{ QUESTION } We are preparing mix design submittals for several recently announced infrastructure projects in our area. The sudden demand may require us to buy aggregate from new sources.

Sensing that aggregate availability could be constrained, many of the bid packages are requesting that we verify and guarantee the structure's durability and service life. We voiced our concerns to the project owner in our pre-bid discussions. Are there any guidelines for inspecting and reviewing new material sources, especially regarding their alkali-silica reactive potential?

{ ANSWER } The susceptibility of structures to alkali-silica reaction (ASR) usually is a regional problem. ASR is directly linked to reactive aggregates. As ASR occurs, the risk is minimal for catastrophic failure. Even so, ASR-induced cracking can cause or allow deterioration mechanisms that occur in frost, deicer, or sulfate exposures.

What was once a local concern is taking on regional implications as known quality quarries become depleted. Concrete producers are often forced to try new sources from outside their normal markets. So to protect their investments, many project owners are asking design engineers to include more quality aggregate specifications in their bid documents.

For the most part, this proactive approach to aggregate prescreening can reduce an owner's and a producer's long-term exposure to ASR-related callbacks. The problem occurs when the specification is poorly written and does not include current industry knowledge.

Consistency counts

To help ensure a consistent manner of accepting aggregates that may be reactive, PCA published the “Guide Specification for Concrete Subject to Alkali-Silica Reactions.” Producers should provide the 2007 document to the specification writer on infrastructure projects. When included in the contract document, all parties are provided a consistent approach to the variety of methods to control ASR, should it be a potential problem. The guide includes a list of test procedures to determine if aggregates are potentially reactive and methods to demonstrate how pozzolans and blended cements can effectively control ASR.

In HPC Bridge Views, Beatrix Kerkhoff, a PCA consultant, reminded engineers and producers that while most aggregates are chemically stable in hydraulic cement concrete without injurious interaction with other concrete constituent materials, it's necessary to be on guard. “While there are accepted procedures a producer may adopt to minimize the potential for expansion and related cracking, the best approach is to identify potentially reactive aggregates during the mix design submittal process,” he wrote.

Should the pre-job tests indicate a problem, producers can mitigate ASR. One useful reference is The Use of Lithium to Prevent or Mitigate Alkali-Silica Reaction in Concrete Pavements and Structures, published by the Federal Highway Administration (See NFHWA-HRT-06-133 at www.fhwa.dot.gov)

This book discusses the mechanism of alkali-silica reaction along with offering testing methods with which to identify potential alkali-silica reactive aggregates. The final two sections cover using lithium, first as an admixture for new concrete construction, and second as a treatment for existing concrete structures affected by ASR.

Another source is “Diagnosis and Control of Alkali-Aggregate Reactions in Concrete,” published by PCA (www.cement.org). The 26-page document explains efficient, effective control of alkali-aggregate reaction. New test methods are critiqued for identifying potentially reactive aggregates and for their ability to demonstrate that supplementary cementing materials and blended cements adequately control ASR. The American Concrete Pavement Association, NRM-CA, and PCA reviewed the document.