In a groundbreaking examination of a century-old concrete sample, provided by Dr. Le Pham, a key contributor to the University of Wisconsin-Madison’s concrete research project, researchers have uncovered intriguing details about the composition and characteristics of the aging material. The concrete exhibited a well-proportioned mix, suggesting an estimated water-to-cement ratio near 0.47, despite the inherent challenges of determining precise values due to the material’s age. Impressively, the compressive strength was estimated between 5,500 and 6,500 psi, showcasing the enduring resilience of this century-old construction material.
Further analysis delved into the cement composition and hydration process. The examination revealed a wide variability in Portland Cement (PC) composition and microstructure, with belite exhibiting different sizes and textures compared to contemporary Portland cement. Despite this, the hydration of the cement appeared advanced or nearly complete. Large cement particles, believed to result from potential sub-water storage or long-term wet curing, displayed characteristics consistent with full hydration. Intriguingly, these particles did not show a noticeable boundary with the surrounding paste, distinguishing them from sand particles in modern concrete.
A notable finding pertained to the low volume of calcium hydroxide, suggesting a correlation with the cement’s low alite (C3S) content and potential leaching over time. The concrete’s aggregate composition included 1-inch top-sized dolomite and siliceous gravel coarse aggregate, along with natural sand fine aggregate, exhibiting a well-graded and uniformly distributed structure without signs of segregation. A surprising revelation was the relatively shallow depth of full carbonation, measuring less than a quarter inch from all surfaces. However, indications of partial carbonation within the body of the cylinder, potentially resulting from strength testing conducted in 2023, were noted. The dolomitic limestone gravel aggregate played a role in internal partial carbonation.
Minor and localized evidence of alkali-silica reaction (ASR) gel and associated microcracks were observed, primarily related to the chert fine aggregate. However, this ASR occurrence did not significantly impact the concrete’s compressive strength. In summary, the examination of this century-old concrete sample has provided valuable insights into its composition and characteristics. The findings contribute significantly to our understanding of old concrete compositions, shedding light on the material’s enduring properties and behavior over time.
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