Science Tackles Concrete Cracks, Saving Billions and Protecting Homes

According to the National Institute of Standards and Technology (NIST), groundbreaking research is addressing widespread cracking issues in concrete that can lead to costly repairs and structural failures. NIST scientists have been studying concrete for over a century, focusing on everything from large-scale building performance to the fundamental interactions of its component molecules. This deep expertise is now being applied to understand and mitigate unintentional chemical reactions that cause concrete to degrade. The outcomes of this research have already resulted in significant cost savings for state governments and are aimed at assisting homeowners struggling with expensive foundation cracks.

Concrete, the second most utilized substance globally after water, is comprised of three primary ingredients: cement, water, and aggregate (small rocks). Cement, a manufactured material created by heating limestone and clay to extreme temperatures, possesses the unique ability to harden when mixed with water. This hardening process involves a chemical reaction where water’s hydrogen and oxygen combine with cement powder to form new, solid cement molecules; the water itself does not evaporate. Aggregate, which constitutes 60% to 80% of concrete, is added for cost-effectiveness and to enhance strength. While generally inert, the minerals present in quarried aggregate can sometimes initiate detrimental chemical reactions within the concrete.

One significant issue is the alkali-silica reaction (ASR), often referred to as “concrete cancer.” This occurs when concrete containing specific types of aggregate remains exposed to moisture for extended periods. The cement, highly alkaline in its fresh state, can leach alkaline substances into internal moisture pockets within the hardened concrete. Certain aggregates react with this alkaline environment, forming a gel that absorbs water and expands. This expansion exerts pressure on the surrounding concrete, initiating hairline cracks that can widen and deepen over time, sometimes accompanied by a yellow discoloration caused by the gel. The damage is progressive and irreversible, potentially compromising the structural integrity of buildings over decades.

The urgency to understand ASR intensified in 2009 when early signs of this reaction were detected at the Seabrook Station Nuclear Power Plant in New England. The Nuclear Regulatory Commission (NRC) sought NIST’s assistance to assess the safety implications and explore mitigation strategies. NIST researchers successfully replicated ASR in laboratory settings, creating large concrete samples with intentionally introduced reactive aggregates. By controlling environmental conditions—specifically maintaining a temperature of approximately 24 degrees Celsius (75 degrees Fahrenheit) and 95% humidity—they accelerated the reaction, enabling them to study its effects over months rather than years. Through rigorous testing involving hydraulic presses to subject the samples to stress, NIST’s findings informed the NRC’s decision. The research concluded that ASR posed no threat to the Seabrook plant’s safety, provided moisture levels were managed. This recertification allowed the plant to continue operations, projecting an estimated saving of over $2 billion for Massachusetts alone in electricity costs and economic activity over a decade.

In more recent years, a different concrete degradation problem, the “pyrrhotite problem,” has affected thousands of homeowners, particularly in Connecticut, starting around 2015. This issue stems from the presence of the mineral pyrrhotite, a compound of iron and sulfur. When exposed to water, pyrrhotite dissolves, releasing iron ions and sulfuric acid into the concrete. The iron ions can form solid byproducts that expand, stressing the concrete, while the sulfuric acid initiates a “sulfate attack” that further degrades the cementitious materials. This reaction can cause extensive cracking in foundations, often necessitating the complete replacement of the concrete foundation, a process that can be more expensive than the value of the house itself.

NIST research, led by Stephanie Watson, is focused on developing reliable methods for detecting pyrrhotite. The challenge lies in identifying the mineral at very low concentrations, as the precise minimum amount required to trigger the detrimental reactions remains unknown. Previous detection methods yielded inconsistent results, making it difficult to accurately quantify pyrrhotite. Watson’s team has identified X-ray fluorescence as a promising technique. This method works by bombarding a sample with X-rays and analyzing the unique pattern of secondary X-rays emitted by the sample’s atoms, akin to an atomic fingerprint.

To facilitate accurate detection, NIST researchers have pioneered a method to create standardized reference material containing pure pyrrhotite. This involves combining iron and sulfur in an oxygen-free environment and heating them to high temperatures in a tube furnace. The resulting reference material, which includes cement, aggregate, sand, and synthesized pyrrhotite, is designed to mimic the composition of concrete affected by the mineral. Once finalized, this material will enable researchers and industry professionals to calibrate their detection equipment and conduct reliable tests on concrete ingredients and existing foundations. Homeowners may be able to request pyrrhotite testing for their foundations through local government channels. While no immediate fix exists for pyrrhotite-affected concrete, NIST researchers plan to investigate potential future treatments, such as moisture-resistant coatings. This research is vital for addressing the structural integrity of thousands of current and future homes.

Article by Mel Anara, based upon information from the National Institute of Standards and Technology (NIST)

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