Query: Can the main mechanism of concrete distress and cracking in a concrete made with ACR-susceptible aggregates be caused by ASR??
Fact: Alkali Carbonate Reaction (ACR) results when brucite, formed in concrete made with dolomitic (ACR-susceptible) aggregate, crystallizes and expands.
Fact: Alkali Silica Reaction (ASR) occurs over time when the alkalis in cement react with microcrystalline silica in aggregate in the presence of water to form an expansive gel.
Answer: We investigated ACR-susceptible concrete extracted from a wharf structure in Quebec, Canada and determined that ASR was the cause of damage.
In “Alkali Silica Reaction (ASR) as a Root Cause of Distress in a Concrete Made from Alkali Carbonate Reaction (ACR) Potentially Susceptible Aggregates,” M. Beyene, A. Snyder, R.J. Lee, M. Blaszkiewicz, (Cement and Concrete Research, September 2013), we present evidence that the main mechanism of concrete distress and cracking in a concrete made with ACR susceptible aggregates is ASR, not ACR.
How and Why of ACR
There are two hypotheses for the mechanism of ACR and the resulting expansion of the concrete: (1) dedolomitization, a breakdown of dolomite that produces crystallization of brucite (magnesium hydroxide which leads to a volume increase); (2) intergranular clay materials in the matrix and the new clay, released during the dedolomitization process, adsorb water and result in swelling. Dedolomitization and the subsequent brucite crystallization appear to be the favored mechanism of ACR. Dolomite is attacked by the alkali hydroxide resulting in alkali carbonate, calcium carbonate, and magnesium hydroxide.
Distress in concrete caused by ACR is not as common as that caused by ASR because aggregates considered to be ACR-reactive have other characteristics such as low strength and durability that make them ill-suited for use in concrete. They also have a characteristic texture easily identifiable by petrographers. Carbonate rocks involved in ACR have a particular textural and mineralogical composition characterized by rhombic shaped crystals of dolomite set in a finer grained matrix of calcite, clay, and silt-sized quartz.
The samples (concrete cores extracted from a wharf structure in Quebec, Canada) were investigated for the cause of distress and cracking. Historically, these types of aggregates are considered to be ACR-susceptible. Thin sections were studied using a petrographic polarized light microscope and then were examined using SEM coupled with qualitative EDS techniques.
Identification of alkali–silica gel in the ACR concrete raised the possibility that ASR was at least playing a role in the ACR reaction, but EDS mapping revealed that the ASR gel was the main reaction product responsible for crack formation in the concrete, and that the gel had a nature common to that found in typical ASR. Our observations also revealed that this particular structure experienced many cycles of wetting and drying indicating that ASR formation in this type of aggregate – with little reactive silica – would likely have occurred over a relatively long period of time before causing severe damage to the structure. The mechanism of distress and cracking in the concrete clearly exhibited that the cause of expansion and cracking in the concrete was Alkali–Silica Reaction (ASR), and not Alkali–Carbonate Reaction (ACR).
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