Deck Protection Systems for Post-Tensioned
Segmental Concrete Bridges

Michael M. Sprinkel, P.E.
Associate Director
Virginia Transportation Research Council
Charlottesville, Virginia 22903
434-293-1941
Michael.Sprinkel@VirginiaDOT.org

Abstract

Conventional deck protection systems for post-tensioned segmental concrete bridges include thin bonded concrete overlays and membranes overlaid with a wearing surface of asphalt. While these systems have performed well on many bridges the risk of early failure is high because of the complexity of the construction procedures. Good surface preparation, low shrink concrete mixtures, and good curing are required for successful thin bonded overlays. A properly installed membrane and rut resistance asphalt are required for a successful membrane and asphalt overlay installation. Both systems are expensive. Cost data based on bid tabulations in Virginia between 1999 and 2002 indicate the average cost of a bonded concrete overlay, including surface preparation and saw cut grooves is $80 per square yard. The average cost of a membrane and asphalt, including grinding is $51 per square yard but on a life cycle basis the cost of the asphalt is more because the concrete overlay typically lasts twice as long.

A monolithic concrete overlay protection system for post-tensioned segmental concrete bridges is simple and low in cost compared to conventional systems. The monolithic concrete is cast at the time the segments are cast. Diamond grinding equipment is used to obtain the desired surface profile. Based on the average cost of bridge superstructure concrete in Virginia of $438 per cubic yard a 2-in thick monolithic overlay would cost approximately $24 per square yard. Diamond grinding and saw cut grooves would each add another $6 per square yard for a total of $36 per square yard. The cost should be much lower because only the cost of the material increases as the overlay portion of the segment is cast.

Thin bonded epoxy overlays have a 27-year record of providing a skid resistance wearing and protection surface for conventionally reinforced concrete bridge decks. The epoxy overlay should perform just as well on post-tensioned segmental concrete bridge decks. The system is easily applied. Two layers of epoxy and aggregate are placed on a shot blasted surface. The average cost is $33 per square yard including grinding and shot blasting. Costing 34 to 45 percent of the conventional systems, monolithic concrete and thin bonded epoxy overlays should be used as deck protection systems for post-tensioned segmental concrete bridges.

In many situations the low permeability concrete used in the segments has sufficient resistance to the penetration of chloride ions that an additional deck protection system is not necessary to prevent corrosion of the deck reinforcement for 100 years or more. The time to corrosion and therefore service life would increase with an increase in concrete cover.

Introduction

Post-tensioned segmental concrete bridge construction provides the most practical and economical bridges for many applications. The effectiveness of the construction increases as the length of the bridge and therefore the number of segments increases. A major concern for bridge engineers and owners is what to do if chlorides reach critical reinforcement in the superstructure. A practical and cost effective repair is not easily envisioned. Consequently, bridge engineers and owners generally agree that a deck protection system is required to prevent the infiltration of chloride ions. The most commonly used deck protection systems are the thin bonded hydraulic cement concrete overlay and the waterproof membrane overlaid with asphalt concrete. These protection strategies are complicated, expensive and come with a high risk of failure. Less expensive systems that have been successfully used on other types of bridges with concrete decks include a thin epoxy concrete overlay, increased cover depth over the reinforcement, and low permeability concrete. Considerable money can be saved by using these systems on post-tensioned segmental concrete bridges.

Objective

The objective of this paper is to compare conventional and new deck protection systems for post-tensioned segmental concrete bridges.

Methodology

Literature was reviewed to identify deck protection systems used on post-tensioned segmental concrete bridges and other types of bridges with concrete decks. Information on the design, construction steps, cost and performance of the systems was obtained. Protection systems selected for comparison included the thin bonded hydraulic cement concrete overlay, waterproof membrane overlaid with asphalt, thin bonded epoxy concrete overlay, monolithically cast concrete, and low permeability concrete segments.

Cost data based on bid tabulations in Virginia between 1999 and 2002 was used to compare the deck protection systems because of the availability of the large amount of data. The data was obtained from the Virginia Department of Transportation bridge office in Richmond Virginia. The data is for protection systems used on many types of bridges with concrete decks (cast in place concrete on beams, prestressed box beams, prestressed slabs, post-tensioned segmental, etc.). While the cost of a protection system will vary with location, access, bridge design, and material quantity, the relationships between the costs of the systems should reasonably approximate most situations.

RESULTS

Design

A sketch of the five systems is shown in Figure 1.

Construction Steps

Construction steps for the five systems are shown in Table 1.

Cost

Average cost data based on bid tabulations in Virginia is shown in Table 2 along with life cycle cost estimates. A zero interest rate was used in the life cycle cost estimates.

Performance

The service life of a deck protection system is the piece of information that is necessary for a life cycle cost analysis. Unfortunately, reaching a consensus on service life is difficult if not impossible. Consequently, the service life values used to compare the deck protection systems (shown in Table 2) are estimates that come from more than 30 years of experience with these systems.

Comparison of Deck Protection Systems

Thin Bonded Hydraulic Cement Concrete Overlays

Thin bonded concrete overlays are the most expensive deck protection strategy based on initial cost. Cost data indicate the average cost of a bonded concrete overlay, including grinding, surface preparation and saw cut grooves is $80 per square yard. Properly constructed, thin bonded concrete overlays can last 30 years or more. Unfortunately, some overlays have cracked and delaminated and had to be replaced before the bridge was opened to traffic. Good surface preparation, low shrink concrete mixtures, and good curing are required for successful thin bonded overlays. Factors that can contribute to premature delamination of the overlay include poor surface preparation, use of mixture proportions with high shrinkage, use of thick overlays, early shrinkage cracking in the overlay, and creep and shrinkage of the newly constructed segmental superstructure. While long lasting overlays have been successfully constructed, they come with a high initial cost and high risk of early failure.

Waterproof Membrane Overlaid with Asphalt

A waterproof membrane overlaid with asphalt is the second most expensive deck protection strategy for post-tensioned segmental concrete bridges based on initial cost. Cost data indicate the average cost of a membrane and asphalt, including grinding is $51 per square yard. The installed membrane accounts for most of the cost. A properly installed membrane and rut resistance asphalt are required for a successful membrane and asphalt overlay installation. The risk of early failure is high because of the complexity of the construction procedures. Rutting and shoving of the asphalt overlay can also be a problem. Replacement of the overlay every 15 years can be expected. The membrane typically has to be replaced when the overlay is replaced. The initial cost of the asphalt overlay on membrane is about 64 percent of a bonded concrete overlay but on a life cycle basis the cost is approximately 20 percent more with 2 overlays and membranes placed over 30 years.

Thin Bonded Epoxy Concrete Overlays

The epoxy overlay is a deck protection system that has been successfully used for 27 years on conventionally reinforced concrete bridge decks. The epoxy overlay has been shown to prevent the infusion of the chloride ions and can be expected to provide a skid resistance wearing and protected system for decks for 15 to 30 years depending on traffic volume.¹ The epoxy overlay should perform just as well on post-tensioned segmental concrete bridge decks. The system is easily applied. Two layers of epoxy and aggregate are placed on a shot blasted surface. The average cost is $33 per square yard including grinding and shot blasting. It costs approximately 65 percent of an asphalt overlay on a membrane. However, on a life cycle basis, the cost can be 32 percent of that of the membrane when the epoxy overlay lasts for 30 years. An additional benefit of the epoxy overlay is that it is only ¼ inches thick and if spalls occur, they do not have a major impact on the ride quality and repairs are easily done. By comparison, the spalling of an asphalt overlay leaves a much deeper hole. The thin epoxy overlay is not prone to cracking and delamination like the hydraulic cement concrete overlay. The epoxy overlay is flexible and will not likely crack and delaminate due to early age creep and shrinkage in the post-tensioned segmental concrete bridge. AASHTO guide specifications for the thin bonded overlay were published in 1995.¹ Thin bonded epoxy concrete overlays should be considered for use as a deck protection system for post-tensioned segmental concrete bridges. For added protection a layer of epoxy could be placed over the joints prior to placing the epoxy overlay.

Monolithic Hydraulic Cement Concrete

The monolithic hydraulic cement concrete protection system is another low cost alternative to the conventional protection systems. The system involves casting an extra 2 inches of concrete on the segments at the time the segments are cast. After all of the segments are post-tensioned, a diamond grinding machine is used to correct surface irregularities and provide the final deck profile. Grooves are saw cut for skid resistance. Good skid resistance is obtained when diamond grinding is used to correct the profile of concrete pavements. Saw cut grooves are not required. If the diamond ground surface is acceptable for concrete pavements it should be acceptable for bridge decks. Elimination of saw cut grooves would save $6 per square yard.

Based on the average cost of bridge superstructure concrete in Virginia of $438 per cubic yard a 2-in thick monolithic overlay would cost approximately $24 per square yard. Diamond grinding and saw cut grooves would each add another $6 per square yard for a total of $36 per square yard. The cost should be much lower because only the cost of the material increases as the overlay portion of the segment is cast. The monolithic hydraulic cement concrete protection system can be expected to protect the deck as long or longer than a quality thin bonded hydraulic cement concrete overlay. At 30 years, if the top 2 inches of integral concrete contains sufficient chlorides to warrant renewal and replacement, a thin bonded hydraulic cement concrete overlay can be placed at that time. However, because of the low permeabilities that are achieved with today's concretes that are prepared with low water to cement ratios and pozzolans or slag, it is reasonable to expect that the monolithic concrete would not contain sufficient chloride ions to warrant removal for more than 150 years (see later discussion on low permeability concrete).² If the monolithic hydraulic cement concrete is replaced at 30 years, (same age as the thin bonded concrete overlay) the life cycle cost is 45 percent of that of a hydraulic cement concrete overlay. If replaced at 60 years, the life cycle cost if 23 percent of the hydraulic cement concrete overlay and if it lasts 90 years, it is 15 percent.

The monolithic concrete overlay protection system has the lowest risk of problems since the concrete is cast on the segments as they are fabricated. Delamination and cracking are not issues. Opponents of the monolithic concrete protection system are concerned that at the time the overlay is removed, the change in the stresses and strains in the superstructure would cause an unacceptable change in surface profile and distress in the superstructure. The magnitude of the change in the stress and strain would depend on the thickness of the concrete removed and the design of the superstructure. The change in the stress and strain in the superstructure would decrease with a decrease in the thickness of the concrete removed and with an increase in the depth of the superstructure and the thickness of the deck. It is reasonable to expect that design guidelines for the thickness of the monolithic concrete removed and superstructure design can be developed to keep distress and changes in surface profile at the time of concrete removal to acceptable levels. A good paper on this issue would provide a major contribution to post-tensioned segmental concrete bridge technology.

Low Permeability Concrete in Segments

Most reports on time to corrosion of reinforcement in decks are based on experiences with older conventional bridge decks that were typically constructed with Portland cement and with a water to cement ratio of 0.45 or higher. The low permeability concretes currently being used to construct segments for post-tensioned bridges have a significantly higher resistance to the penetration of chloride ions and moisture than the concretes in these older decks. Table 3 shows the permeability to chloride ion at 1 year of concrete deck mixtures.² Mixtures with pozzolans and slag and water to cement ratios of 0.35 and 0.4 have a permeability to chloride ion that is approximately one forth to one tenth of that of mixtures with Portland cement and a water to cement ratio of 0.45. The diffusion constant for the conventional deck concrete was 5 x 10^-8 cm^2/sec.² The diffusion constants for the low permeability concretes were 0.2 to 1 x 10^-8 cm^2/sec.² The diffusion constants for the low permeability concretes are approximately one fifth to one twenty fifth of that of the conventional deck concretes.Chloride corrosion induced spalling could be expected in these older decks in 37 years.³ The deck of segments constructed with low permeability concrete (water to cement ratio of 0.35 to 0.4 and pozzolans or slag) and free of cracks can be expected to be free of chloride corrosion induced spalling for 4 to 10 times longer. Since post-tensioned structures are free of cracks, the low permeability concrete should not have chlorides present in sufficient quantities to cause corrosion of reinforcement with a 2-in cover for 150 to 370 years. The use of a protection strategy other than casting the segments with low permeability concrete is difficult to justify.

Conclusions

1. Use of low permeability concrete in segments is by far the most practical and economical of the deck protection systems. Use of concretes with low water to cement ratios and pozzolans or slag should provide a service life of more than 150 years.
2. For designs in which a sacrificial concrete cover is wanted, the monolithic concrete deck protection system is by far the most practical and economical. Issues related to distress at the time the concrete is removed should be resolvable.
3. For designs in which an impermeable layer on the deck surface is wanted, the thin epoxy overlay deck protected strategy is by far the next most practical. The low cost and ease of maintenance make it ideal for post-tensioned segmental concrete bridges.
4. Use the thin bonded hydraulic cement concrete overlay and the waterproof membrane overlaid with asphalt concrete should be reduced. These protection systems are the most complicated and expensive and come with a high risk of failure.

References

1. "Guide Specifications for Polymer Concrete Bridge Deck Overlays", American Association of State Highway and transportation Officials, Washington D. C. 1995.

2. Ozyildirim, Celik, "Permeability Specifications for High-Performance Concrete Decks" Transportation Research Record 1610, Transportation Research Board, Washington D. C. 1998.

3. Pyc, Wioleta A., Weyers, Richard E., Weyers, Ryan N., Mokarem, David W., Zemajtis, Jerzy, Sprinkel, Michael M., Dillard, John G., "Field Performance of Epoxy Coated Reinforcing Steel in Virginia Bridge Decks", VTRC 00-R16, Virginia Transportation Research Council, Charlottesville, Virginia, 2000.

Figure 1. Deck protection systems for post-tensioned segmental concrete bridges