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Virginia's Approach to ASR
D. Stephen Lane, Virginia Transportation Research CouncilAlkali-silica reaction (ASR) was first reported to have occurred in Virginia in 1941, when Kammer and Carlson revisited the cause of expansion and cracking in a dam attributed a decade earlier to an unidentified cement-aggregate interaction (Lane, 1993). In the late 1950s and mid 1960s, occurrences were noted in pavements of the Pentagon network and the R. E. Lee Bridge in Richmond constructed in the mid 1930s (Lane, 1993). The reactive constituent in the aggregates was microcrystalline or strained quartz, which is more slowly reactive than other reactive constituents such as opal, chalcedony, or volcanic glass. Despite these early identified occurrences, ASR was not considered to be a significant concern in Virginia until much later, in part because the focus of early work on ASR revolved around the rapidly reacting materials.
In the late 1980s, a stretch of interstate pavement placed in the Charlottesville, VA, area in the early 1970s was replaced because of significant cracking caused by ASR. Other stretches, less severely affected, were ultimately overlaid by the mid 1990s. Two coarse aggregates, a metabasalt and a granitic gneiss were involved, both containing microcrystalline and/or strained quartz as the reactive component. This coincided with a growing regional and international recognition of the potentially deleterious nature of aggregates containing varieties of quartz as the reactive constituent, greatly extending the areas where problems might be encountered. In 1989, the National Ready Mixed Concrete Association and the National Aggregates Association organized the Mid-Atlantic technical committee, composed of industry representatives; state Departments of Transportation of Maryland, North Carolina, Delaware, Pennsylvania, and Virginia; Federal Highway Administration; and the local mass transportation agency to serve as a working group to study the problem and develop solutions.
While the 1980s was a period of reawakening regarding the impact of ASR, it was also a period when the use of pozzolans and slag cement experienced great growth in the United States concrete industry. The Virginia Department of Transportation (VDOT) had revised its concrete specifications to allow the use of fly ash and slag cement in 1984 and 1985, respectively. Although economic considerations were the primary driving forces at this point, numerous studies had demonstrated the beneficial attributes that these materials could provide with respect to mitigation of both ASR and chloride-induced corrosion.
In 1990, VDOT launched a study to determine the extent of its ASR problem and to develop measures to prevent further problems. It quickly became clear that the local availability of Class F fly ash and slag cement provided an economical solution to both the ASR and chloride-induced corrosion problems, without any real downside. In 1991, VDOT revised its concrete specification to require 15% replacement plus 5% addition of Class F fly ash or 25 to 50% slag cement unless Type II cement with an alkali content less than 0.40% Na2O equivalent = Na2O + 0.658K2O was used.
The outcome of the initial study supported the interim specification because potentially reactive aggregates were in widespread use and test methods capable of clearly distinguishing between non-deleterious and deleteriously reactive aggregates were not available. Also, it seemed that the primary purpose behind identifying non-deleterious aggregates was to avoid having to use fly ash or slag cement in the concrete, which would be less economical and leave it more susceptible to chloride-induced corrosion because of concrete permeability.
A follow-up study (Lane and Ozyildirim, 1995) focused on determining the amounts of pozzolans or slag cement needed to prevent deleterious reactivity using a standard reactive material (borosilicate glass). The work showed that the amount of a given mitigating material needed was a function of the alkali content of the portland cement with which it was used. Based on this study, VDOT revised its concrete specification to a sliding scale of minimum replacement of portland cement with Class F fly ash, slag cement, or silica fume as a function of the cement alkali content as shown the following table.
VDOT ASR Mitigation Requirements—1995 Revision
* Replacement of portland cement by mass
The above table lists the minimum percentages of fly ash, slag cement, or silica fume to be used depending on the cement alkali content.
A subsequent study (Lane and Ozyildirim, 1999) was then conducted to verify the findings by testing concretes produced with a reactive aggregate used in Virginia construction. This study included other durability factors in addition to ASR and recommended adjustments to the earlier specification with the intent of providing adequate mitigation of both ASR and chloride-induced corrosion.
VDOT Recommendation to Provide ASR Mitigation and Low Permeability Concrete
* Minimum percentage cement replacement by mass
The above table lists the minimum percentage of cement replacement using fly ash, slag cement, or silica fume for portland cement alkali contents less than or equal to 0.75% and greater than 0.75%.
The use of pozzolans or slag cement has served VDOT well over the years in preventing significant early damage resulting from ASR. Assuring compliance has been straightforward, relying primarily on mill certifications of the cementitious materials. It has allowed VDOT to avoid the much more difficult and larger task of developing and maintaining a program of testing aggregates for ASR potential, which would impose much greater management and manpower demands.
While the ideal is to avoid ASR-related damage and the specifications in place for over fifteen years appear to be accomplishing that (Lane, 2006), structures built earlier may require periodic repair or rehabilitation. VDOT has overlaid damaged bridge decks with latex-modified concrete since 1970, polymer mortar since the early 1980s, or silica fume concrete (now low-permeability with pozzolans or slag cement) since the early 1990s. These systems have served as primary maintenance and rehabilitation tools. A number of these decks were undoubtedly damaged by ASR but the cause(s) never clearly defined. VDOT has had excellent success with these overlay systems in extending bridge deck service life.
Lane, D. S., "Alkali-Silica Reactivity in Virginia," Final Report, Virginia Transportation Research Council, Report No. 94-R17, 1993.
Lane, D. S. and Ozyildirim, H. C., "Use of Fly Ash, Slag, or Silica Fume to Inhibit Alkali-Silica Reactivity," Final Report, Virginia Transportation Research Council, Report No. 95-R21, 1995. 41 pp.
Lane, D. S. and Ozyildirim, C., "Combinations of Pozzolans and Ground, Granulated, Blast-Furnace Slag for Durable Hydraulic Cement Concrete," Final Report, Virginia Transportation Research Council, Report No. 00-R1, 1999, 22 pp.
Lane, D. S., "An Evaluation of the Performance of Concretes Containing Fly Ash and Ground Slag in Bridge Decks," Virginia Transportation Research Council, Report No. 07-R7, 2006, 21 pp.
For further information on VDOT's approach, please contact the author at 434-293-1953 or email@example.com.
HPC Bridge Views, Issue 51, Sept/Oct 2008