FHWA LTBP Summary-Current Information on the Use of Overlays and Sealers

Introduction

The Federal Highway Administration’s (FHWA’s) Long-Term Bridge Performance (LTBP) Program is a long-term research effort, authorized by the U.S. Congress under the Safe, Accountable, Flexible, Efficient Transportation Equity Act legislation, to collect high-quality bridge data from a representative sample of highway bridges nationwide that will help the bridge community better understand bridge performance. The products from this program will be a collection of data-driven tools, including predictive and forecasting models that will enhance the abilities of bridge owners to optimize their management of bridges.

Bridge performance is defined by the LTBP Program in its LTBP Bridge Performance Primer as follows:

“Bridge performance encompasses how bridges function and behave under the complex and interrelated factors they are subjected to day in and day out—traffic volumes, loads, deicing chemicals, freeze-thaw cycles, rains, or high winds. Bridge design, construction, materials, age, and maintenance history also play roles in performance.”⁽¹⁾

The LTBP Bridge Performance Primer further states, “Performance is usually associated with some set of standards, whether absolute or relative, and performance can be measured against those standards.”⁽¹⁾

To better define the most important issues to investigate in the LTBP Program, the LTBP Program team conducted in-depth interviews with 16 States in 2008 and 2009 and discussed the bridge issues most vexing State transportation departments. The LTBP Program also convened a special workshop to identify bridge substructure issues. These processes are described in two FHWA publications, Long-Term Bridge Performance High Priority Bridge Performance Issues (Report No. FHWA-HRT-14-052) and TechBrief: FHWA LTBP Workshop to Identify Bridge Substructure Performance Issues (Report No. FHWA-HRT-13-049).^{(2, 3)} These discussions resulted in the identification of more than 20 important bridge issues, including the following 6 deemed by the LTBP Program as high priority:

• Untreated decks (no overlays).
• Treated decks (decks with overlays).
• Joints.
• Bearings.
• Coatings for structural steel girders.
• Verification of the condition of prestressing strands and tendons in prestressed concrete bridge members.

These six high-priority bridge performance issues will be investigated by collecting and evaluating field data on bridges throughout the United States. This is only possible through strong partnerships with the 52 State transportation departments (includes the District of Columbia and Puerto Rico). Each agency identified an individual who will serve as that State’s coordinator with the LTBP Program.

Once a year, the LTBP Program State coordinators meet in person to receive program updates and to discuss bridge issues in their States. During this annual meeting, breakout sessions are held, during which State coordinators describe bridge practices in their States on key topics; afterward, the breakout groups report back to the group as a whole. One of the topics discussed during the August 2013 LTBP Program State coordinators’ meeting was bridge overlays, and the most important trends from the meeting are presented in this publication.

Many different types of overlays have been used on bridge decks. This document focuses on the current use (as of August 2013) of overlays and sealers by State transportation departments. Future LTBP Program publications will focus on the historical development of overlays on a national basis, as well as the historic use of overlays by State transportation departments. In addition, this document discusses the current means (as of August 2013) of evaluating the performance and condition of the overlays and sealers.

Types of Overlays and Sealers Used by the States

The top ten overlays and sealers used in 2013 by the State transportation departments were ranked as most prevalent by the number of States that had tried each overlay type and are shown in figure 1:

1. Asphalt overlay (with or without a membrane).
2. Latex modified concrete (LMC) overlay.
3. Epoxy polymer concrete overlay.
4. Membranes (act as sealers).
5. Portland cement concrete overlay.
6. Silica fume (microsilica) concrete overlay.
7. High molecular weight methacrylate (HMWM) sealer, prime coat, or overlay.
8. Polyester polymer overlay.
9. Silane sealers.
10. Low slump/dense concrete overlay.

Note that this list includes rankings from States that have tried the overlay types even if they have stopped using them. Each of these types of overlays and sealers is discussed in the following subsections.

Asphalt Overlays

Asphalt overlays have been used across the United States, both with and without a membrane. (Membranes are discussed in detail in the subsection entitled Membranes (Used with Asphalt Overlays).) The benefits of asphalt overlays are their ease of use, relative low cost, and improvement to smoothness of ride (“rideability”). The challenges for using asphalt overlays are that they add dead load to the bridge, disbond from the concrete deck, and trap water and/or chlorides beneath them.⁽⁴⁾

As seen in figure 2, 38 of 52 State transportation departments have used asphalt overlays. Of these 38 States, 9 rated the use of asphalt overlays as successful, and 1 of the 38 States stopped using asphalt overlays. This likely indicates that the performance of asphalt overlays has been mixed, with some good performance and some less than successful performance, within a majority of the responding States.

LMC Overlays

The American Concrete Institute defines latex as “a dispersion of organic polymer particles in water” and further defines LMC as “hydraulic cement and aggregates combined at the time of mixing with organic polymers that are dispersed or redispersed in water.”^(5,6)

LMC overlays typically differ from conventional concrete in the following ways:

• LMC overlays contain a latex admixture, typically consisting of styrene-butadiene polymers in an emulsion.⁽⁷⁾
• The latex admixture has separate specifications, such as the combination of ASTM International standards ASTM C1438 and ASTM C1439, or the prequalification requirements in FHWA Report FHWA-RD-78-35.^(8,9,7)
• The water content of the latex emulsion is counted as part of the water in the concrete mixture.^(6,10)
• Air entraining admixtures are not typically used in LMC. “Latex loses its stability with air entrainment; hence, no air entraining admixture is used …. In fact, it is a good idea to add an anti-foamer to the latex before placing it in the mixer.”⁽¹¹⁾
• A mobile mixer is typically used for mixing the LMC concrete.
• LMC overlays are typically moist-cured for a period of hours (such as 48 to 60 h), and then air-cured until the required concrete compressive strength is reached. Conventional concretes are typically moist-cured for the entire length of the curing period.⁽⁶⁾

One major benefit of using LMC overlays is the performance. LMC overlays have much lower chloride permeability than conventional concretes.⁽⁷⁾ However, one challenge for using an LMC overlay is the comparatively longer time that it takes for an LMC overlay to attain the required strength for opening the bridge to traffic.⁽⁴⁾

LMC overlays were developed prior to low slump and silica fume overlays. Both of these later overlay types are generally less costly than LMC. All three overlay types require specialized equipment: LMC overlays require mobile mixers, low slump overlays require special vibrating finishing screeds, and silica fume overlays require bagged additions of silica fume to the ready-mix concrete truck or a special storage tank at the concrete plant. Some State specifications allow the contractor to choose either LMC overlays or one of the other overlay types, and those States that allow this choice may have stopped using LMC overlays because of contractor choice of the least costly option. That could mean that LMC overlays have good performance but their use was stopped for reasons other than performance. Some of the more recent LMC overlay mixes have very rapid curing, so a resurgence of LMC overlays is possible even though the cost may be higher because the reduced curing time allows vehicles to drive on the overlay sooner and therefore may justify increased usage of LMC overlays.

As seen in figure 2, 36 of 52 State transportation departments have used LMC overlays. Of these 36 States, 12 rated the use of LMC as successful, while 7 States have stopped using LMC overlays.

Epoxy Polymer Concrete Overlays

For cast-in-place concrete decks, the binder used in the concrete mixture to hold the other ingredients together is portland cement. However, in polymer concrete overlays, portland cement is not used as the binder—polymers are used as the binder.

Polymers are substances made up of monomers chemically bonded together. A monomer can be defined as “an organic molecule of relatively low molecular weight that creates a solid polymer by reacting with itself or other compounds of low molecular weight or both.”⁽¹²⁾ When monomers of low molecular weight chemically react together, they form a polymer. This polymer has the same chemical makeup and proportion of constituent ingredients as the monomers, but it has high molecular weight.⁽¹²⁾

The molecules of polymers can react together chemically to form prepolymers. Hundreds or thousands of polymers can be linked together to form a prepolymer. Prepolymers can then be combined to form a viscous or soft solid substance known as a resin. The resin is then partnered with a particular curing agents (hardening agents) to react together to form the final polymer binder of a higher molecular weight. When aggregates are added to the binder, a polymer concrete is formed. ^(12,13,14)

The type of binder used to create the polymer defines the name and material properties of the resulting polymer. Four main types of binders are used in polymer concrete overlays: epoxies, polyesters, polyurethanes, and methacrylates.⁽¹³⁾ Material properties for two types of polymer overlays, epoxy polymer concrete and polyester polymer concrete (see page 8), are provided in table 1.

n/a = Not available at this time.
cP = centipoise.

The epoxy polymer forms the binder for this type of concrete overlay. The binder actually consists of two components blended together—a component with the epoxy resin and a component with the curing (or hardening) material. The aggregates are then added to form the concrete overlay. Typically, no primers are required for this type of overlay.⁽¹³⁾

As seen in table 1, epoxy polymer concrete overlays have short working lives (30 to 60 min) and short curing durations (about 3 h). They also provide good compressive strengths (approximately 5,000 psi). Therefore, one benefit of this type of overlay is that it can be opened to traffic rather quickly after placement. Another benefit is that “cured epoxy binders are resistant to water, deicing chemicals, dilute acids, gasoline, and other petroleum products.”⁽¹³⁾ These overlays are relatively easy to place, typically using a broom and seed method, which consists of repeating layers of resin and coarse aggregate placed until the specified thickness is reached.⁽¹⁵⁾ However, these overlays are thin (0.25- to 0.5-inch thickness), and their life may be limited because of high traffic volume and studded tire or tire chain use.

As seen in figure 2, 33 of 52 State transportation departments have used epoxy polymer concrete overlays. Of these 33, 16 States rated the use of epoxy polymer concrete overlays as successful, and 2 States had stopped using epoxy polymer concrete overlays.

Membranes (Used with Asphalt Overlays)

Membranes are used on bridges by State transportation departments primarily for waterproofing decks, and many States use them in conjunction with asphalt overlays. The typical membrane-overlay system consists of the following layers:

1. Primer—Primers (typically with a methacrylate, epoxy, or polyurethane resin) are used to help the membrane bond to the concrete bridge deck effectively.
2. Membrane—Either a sheet membrane or a liquid-applied membrane is used for waterproofing.
3. Tack coat—Tack coats are used to help the asphalt overlay bond to the membrane.
4. Asphalt overlay—The asphalt overlay serves as the wearing surface on the bridge.⁽¹⁶⁾

Primers typically are liquid and may be applied using a pump or with a squeegee. Surface preparation is necessary so that the primer is applied to a clean, dry deck.⁽¹⁷⁾

Sheet membranes are applied using either heat (torch-applied) or by using an adhesive coating on the back of the sheet. To cover the entire bridge deck, sheets overlap one another. Care must be taken to ensure that there are no pathways for leakage in the overlapping regions.⁽¹⁷⁾

Liquid membranes may use a layer of reinforcing fabric. Newer membranes are applied via hand spraying or robotic equipment. The bond of the liquid membrane to the asphalt is key to a successful installation.^(16,17)

Some States employ tack coats or tack coats with aggregates spread over them on top of membranes to help achieve bond between the membrane and the asphalt. Care must be taken in both selection of the tack coat to make sure that it bonds well with the membrane and the asphalt and in allowing the tack coat to cure properly.⁽¹⁷⁾

As seen in figure 2, 28 of 52 State transportation departments have used membranes. Of these 28States, 7 States rated the use of membranes as successful, and 1 State had stopped using membranes.

Portland Cement Concrete Overlays

Portland cement concrete overlays are typically the same concrete mixture that is used for the portland cement concrete deck and therefore typically use portland cement as the main binding agent for the overlay. The overlay may or may not have reinforcement in it. If the portland cement concrete overlay is placed concurrently with the deck, then the overlay is said to be “monolithic.” If the overlay is placed at a later time than the deck is cast, then the overlay is said to be “integral.”⁽¹⁸⁾

Advantages of using portland cement concrete overlays are that the same materials are used for the reinforced concrete deck and the overlay and that the mechanical and thermal properties of the deck and overlay will be similar. Challenges of using this type of overlay, especially for integral overlays (wearing surfaces), are that the overlay material may shrink differently from the deck itself, and if the deck concrete is providing poor performance, then the overlay of the same type of concrete may provide the same poor performance.⁽⁴⁾

As seen in figure 2, 26 of 52 State transportation departments have used portland cement concrete overlays. Of these 26, 7 States rated the use of portland cement concrete overlays as successful, while 2 States had stopped using portland cement concrete overlays.

Silica Fume Concrete Overlays

Silica fume concrete overlays consist of a conventional concrete mixture with silica fume (microsilica) added to the mixture. The silica fume may be added as a supplement to the portland cement in the mixture, or it can be used as a replacement for part of the cement in the mixture. In addition, there may be a change in the amount of water in the mixture compared to conventional concretes. A State transportation department may keep the same amount of water for the mixture or may reduce the amount of water in the mixture—this is dependent on the water-to-cementitious materials ratio the State is trying to achieve and on whether or not the silica fume is added as a supplement or as a replacement for the cement. Curing of silica fume overlays may take 7 or more days.^(19,20)

One primary benefit of a silica fume overlay is that it typically has a very low permeability. The challenges of using silica fume overlays are that they are a somewhat stickier mixture while mixing and placing the overlay, and they require special curing procedures to reduce shrinkage cracking of the overlay.⁽⁴⁾ In addition, silica fume overlays require bagged additions of silica fume to the ready-mix concrete truck or a special storage tank at the concrete plant.

As seen in figure 3, 23 of 52 State transportation departments used silica fume overlays. Of these 23, 8 rated the use of silica fume overlays as successful, while 6 States had stopped using silica fume overlays. This statistic may not be reflective of poor performance but may be because of relatively long curing times for silica fume concrete overlays (days) compared with the epoxy polymer and polyester polymer overlays (hours).

HMWM Sealers, Prime Coats, and Polymer Concrete Overlays

HMWM resin is a chemical that is used by State transportation departments as a surface sealer for concrete bridge decks, as a crack sealer for concrete bridge decks, as a prime coat (bond coat) placed on a concrete bridge deck before a polyester polymer overlay is put in place, and as a polymer concrete overlay. HMWM resin systems can be “…a three component product composed of a monomer, a cumene hydroperoxide initiator and a cobalt naphthenate promoter.”⁽²¹⁾ They can also be a two-component system, with the promoter included with the resin (a promoted resin) and an initiator.⁽¹³⁾

In terms of benefits, “HMWM resins have been effective, when applied properly, in bonding and preventing infiltration of deicing solutions into both wide and hairline cracks.”⁽⁴⁾ In terms of cautions, HMWM is sensitive to temperature, and it produces airborne emissions.⁽²²⁾ At least one State required an airborne emissions monitoring plan when it was used.

As seen in figure 3, 21 of 52 State transportation departments had used methacrylate as a surface sealer, as a crack sealer, as a prime coat, or in a polymer concrete overlay. Of these 21 States, 8 States rated the use of methacrylate as successful, and no States had stopped using methacrylate. Successful use as a sealer should not be inferred as successful use in a polymer concrete overlay because the product performance may differ in some States.

Polyester Polymer Concrete Overlays

The polyester polymer forms the binder for this type of concrete overlay. The binder actually consists of two components blended together—a component with the polyester resin and a component with the hardening material (also called an initiator). The aggregates are then added to form the concrete overlay.⁽¹³⁾

As seen in table 1, polyester polymer concrete overlays have short working lives (10 to 60 min) and short curing durations (1 to 5 h). The working life of polyester polymer concrete overlays “…can be easily adjusted to almost any range by varying the amount of initiator and promoter.”⁽¹³⁾ Temperature also plays a role in working life: “…as a general rule, polyester [polymer concrete overlays] should not be used at application temperatures below 50°F… unless recommended by the manufacturer.”⁽¹³⁾ Polyester polymer concrete overlays also provide good compressive strengths (approximately 4,000 psi). Therefore, one benefit of this type of overlay is that the roadway can be opened to traffic rather quickly after placement. Another benefit is that “cured polyester binders are resistant to water, deicing chemicals, dilute acids, gasoline, and other petroleum products.”⁽¹³⁾

Primers must be used for this type of overlay. In addition, care must be taken while mixing the polyester binders because both the polyester resin and the hardening agent are flammable. “Inert liquids or fillers are incorporated by the manufacturer to minimize the explosion hazard.”⁽¹³⁾ These materials are normally premixed and placed in thicker layers (0.5- to 1-inch layers) than epoxy polymer overlays and therefore have the potential to last longer.

As seen in figure 3, 16 of 52 State transportation departments used polyester polymer concrete overlays. Of these 16 States, 10 rated the use of polyester polymer concrete overlays as successful, and no States had stopped using polyester polymer concrete overlays.

Silane Sealers

Sealers for concrete bridge decks are available in two main types: penetrating sealers and film-forming sealers. The penetrating sealers travel down into the pores of the concrete surface, whereas film formers create a layer or film over the deck that prevents water and chloride ions from entering the concrete deck. The challenges with the penetrating sealer are how deep they are able to penetrate, and some have volatile organic compounds contained within them. The challenges with film formers are a potential reduction in skid resistance on the deck, and they may wear out owing to abrasion from vehicles.⁽²³⁾

Silane is a penetrating sealer and is actually deemed a reactive penetrant sealer because silane reacts with the concrete and forms a layer that resists water entering the pores.⁽²³⁾

In terms of performance, silane sealers have been noted as good performers. “Although silanes and siloxanes have similar water repellent abilities, silane molecules are smaller than siloxane molecules, so they penetrate deeper into the concrete. Therefore, they provide longer-lasting protection to concrete exposed to abrasion. The higher the solids content of silanes, the deeper the penetration and the better the performance.”⁽²³⁾

As seen in figure 3, 14 of 52 State transportation departments used silane sealers. Of these 14 States, 2 rated the use of silane sealers as successful, and none of the 14 States had stopped using silane sealers.

Low Slump Concrete Overlays

Low slump concrete overlays are also known as low slump dense concrete overlays. State transportation departments may have a specification prescribing the mix ingredients or may have a performance-based specification for the overlays. Slumps for this type of overlay are generally less than or equal to 1 inch, and a mobile mixer is frequently used.

Low slump dense concrete overlays typically have lower permeabilities than conventional concretes. However, low slump dense concrete overlays have long cure times and may be difficult to place.⁽⁴⁾

As seen in figure 3, 12 of 52 State transportation departments used low slump concrete overlays. Of these 12 States, 4 States rated the use of low slump concrete overlays as successful, while 1 State had stopped using low slump concrete overlays.

How the States evaluated Overlays and Sealers

While all States used visual inspection to evaluate bridge decks and bridge deck overlays, some States implemented additional measures when they suspected a problem. As seen in table 2, the predominant method of assessing bridge decks and overlays after or concurrent with a visual inspection was by chain drag and/or hammer sounding.

The following six different types of nondestructive evaluation techniques were used by the States: ground penetrating radar (GPR), infrared thermography, high-resolution imaging, electrical resistivity (ER), a nuclear density gauge, and impact echo (IE).

Conclusions

Many different types of overlays were used on bridge decks for the 52 State transportation departments. This document focuses on the currentuse (as of August 2013) of overlays and sealers, as well as the methods of evaluation employed by the State transportation departments. This information will be used in planning upcoming evaluations of bridges with treated decks as part of FHWA’s LTBP Program.

References

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