with water that hardens after compaction.   This method is commonly used in flexible or rigid roadway pavements.  The amount of cement and coarse aggregates will directly relate to the final strength and stiffness of the subbase.  CTAB can show elastic, slab-like response to loading. 

Table 1.  Assumptions used for materials in this study 

Material properties were determined from available field data and also from values documented in the literature when field data are inaccessible. The moduli of elasticity for cement-treated and lime treated clays were assumed to be 1.55 GPa and 517 MPa, respectively (Bhattacharja and Bhatty, 2003; Tuleubekov and Brill). The corresponding untreated soil is a Texas soil which has a Young's modulus of 10 MPa, a liquid limit of 62 and a plastic limit of 22 (Bhattacharja and Bhatty, 2003; GeotechData.info). The clay subgrade has a thickness of 10 feet. 

Slab loading is by using the Combi-lift C30,000 ground pressure. This machine when loaded with 15,000 lbs (total GVW of 30,000) results in 147psi at dual front tires and 133 psi at rear single tire, see image in Current Specification section of this document. 

Soil Mechanics Analysis 

The deflection on the pavement surface describes the degree to which the pavement structure is displaced under a load. Using one specific level of loading, a greater deflection indicates a lower stiffness of the system which consists of pavement, base, and subgrade layers. To characterize the stiffness of the system, a “composite” stiffness is defined in a way similar to that of the modulus of subgrade reaction, for which a plate is placed on the pavement for loading. In this case, we replaced the plate with a ground pressure of 147 psi, which is caused by the COMBiLift vehicle. The "composite" stiffness is calculated as the ratio between the pressure and the maximum deflection measured on the pavement surface. For simplicity, a circular loaded area with a radius of 0.1 m is chosen to approximate both the ground pressure cause by a normal vehicle tire and the loading used for modulus of subgrade reaction and for the falling weight deflectometer tests.  

Thus the deflection of the pavement structure and underlying subgrade is a very typical multi-region linear elastic problem. For this elastic material, the general constitutive relationship is Hooke's law: 
σ = C : ε

where σ is the Cauchy stress tensor, ε is the infinitesimal strain tensor, and C is the fourth-order stiffness tensor. However, in engineering applications, we usually do not deal with the above equation using the stress and strain as second order tensors and the stiffness as the forth order tensors.  Here we assume both concrete and soils including treated one are homogeneous and isotropic materials, then the stiffness tensor can be written using the Vigot notation.

Three-dimensional finite element analysis was conducted for a computational domain of 100 m by 100 m by 5 m. Such a large computational domain was first selected to ensure that the area to be analyzed is large enough to eliminate the boundary effect. The trial simulation results indicated that deformation caused by the aforementioned load only obviously impacts an area that is 20 to 30 m away from the center of the load, see Figure 4. Then the above 3D simulation is reduced into equivalent 2D simulation based on axial-symmetry of the problem.  

Table 2.  Results of soil mechanics analysis 

Soil Mechanics Analysis Discussion    

Proposal 1 and 2 

Analysis shows that the thickness of the pavement layer is the dominant factor for determining the “composite” stiffness.  Proposal 1 and Proposal 2 are modeled with six inches of concrete vs. ten inches of concrete and the resulting “composite” stiffness values where lower than that of the current specification.  The thickness and stiffness of the lime-stabilized clay layer had minimal effect on the “composite” stiffness because the stiffness of the lime-stabilized clay is less than 100 psi, Table 1, and this is many times less than the stiffness of the concrete at 4,074 psi for 5,000 psi concrete.  Even using a cement-treated clay subbase it will be difficult to match the “composite” stiffness that 10 inches of concrete can offer.  This is based on the literature searching of values that offer up to 225 psi, Table 1, elastic modulus.   

It should be noted that even though the “composite” stiffness is much less than the current specification that the deflection and slab maximum stress is still relatively low.  The maximum stress values are lower than the modulus of rupture, 530 psi, for 5000 psi compressive strength concrete.  As can be seen in Table 3 this creates a safety factor of 1.8 for Proposal 1 and a safety factor of 2.2 for Proposal 2.  This means that the slab will not fail under this loading.  When considering slab deformations ride quality is often an important factor.  When considering the type and quantity of traffic this slab would see, the deformations seen in Table 2 should not be an issue. 

Table 3.  Safety Factor when considering slab maximum stress and modulus of rupture 

Proposal 3 

Whether using 3500 psi or 5000 psi compressive strength concrete, Proposal 3 offers an alternative that can closely match the “composite” stiffness of the current specification with 10 inches of concrete.  Using a CTAB modulus of 1,450 psi the desired stiffness can be achieved (Lim and Zollinger, 2003).  This is a significant increase in stiffness that cannot be achieved by simply treating the existing clay. As Table 3 shows that the safety factor, when considering modulus of rupture for the 3,500 psi and 5,000 psi concrete, will exceed the current specification.  

Neuvokas believes that some amount of aggregate (crushed limestone) could be added to the existing clay, with cement then added as well, to create a base somewhere between CTB and a complete CTAB.  Testing would be needed to confirm the elastic modulus of the final mix design for this base. 

This type of base will increase the coefficient of friction between the concrete and base material.  Typically this can create more cracking in the slab-on-grade (Chen, 2003), but using Neuvokas GatorBar will reduce the amount of cracking that could occur from a base material such as CTAB. 

Rebar 

Neuvokas GatorBar is an alternative to black steel rebar in a many applications.  It offers the performance advantages such as zero rust and lower weight.  Table 4 shows one advantage of this weight savings, #5 steel rebar only offers 1,912 feet of rebar per ton where #3 GatorBar can offer 21,858 feet of rebar per ton.   

Assuming #5 steel rebar at $720/ton. 

Table 4.  Steel rebar vs. GatorBar product information. 

As can be seen in Table 5, calculations show that #5 black steel rebar (60 ksi tensile) on 24” centers provides 18,420 lbs of tensile restraint to the slab at a reinforcement ratio of .51%. The same calculation shows that #3 GatorBar (145 ksi tensile) on 16” centers provides 23,925 lbs of tensile restraint to the slab at the same reinforcement ratio of .46% (when considering the reduction in concrete slab thickness).  This 23% increase in tensile strength combined with the reduced internal stresses (provided by its low tensile modulus) will provide improved slab performance.  The benefits of this reduced tensile modulus are presented in the attached article titled: FRP Rebar in Slabs on Grade Benefit from Low Modulus of Elasticity by Steven E. Williams, P.E..  To summarize this paper the reduced tensile modulus of GatorBar will result in a larger crack spacing and larger crack widths. Neuvokas will typically review customer applications to verify that crack widths stay with American Association of State Highway Transportation Office (AASHTO) guidelines.   

When considering crack control using a smaller diameter GatorBar with a smaller spacing will help reduce the quantity and spacing of cracks.  By presenting more restraint over a greater surface area (2.2 to 1 increase) to the slab, the stress concentrations at each reinforcement spot are reduced.  This results in lower chances of punch-out failure and keeps the crack width small. 

GatorBar’s lower tensile modulus will combat curling caused by internal stresses and because it doesn’t rust, spalling from corrosion is not a concern if there are relatively more cracks.   

Site requires 31,460 yds2
Reinforcement ratio based on concrete thickness and OCBW

Table 5.  Steel rebar vs. GatorBar project comparison 

Economic Feasibility / Cost Analysis 

Table 4 provides a summary of the concrete, base, and subbase cost for each proposal.  It is assumed that all approaches use essentially the same amount of labor as the currently specified design. Proposal 1 would cost $27,437 more in subgrade cost, Proposal 2 would cost $101,054 more in subgrade cost, and Proposal 3 would cost $162,401 more in subgrade cost.  Each of these proposals results in significant cost savings for the customer when considering the reduction in concrete cover that can be achieved.  This cost is based on the summary of costs listed in Table 6 and 7. 

Note: Subbase cost in proposal 3 assumes importing 6" of crushed limestone.

Table 6.  Cost Summary 

Table 7.  Cost Summary of line items 

Conclusion and Summary 

The composite stiffness of a CTB base, whether Proposal 1 or Proposal 2, cannot match the current specification with ten inches of concrete.  While this is understood, the maximum stress within the concrete in any of these proposals is still lower than the modulus of rupture so the concrete in any of them should not crack with this loading.  Proposal 3 utilizing a crushed limestone CTAB can match or exceed the composite stiffness of the current specification while utilizing only six inches of concrete.  Each of these proposals have been specified and used in Texas on a variety of projects, and do not represent something that has not been done before.  The end result of this approach will be a slab with a high stiffness due to its six inches of “structural” base and increased concrete stiffness.   

By placing the GatorBar above the centerline in the slab thickness the rebar will truly serve as crack control for Houston Texas ground thrust due to expansive clay conditions.  Lastly because the subbase is more rigid the slab will benefit from increased resistance to potential pumping at expansion joints during rain events.  

Viewed holistically this approach will nominally change many aspects without making a major change in any one place.  The degree of difficulty to the concrete contractor installing the pavement is not significantly altered.  The cost of the materials involved for any of the proposals will result in money saved.  The Customer receives a superior slab that will carry the loads specified.   

Remaining Concerns / Further Analysis 

All calculations in this paper are for stiffness or deformation only. So they are not directly related to strength properties such as "3500 psi" or "5000 psi" for concrete strength. The stiffness only reflects the relationship between load/stress and deflection/deformation in small strain conditions. In such conditions, no plastic deformation or cracking occurs.  Besides, there are many other concerns in pavement design besides deformation, such as drainage and settlement. Secondly, time and the savings available needs to be mentioned when the above evaluation is conducted. This is because the pozzolanic reactions in lime-stabilized soils are much slower than the cement hydration reactions in cement-stabilized soils allowing contractors to begin work much sooner on cement treated base. Thirdly, changes in material properties may change the above results considerably. 

Another potential issue is differential settlement. This could be caused by either the variability of soil properties across the region or by a long-term load distributed over a specific area. Assuming the FAT clay has a medium to high compressibility, that is, a compressibility ratio of 0.2. The 10 feet clay layer could generate a consolidation settlement from several inches to a couple of feet, which, over the time scale of several months to several decades, could manifest itself, depending on the level of loading, the coefficient of consolidation of the clay, and local drainage condition. If a pile of steel is placed over an area for a long time, the underlying clay layer could produce significant consolidation settlement while that below other unloaded areas produce much less or even negligible settlement. This differential settlement can bend the concrete slab. Depending on the magnitude of the differential settlement, cracks could be initiated and propagated along the margins between the loading and unloading areas.  

Supporting Data 

In its preliminary research Neuvokas uncovered several articles concerning projects that used these approaches to great effect.  In particular the San Antonio Texas spur 66/Watson Road project and the West Virginia Route 9 project outside Martinsville have well documented the use of CTB.  Various sources also list different cement treatment levels and methods to place the Portland cement.  Document TM5-822-14 from the Army and Air Force further explain this method for soil stabilization.   TxDOT has published specification 275 that explains the placement of CTB as well. 

References 

Little, Dallas.  Evaluation of Structural Properties of Lime Stabilized Soils, National Lime Association, Jan. 1999. 

Muthunthan, Balasingam and Farid Sarioesseiri, Interpretation of Geotechnical Properties of Cement Treated Soils, Washington State Department of Transportation, July 2008. 

Lim, Seungwook and Dan Zollniger, Estimation of the Compressive Strength and Modulus of Elasticity of Cement-Treated Aggregate Base Materials, TRB 2003 Annual Meeting, 2003. 

Bhattacharja, Sankar and Javed Bhatty, Comparative Performance of Portland Cement and Lime Stabilization of Moderate to High Plasticity, Portland Cement Association, 2003. 

AASHTO, AASHTO Guide for Design of Pavement Structures, American Association of State Highway and Transportation Officials, Washington, D.C. 

ACPA, Stabilized Subbases, Concrete Pavement Technology Series,. 2008 

Army, TM 5-809-1, Concrete Floor Slabs on Grade Subjected to Heavy Loads, , Chapter 5 page 5-1 to 5-3, August 1987.

Little, Dallas, Recommended Practice for Stabilization of Subgrade Soils and Base Materials. NCHRP, August 2009. 
http://www.cement.org/think-harder-concrete-/paving/cement-modified-soils-(cms)/cms-case-histories/cement-speeds-up-stabilizes-txdot-san-antonio-project