International Concrete Abstracts Portal

Due to global warming, the temperature of earth surface increased by 0.95 to 1.20℃ in the past 4 decades. The increase in temperature has significant effects on the concrete industry, causing alterations in concrete curing conditions and degradation in strength and durability properties. The understanding of changes in concrete properties due to variations in curing conditions from climate change is an imminent task that has to be resolved. Among the durability properties of concrete, freeze-thaw (FT) resistance is most directly affected by climate change. However, in all of the studies conducted on the FT behavior of concrete, the dramatic changes in environmental conditions due to climate change were not considered. Therefore, the focus of this study is to understand the FT performance of concrete from extreme temperature and relative humidity (RH) changes in curing conditions. To find the relationship between the curing condition change and FT resistance levels as a function of time, a 3-D satisfaction surface graph was developed using the Bayesian probabilistic method. Then, an example of drawing the 3-D satisfaction surface diagrams for FT resistance based on the weather conditions in New York City between 2001 and 2100 was shown. Furthermore, considering the reduction rate of the average annual FT cycle due to climate change, this study confirmed that FT resistance performance increased. This approach contributes to a performance-based evaluation (PBE) strategy for concrete exposed to FT cycles under various environmental conditions. The study details and results are discussed in the paper.

To address the autogenous shrinkage issue of ultra-high-performance concrete (UHPC), internal curing technology has shown great potential in resolving this challenge by providing additional moisture. To further improve its curing efficiency, this study proposes an innovative internal curing technology that can significantly reduce autogenous shrinkage without increasing the amount of internal curing water or compromising mechanical strength. This approach utilizes perforated cenospheres (PCs) as internal curing agents while substituting internal curing water with urea solutions. In addition to replenishing water, urea solutions, once released into the cement paste, can react with portlandite. This reaction generates CaCO₃; owing to the intrinsic properties of CaCO₃, it has a larger macroscopic volume and a much higher elastic modulus than portlandite. This approach effectively reduces chemical shrinkage while concurrently increasing the stiffness of the cement paste, thereby achieving a significant reduction in autogenous shrinkage. As a result, replacing water with 3% urea solution in PCs enhances the autogenous shrinkage of UHPC, reducing it from less than 50% to over 90%.

The cement industry’s strategy in many countries is to reduce its CO2 emissions to diminish the greenhouse effect. This strategy aims to reduce these emissions by decreasing the clinker content in their new formulations, replacing it with supplementary cementitious materials (SCMs) or inert fillers. One of the most-used additions in Latin America’s cement industry is inert limestone fillers, which is the most inexpensive one. In North America, there are restrictions on using this inert addition in portland cement, with a maximum allowable content of 15% as limestone content (LSC). Nevertheless, in Latin America and other countries, this limestone filler content restriction is not as strict, allowing contents of up to 35%. This investigation includes experimental results obtained from portland cement mortars where inert limestone fillers were used at a replacement level between 20 and 30% by clinker, and only 24-hour curing was considered. Results obtained include mechanical (compressive strength), physical (electrical resistivity, total void content, and capillary porosity), and chemical (carbonation after 1 year of natural exposure) performance of such mortars. The carbonation coefficients (kCO2) obtained after 1 year of exposure in a natural urban environment were 17.3, 22.9, and 24.5 mm/y1/2 for 23%, 27%, and 29% LSCs, respectively. These results were higher than typical kCO2 values of approximately 4 mm/y1/2 obtained from ordinary portland cement (OPC)-based mortars with a 90 to 95% clinker content, and standard 28-day water curing.

Crack densities obtained from on-site surveys of 74 bridge deck placements containing concrete mixtures with paste contents between 22.8 and 29.4% are evaluated. Twenty of the placements were constructed with a crack-reducing technology (shrinkage- reducing admixtures, internal curing, or fiber reinforcement) and 54 without; three of the decks with fiber reinforcement and nine of the decks without crack-reducing technologies involved poor construction practices. The results indicate that using a concrete mixture with a low paste content is the most effective way to reduce bridge deck cracking. Bridge decks with paste contents exceeding 27.3% had a significantly higher crack density than decks with lower paste contents. Crack-reducing technologies can play a role in reducing cracking in bridge decks, but they must be used in conjunction with a low-paste-content concrete and good construction practices to achieve minimal cracking in a deck. Failure to follow proper procedures to consolidate, finish, or cure concrete will result in bridge decks that exhibit increased cracking, even when low paste contents are used.

Portland-limestone cement (PLC) currently has gained widespread use as the most accessible and sustainable blended cement in the market. However, in many African countries, including Ghana, the use of clay pozzolana (CP) in the concrete industry has primarily relied on ordinary portland cement (OPC). In this study, PLC Type II/B-L was partially replaced with CP at levels ranging from 10 to 50% by weight. The investigation included compressive strength testing, nondestructive evaluations using electrical surface resistivity, pulse velocity, and chloride penetration tests, targeting a characteristic strength of 30 MPa (4351.13 psi). Additionally, an environmental impact assessment based on the carbon footprint of both control and CP concretes was conducted. The mixture design followed the EN 206 standard. A total of 72 cubic molds were produced for the strength test. The results showed that CP concretes with between 10 and 20% replacement achieved strength values of 35 and 33 MPa (5076.4 and 4786.32 psi), respectively, higher than the target of 30 MPa (4351.13 psi) strength at 28 days. However, mixtures with 30 to 50% replacement required extended curing periods of 60 to 90 days to reach the desired strength. At extended curing, 10 to 50% CP replacement attained strength between 32 and 41 MPa (4641.28 and 5946.64 psi). Nondestructive test results showed no direct correlation with compressive strength, confirming that different factors govern strength, resistivity, and pulse velocity. The environmental impact assessment revealed a 14 to 51% reduction in carbon strength index (CSi) and a 19 to 36% increase in carbon durability index (CRi) with 10 to 50% CP (for CSi) and 10 to 40% (for CRi). The thermodynamic modeling also revealed that pozzolana contents below 30% primarily promoted pozzolanic reactions, enhancing performance compared to the control mixture. Based on these results, 20 to 30% CP replacement is recommended to ensure reliable performance, while higher levels (>30%) require further durability evaluation for long-term use.