It’s no secret that the nation’s highway and bridge infrastructure is in need of rehabilitation after decades of wear and tear. Bridges have unique challenges due to the combination of mechanical assault (deflection, freeze/thaw) and chemical assault (deicing salts) to which they’re exposed. Additionally, bridge shutdowns are very disruptive due to limited routing alternatives. So when they are shut down for repairs, the repairs need to be optimized for efficiency, longevity and, of course, life-cycle costs.
A cost-effective way to rehabilitate concrete bridge decks that are structurally sound is to remove the damaged wearing surface and install a concrete-based overlay. The three most popular concrete overlay options in the U.S. are latex-modified concrete (LMC), microsilica concrete (MSC) and low-slump concrete (LSC).
Latex-modified concrete was developed in the 1960s for bridge deck overlays, and since then over 10,000 bridges have been successfully rehabilitated. LMC also has been used to renovate roadways, parking decks, marine structures and even major league sports stadiums.
In the 1980s, silica fume, or microsilica, was introduced in the U.S. as a concrete admixture reported to provide similar benefits as latex at lower cost. However, microsilica, a waste product recovered from ferrosilicon manufacturing, behaves very differently than latex when added to portland cement concrete.
We’ll examine some of the differences between latex, microsilica and low-slump concretes and their effect on bridge deck longevity.
Tiny and tough
Latex produced for use in concrete modification is a suspension of tiny (0.2 micron average diam.) styrene-butadiene (S-B) polymer particles in water, typically about 50% polymer solids. Styrene-butadiene polymers are known for their hydrophobicity or excellent water resistance. The polymer particles coalesce or fuse together when in intimate contact to form a highly waterproof polymer film.
A typical LMC formulation contains 15% latex solids as a percent of cement solids, or 24.5 gal of latex per cu yd of concrete.
It’s useful to visualize the structure formation of LMC (see chart above). S-B latex is composed of trillions of tiny polymer beads suspended in water. These polymer beads are hydrophobic by nature and prefer to reside on a nonaqueous surface, such as cement, sand, aggregate or a prepared concrete deck. As soon as LMC mixing and pouring begin, latex particles quickly coat every available inorganic surface they can find, including the existing deck. There are approximately 1 million trillion latex particles per cu yd of LMC, all looking for an available surface, as free water leaves the system by evaporation or cement hydration. A monolayer of latex particles in 1 cu yd of LMC would make a continuous film covering six football fields. As the particles form close pack arrays, they coalesce, or knit together, to form a highly waterproof polymer film. Because of their small size, the particles can enter and seal the capillaries formed as free water leaves the concrete. The polymer film essentially waterproofs the concrete.
In the microstructure and fracture behavior of LMC and portland cement concrete (PCC), the fracture running through the PCC specimen is brittle, as expected. The fracture in the LMC specimen also is ultimately brittle, but there are multiple strands of elongated polymer clearly visible. These domains of relatively soft, plastic polymer throughout its structure are the reason LMC exhibits reduced crack propagation, higher flexural strength and lower elastic modulus than other cementitious materials, including microsilica and low-slump concretes. Additionally, the PCC specimen is sponge-like with porosity, while there is little visible porosity in the LMC specimen.
Film fits the thin
Once the decision has been made to repair rather than replace a concrete structure, the question becomes one of choosing the best repair option. Portland cement concrete is a wonderful construction material, but has several well-documented weaknesses, including:
- Low strength in tension or flex;
- High porosity and permeability;
- Low resistance to crack propagation; and
- Poor adhesion to other surfaces, including itself.
These weaknesses make conventional concrete a poor choice as a thin overlay or patching material for existing concrete structures. However, by adding a low-modulus, film-forming polymer component, a latex modifier, concrete’s properties are dramatically improved in each of these areas.
For a relatively thin concrete overlay to last at least 20 years, it must be fully bonded to the substrate, so the deck acts as one, rather than two separate decks. Reduced modulus of elasticity is important to minimize stresses in the overlay during deflection of the deck. Low permeability is necessary to protect rebar from corrosion and to minimize freeze/thaw damage. Reduced crack propagation is important, due to the tendency of concrete to crack when bridge decks undergo deflection and tensile load. And finally, the surface should be wear-resistant to maintain a quality, nonskid riding surface over the long term.
Free to choose
Cementitious bridge deck overlay options specified by DOTs today include latex-modified concrete, microsilica concrete and low-slump concrete. Rapid-setting concrete is another option for special situations and will be addressed separately. State specifications vary, but some states permit contractors to choose among the three options, essentially as equivalents, for bridge deck overlays. Yet, are they equivalent in properties, performance and life-cycle costs?
Let’s consider the similarities and differences between LMC, MSC and LSC.
LMC, MSC and LSC all provide low water-to-cement ratios, which is important for reduced shrinkage and reduced stresses at the bond interface and in the overlay itself. W/C ratios of 0.4 or less can be achieved by all three systems.
- Compressive strength and elastic modulus
LMC, MSC and LSC all provide adequate compressive strength development. It should be cautioned that specifying higher strengths than necessary for an overlay or patching material generally does not make good sense. Such designs often create higher elastic modulus or stiffer materials, which in turn cause higher stresses to develop in the overlay during deck deflection. Higher-strength, higher-modulus materials provide potential advantages for full deck designs, but not for thin overlays. The goal is to minimize stresses caused by deck deflection, which is achieved by reducing, not increasing, the elastic modulus of the overlay material.
The addition of 15% latex, based on cement solids, to concrete typically reduces the elastic modulus by about 15%. Latex does not react with concrete components, but is present as a distinct, low-modulus phase.
Of course, it is important that an overlay material minimize penetration of water and salt into the underlying deck. LMC and MSC both reduce the permeability of concrete by physically blocking the capillary structure normally present in concrete. LMC and MSC are both protective when they are intact and crack-free. LSC has no such mechanism for blocking its capillary porosity.
Is LMC’s permeability protection maintained over time? Fourteen bridges in Virginia with LMC overlays ranging from 2 to 20 years of age were examined for overlay condition and performance. Cores were taken in the shoulder, wheel track and center areas and examined for permeability and bond strength. Consideration of the average permeabilities of the LMC overlays relative to the base concretes demonstrates the continuing effectiveness of the overlays after up to 20 years.
- Bond strength
For a 11?2 -in. to 3-in. overlay to last at least 20 years, it must act as one with the substrate deck. In the event of de-bonding or de-lamination, the overlay will certainly fail prematurely.
Styrene-butadiene latexes, such as Modifier A, are excellent adhesives, and similar products are sold as adhesives for other applications. As LMC is applied to a well-prepared concrete deck, latex quickly coats the aged concrete and, in effect, forms an in-situ adhesive for the overlay. The adhesive power of latex creates bond joints that are generally stronger than either the base concrete or the overlay itself.
To examine bond strength in the Virginia study, cores were broken in shear to determine relative bond strength of the overlay. The data for LMC overlays show the cores failed primarily in the base concrete, to a lesser extent in the overlay and very little in the bond joint area.
It is interesting to note that there were two bridges in the study where both conventional concrete (PCC) and LMC overlays were installed at the same time. For the conventional concrete overlays, the core failures occurred primarily at the bond interface or in the overlay itself. For the LMC overlays on the same bridge, failure occurred primarily in the base concrete. This is dramatic evidence of the long-term integrity of LMC’s in-situ adhesive bond. The shear bond rupture strengths of the LMC overlays on these two bridges averaged 1,092 psi, compared with 614 psi for the PCC overlays installed at the same time.
- Crack propagation
Cracks propagate easily in conventional concrete under high tensile or flexural loads. LMC is filled with domains of low-modulus polymer that provide stress relief at crack tips, thus decreasing the tendency for cracks to propagate. Proof that this mechanism is in play is demonstrated by LMC flexural strengths up to 50% higher than conventional concrete. This feature is unique to polymer-containing concretes.
Latex will not prevent concrete cracking caused by deck design flaws or poor curing practices; however, reducing crack propagation in an overlay or repair could mean many years of added service life.
- Wear resistance
It seems intuitive that the wear resistance of concrete should improve with increasing strength. However, this is often not the case with brittle materials, such as concrete. Toughness and elasticity are at least as important when designing for wear resistance. For example, many materials are much stronger and harder than rubber, but how many of them could last 50,000 miles as automobile tires do? Few, if any.
Researchers have noted “a tremendous increase in the abrasion resistance of latex-modified mortars and concretes.” One reported a 90% reduction in abrasion wear, comparing a 10% S-B latex-modified mortar with an unmodified mortar.
Kuhlmann tested mortars modified with latex and microsilica for wear resistance in the Los Angeles machine, which is a measure of both abrasion and impact. After only 2,000 cycles, the latex-modified mortar cubes had lost 35% less weight than the microsilica cubes.
Recovering the money
At first look, LMC seems an expensive proposition, since latex adds about $110 material cost to a cu yd of concrete. However, material cost is a relatively small part of the total picture. The important question is “What is the impact of material choice on life-cycle cost of a roadway or bridge?”
Michael Sprinkel, Virginia Transportation Research Council, determined that the cost of an installed 2-in.-thick LMC overlay in Virginia in 2003 averaged $130/sq yd. This covered all costs, including traffic control at $46/sq yd. The cost of latex was $6.12/sq yd, or 4.7% of the total. This is consistent with earlier analyses by the Dow Chemical Co., where latex cost averaged 2-4% of overlay cost, depending upon job size.
Let’s take an example:
- $500,000 bridge overlay job (total cost of nonlatex bridge overlay)
- 15 years overlay design life
$500,000/15 yrs = $33,300 per year of service life in today’s dollars
Assume latex added to job @ 5%, or $25,000 additional cost.
If LMC adds just one year to service life (valued at $33,300/year), its cost ($25,000) is recovered. In practice, LMC has added multiple years of service life to bridge decks throughout the U.S. In a follow-up article, operational bridges with original LMC overlays 25-30 years old will be reviewed.
A relatively recent addition to the concrete family is rapid-setting concrete. These products are low-shrinkage, nonportland cement concretes, which harden fast enough to permit traffic in hours, rather than days. Commercial products, such as Rapid Set, are especially useful for overlays and patching jobs, where traffic disruption or worker exposure must be minimized. Latex provides the same benefits of low permeability, adhesive bond, reduced modulus and improved wear resistance.