Insights into the Composition and Characteristics of 100-Year-Old Concrete

In a recent examination of a 100-year-old cylinder concrete sample provided by my colleague, Dr. Le Pham , who has contributed to the University of Wisconsin-Madison's concrete research project, several noteworthy observations were made. This article will describe the findings and educated speculations derived from these observations.

Concrete Characteristics: The concrete appeared to exhibit a well-proportioned mix, with estimated water-to-cement ratio (w/c) near 0.47 (somewhat speculative due to the age). The compressive strength was estimated to range roughly between 5,500 and 6,500 psi. The concrete showed a uniform distribution of constituents and is free from large entrapped or compaction voids.

Cement Composition and Hydration: The analysis revealed a wide-ranging variability in portland cement (PC) composition and micro structure. Of notes, belite appeared to be of varying sizes and exhibit much different texture from today's portland cement. Alite has a much lower content than today's PC. Despite this variability, the hydration of cement appeared to be advanced or almost complete. The coarsely ground, large cement particles displayed characteristics consistent with nearly full hydration, suggesting potential sub-water storage or long-term wet curing (no detail exposure information was provided).

The large cement particles in the old concrete often did not exhibit an interface transition zone (ITZ) or a noticeable boundary with the surrounding paste. This distinction sets them apart from sand particles in concrete. Therefore, coarse cement particles do not function as fine aggregate particles.

Calcium Hydroxide: A notable finding was the low volume and/or small crystals of calcium hydroxide. I suspected this can be attributed to the cement's low alite (C3S) content and potential leaching over time.

Aggregate Composition:

The concrete contains 1 inch top-sized dolomite and siliceous gravel coarse aggregate, and natural sand fine aggregate. The aggregate is visually well-graded and uniformly distributed. No segregation was observed.

Carbonation and Partial Carbonation: A surprising discovery was the relatively shallow depth of full carbonation, measuring less than a quarter inch from all surfaces. However, there were indications of partial carbonation within the body of the cylinder (based on thin section examination), potentially occurred along cracks or microcracks created by the strength testing that was conducted in Year 2023. The dolomitic limestone gravel aggregate has also played a role in the internal partial carbonation. As shown in one of the thin section photos, dedolomitization of aggregate and associated paste carbonation (halos in the paste) has occurred along the rim of an aggregate particle. It is worth noting that distinguishing between partially carbonated and non-carbonated concrete using phenolphthalein staining alone proved difficult.

Alkali-Silica Reaction (ASR): Minor and localized evidence of ASR gel and its associated microcracks were observed, primarily related to the chert fine aggregate. However, this ASR occurrence did not appear to impact the concrete's compressive strength.

Summary: The examination of the 100-year-old concrete sample provided valuable insights into its composition and characteristics. The concrete displayed a dense paste/minimal carbonation, uniform distribution of constituents, and well-graded aggregate. The observations of carbonation, ASR, cement hydration, and calcium hydroxide provide further understanding of the concrete's behavior over time. Overall, this limited study contributes to our knowledge of old concrete compositions. Stay tuned for future update with some SEM work.