Alkali-Silica Reaction
Guest Author
Rachel Detwiler, Ph.D., P.E.

Figure 1.
Alkali-silica reaction is identified by the classic three-armed cracks on the surface of the concrete. It should be verified by petrographic examination (PCA Index No. 55075).

Figure 2.
Reaction rim around aggregate particle (arrrow) (PCA Index No. 43090).

Figure 3.
An aggregate particle that has reacted with the alkalis in the cement. The crack (arrow) started within the aggregate particle. (Photo courtesy of Laura Powers, CTL.)

Figure 4.
The aggregate particle has reacted with alkalis in the concrete. Gel from the reaction fills the crack (blue arrow) that originated in the aggregate particle (yellow arrow) and propagated into the cement paste. (Photo courtesy of Ron Sturm, CTL.)

What is ASR?
Alkali-silica reaction takes place between reactive siliceous minerals in certain aggregates and OH- ions in the cement paste. Alkalis (Na+ and K+) from the cement, mixing water, or environment increase the concentration of OH- ions in the concrete. The OH- ions attack susceptible aggregate minerals. The damaged framework forms a gel that absorbs water from the surrounding concrete. The gel expands, generating pressures that can crack the concrete. The damage may not be visible to the naked eye for years after the concrete has been placed.

What does it look like?
Alkali-silica reaction is identified by characteristic three-armed cracks (Figure 1) appearing on the surface of the concrete. The crack pattern may be altered due to restraint in one direction (as in a pavement), or due to other stresses from imposed loads. Sometimes you can see gel oozing from the cracks. A closer look shows that the cracks start from the aggregate particles.

There are other possible causes of three-armed cracks. To confirm that the cracks are caused by alkali-silica reaction, have a petrographer examine the concrete. A slightly magnified view of a polished surface will show reaction rims (Figure 2) around the reactive aggregate particles. In a thin section (Figures 3 and 4), signs of ASR include cracks originating from the reactive aggregate particles, discolored areas around the aggregate particles where ASR gel has stained the surrounding paste, and gel filling the cracks. The petrographer will also be able to identify the type(s) of reactive mineral(s) causing the problem.

Where does it occur?
ASR-susceptible aggregates are found in most states in the US. However, when engineers are aware of susceptible aggregates, they can take appropriate preventive measures.

How is ASR Detected?
The first step is to determine whether the aggregates to be used on a project are susceptible to ASR. CTL can provide any or all of the tests required, as well as recommendations for how best to control any harmful expansions.

The Portland Cement Association recommends analyzing the aggregate according to ASTM C 295, "Standard Guide for Petrographic Examination of Aggregates for Concrete." If the aggregate contains more than the following quantities of any of these reactive minerals, it is considered potentially reactive:

  • Optically strained, microfractured, or microcrystalline quartz exceeding 5.0%
  • Chert or chalcedony exceeding 3.0%
  • Tridymite or crystobalite exceeding 1.0%
  • Opal exceeding 0.5%
  • Natural volcanic glass in volcanic rocks exceeding 3.0%

It is helpful to give this list to the petrographer so s/he knows how much of each constitutent is considered problematic.

In addition, the aggregate should also be tested according to ASTM C 1260, "Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method)." Any aggregate having a 14-day expansion greater than 0.10% is considered potentially reactive.

If the aggregate is determined to be potentially reactive by either of these tests, it may be further evaluated by ASTM C 1293, "Standard Test Method for Determination of Length Change of Concrete Due to Alkali-Silica Reaction." An aggregate having a 1-year expansion greater than 0.04% is considered potentially reactive.

How to minimize the effects of ASR
If the aggregate is shown to be potentially reactive by these tests, some mitigation measure must be used to control the expansion and cracking. Low-alkali cements are sometimes recommended for this purpose, but they are not always the best or even an adequate solution. Some aggregates are too reactive to be controlled by a low-alkali cement. Also, alkalis can come from other sources besides the cement. It is better to use supplementary cementitious materials in some form to control the expansions:

  • Blended cements containing Class F fly ash, natural pozzolans, calcined clay, silica fume, or slag may be used either alone or in combination with additional supplementary cementitious materials of the same or different type.
  • Portland cement may be used with one or more supplementary cementitious materials. Lithium admixtures may also be used.

You should also verify that the control measure you have selected will do the job. For supplementary cementitious materials, conduct ASTM C 1260 again, this time with the job cement and supplementary cementitious material(s) in the proportions proposed. Not sure how much to use? In that case, test several combinations at the same time. For Class F fly ash, try mixes containing 15%, 20%, and 25% by weight of cement; for slag, try 40%, 45%, and 50%.

For lithium admixtures, the appropriate dosage must be verified by ASTM C 1293. Consult the supplier or CTL for recommendations.

About the author
Rachel Detwiler joined Contruction Technology Labs (CTL) in 1993 and is currently Principal Engineer in Materials Consulting. She conducts forensic investigations of materials-related failures and evaluates the performance of concrete component materials, particularly with regard to their effects on durability and mitigation of expansions due to alkali-silica reactivity. Her primary areas of expertise are supplementary cementitious materials, forensic investigations, transport properties, microstructure, and test methods. Dr. Detwiler received her Ph.D. in 1988 from the University of California at Berkeley, after which she spent one year as a postdoctoral fellow at Norges Tekniske Høgskole in Trondheim, Norway. From 1990 to 1993 she was Assistant Professor at the University of Toronto.

You can e-mail Rachel Detwiler at rdetwiler@ctlgroup.com or contact her at CTL, 847-965-7500.

Is ASR a problem in your area?
During preparation for this edition of the Concrete News it was learned that the latest map showing the location of ASR problematic aggregate was out of date.

We would like to poll the readership to determine if there is an updated map or data existing on a local or national level.

If you have any pertinent information about ASR, please contact:
Phil Smith, L&M Construction Chemicals, Inc.
14581 Calhoun Rd, Omaha, NE 68152
phone 1-800-362-3331 or
e-Mail: Philsmith@lmcc.com

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