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April 28, 2015

Thermoplastic polymer-modified asphalt overlays work in Va.

Overlays are placed on bridge decks to reduce infiltration of water and chloride ions and to improve skid resistance, ride quality and surface appearance.

Hydraulic cement concrete (HCC) overlays used in Virginia include latex-modified concrete (LMC), first used in 1969, 7% silica fume concrete (SFC), first used in 1987, and LMC prepared with very early hardening cement (LMC-VE), first used in 1997.  

DOTs are increasing the use of overlays that can be constructed rapidly to minimize traffic congestion and delays. Asphalt overlays also can be installed on bridge decks with a short lane closure time. However, asphalt overlays are rarely used by the Virginia Department of Transportation (VDOT) because conventional asphalt is permeable, particularly when placed on bridge decks, because polymer modifiers are not typically used and vibratory compaction is not allowed. To protect the deck, a membrane must be installed before the overlay is placed. Unfortunately, DOTs have reported that membranes often leak. An epoxy overlay is a rapid option, as it is typically placed at night or on weekends with minimal disruption to traffic, because it can be driven on after only three hours of curing.

A thermoplastic plant-mix additive can be mixed with asphalt to provide an overlay that can be used as a wearing surface and protection system on bridge decks without the use of a waterproof membrane and without the need for vibratory compaction. This type of overlay is considered a thermoplastic polymer-modified asphalt (TPMA) overlay because of the polymer modifier and higher asphalt content than VDOT uses for typical asphalt mixtures. To prepare for the use of TPMA, VDOT staff contacted staff of the Wisconsin DOT, Kentucky Transportation Cabinet, Massachusetts Turnpike Authority and the Port Authority of New York and New Jersey. These agencies had used TPMA on a number of projects and were generally satisfied with the installations and reported installed costs of approximately $400 to $500 per ton. Since TPMA had never been used in Virginia, its suitability as a wearing surface and protection system was evaluated in the two experimental-features projects described in this article.

A TPMA overlay was first used in Virginia on a bridge deck on I-85 in the northbound lanes (NBL) over Rte. 629. The parallel bridge in the southbound lanes (SBL) received a conventional epoxy waterproof membrane and asphalt overlay. The structures, built in 1964, are 124.5 ft long with 20-ft approach slabs and three simple concrete T-beam spans with a 27° skew. The average daily traffic was approximately 12,388 vehicles per day in 2005 with 13% truck traffic. 

Prior to placement of the overlays, the bridges were patched, and the two transverse joints separating the spans were removed and the spans made continuous. A very rapid-hardening concrete prepared with calcium sulfoaluminate cement, called Rapid Set, was used for the patching and joint replacements. 

One lane was closed in each direction to allow for the repairs and the installation of the asphalt overlays.

A second TPMA overlay was placed on span 22 of the Norris Bridge. The bridge is a two-lane structure approximately 9,985 ft long and 23 ft wide and carries S.R. 3 over the Rappahannock River between Middlesex and Lancaster counties in Virginia. The deck includes a steel grid that is filled with a lightweight concrete and an LMC overlay that is delaminating from the top of the grid in many areas.

A TPMA overlay was considered to be the optimum wearing and protection system, because the overlay could be placed with the least amount of lane closure time and the overlay would be impermeable. VDOT prepared a life-cycle-cost analysis, which indicated a TPMA overlay was more economical than an LMC-VE overlay or an asphalt overlay on a liquid membrane. To gain more experience with TPMA and the preparation of contract documents for placing an overlay on the rest of the bridge, VDOT awarded a contract to replace the overlay on span 22, the span considered to be in the worst condition, with 46.4% delaminated and 27.8% spalled or ready to spall.

Looking for speed, ease

The purpose of this research was to evaluate the construction, initial condition and cost of the TPMA overlays placed on two bridges and to compare the TPMA installations with the conventional asphalt overlay and membrane system placed on an adjacent bridge. Emphasis was placed on comparing the wearing and protection systems with respect to speed and ease of construction (including lane closure time); the initial condition as indicated by physical properties, protection and skid resistance; and cost. An objective also was to compare these protection systems with HCC overlays of LMC-VE, LMC and SFC and epoxy overlays. Information on the HCC and epoxy overlays was taken from published reports and data available from project files. 

Eye on the Norris

VDOT’s modified EP-5 epoxy overlay waterproof membrane was placed on the travel and passing lanes of the SBL bridge on I-85. The deck was shot-blasted prior to placement of the epoxy. Two TPMA control strips were constructed in the parking lot at the VDOT South Hill residency. The automatic equipment that added the TPMA powdered rubber additive was not calibrated properly, and insufficient TPMA was added to the mixture, requiring a second placement. TPMA was placed on the passing and travel lanes of I-85 NBL.

The EBL and westbound lane (WBL) of the Norris Bridge were overlaid with TPMA. Laboratory tests were conducted on samples of the SM-9.5 mixture and TPMA taken during the construction of the overlays on I-85 and on the two samples (10-1014 and 10-1015) taken during the construction of the TPMA overlay on the Norris Bridge. Each of the three bridges was overlaid in two days. The epoxy membrane placed on the I-85 SBL required an additional two days. The installations indicated that both the SM-9.5 mixture and membrane (SM-9.5 + membrane) system and TPMA are rapid options for placing a wearing and protection system on a bridge deck. 

On the Norris Bridge, the latex overlay was milled within 0.5 in. of the top of the grid in two days. Small impact hammers were used to remove the 0.5 in. of overlay that was bonded. Areas with delaminated overlay typically had low areas (valleys) in the grid. The valleys in the grid were filled with a rapid-hardening mortar. Approximately six weeks was required to remove the bottom 0.5 in. of the well-bonded areas of overlay. The delaminated sections were easily removed.

Table 1 provides a summary of the asphalt mixture properties. The mixtures on I-85 failed the permeability test but passed all other tests. TPMA used on the Norris Bridge was much more rut-resistant than the SM-9.5 mixture used on the I-85 SBL. The VDOT generic specification for a TPMA polymer-modified asphalt waterproofing mix is given in a VCTIR report.

Skid resistance was measured with bald-tire tests (ASTM E524) done with the VDOT skid trailer. The numbers ranged from 29 to 49, with the lowest number reported for the TPMA on the Norris Bridge EBL. The EBL overlay had a much higher binder content than the WBL and both lanes of I-85. HCC overlays typically have a bald-tire skid number of 45 to 53. By correlation, the LMC overlay on the Norris Bridge had a skid number of about 43, which is close to the lower end of the range for HCC overlays. New polymer-concrete overlays typically have a bald-tire skid number of 50 to 60 (ASTM E524), and the number levels off in the 30s or 40s (similar to asphalt) after 15 to 20 years in service.

Reasonable design service life for the overlay systems are shown in Table 2. Life-cycle costs (LCCs) for the overlay systems also are shown in Table 2. The LCCs are based on a 30-year design period and the service life shown in Table 2 without any correction for discount or inflation rate. Epoxy has the lowest LCC with two installations in 30 years. The HCC overlays have the next lowest LCCs with one installation in 30 years. The SM-9.5 mixture will likely have to be replaced every 10 years, but if the membrane at $30/sq yd lasts 30 years, the LCC is the next to the highest at $255/sq yd. TPMA is the most expensive option based on the cost for the first two projects. At $590 per ton, TPMA would have the same initial cost as the SM-9.5 + membrane based on costs in Table 2 and would cost less based on prices from other agencies. 

Approximate road-user costs for HCC overlays have been reported. The LMC-VE overlay was constructed over two long weekends, and the road-user cost savings were $518,984 compared with the cost for constructing SFC or LMC overlays over two two-week periods. LMC-VE overlays are much more economical than LMC and SFC overlays when road-user costs are included at high-traffic locations. Table 2 shows that lane closure times and consequently road-user costs are the same for TPMA and epoxy overlays. Lane closure times double when SM-9.5 + membrane or LMC-VE is used and increase by a factor of about 17 compared with TPMA and epoxy when LMC and SFC overlays are constructed. However, additional road-user costs need to be considered when replacing the SM-9.5 overlay after 10 and 20 years and the epoxy and TPMA overlays after 15 years.

Fast service

The following conclusions were made as a result of this study:

TPMA, epoxy, SM-9.5 + membrane and LMC-VE overlays can be constructed more rapidly than conventional HCC overlays such as SFC and LMC overlays; 

The TPMA on the Norris Bridge is less permeable and more fatigue- and rut-resistant than the SM-9.5 mixture on I-85 and should last longer; and

TPMA is expensive compared with the other overlays, but this is likely because it is a new product in Virginia. Based on costs reported by other agencies, TPMA would be competitive with other protective systems.

Consider using TPMA when a rapid protection system is required but only if the price is cost-effective, i.e., not at the prices reflected by the first two VDOT projects. AT

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