International Concrete Abstracts Portal

The cement industry´s strategy in many countries is to reduce its CO2 emissions to diminish greenhouse effects. This strategy is to reduce these emissions by decreasing the clinker content in their new formulations, replacing it by using supplementary cement materials 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, defining as 15% the maximum allowable content as limestone cement (LSC). Nevertheless, in Latin America and other countries, this limestone filler content restriction is not that strict, allowing contents as much as 35%. This investigation includes experimental results obtained from Portland cement mortars where inert limestone fillers used were between 20% and 30% by clinker replacement, and only 24-hour curing was considered. Results obtained include mechanical (compressive strength), physical (electrical resistivity, total void content, capillary porosity), and chemical (carbonation after one-year natural exposure) performance of such mortars. The carbonation coefficients (kCO2) obtained after 1-year exposure in a natural urban environment were 17.3, 22.9, and 24.5 mm/y½ for 23%, 27%, and 29% LSCs, respectively. These results were comparable higher than typical kCO2 values of ~ 4 mm/y½ obtained from ordinary Portland cement-based mortars having 90 to 95% clinker content.

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) 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 in the concrete industry has primarily relied on Ordinary Portland Cement (OPC). In this study, PLC Type II/B-L was partially replaced with clay pozzolana at levels ranging from 10% to 50% by weight. The investigation included compressive strength testing, non-destructive evaluations using electrical surface resistivity, pulse velocity, and chloride penetration tests, targeting a characteristic strength of 30 MPa. Additionally, an environmental impact assessment based on the carbon footprint of both control and clay pozzolana concretes was conducted. The mix design followed the EN 206 standard. A total of 72 cubic moulds were produced for the strength test. The results showed that clay pozzolana concretes with between 10 and 20% replacement achieved strength values of 35 and 33 MPa, 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, 10-50% clay pozzolana replacement attained strength between 32 and 41 MPa. Non-destructive 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 CSi and a 19 to 36% increase in CRi with 10 to 50% clay pozzolana (for CSi) and 10 to 40% (for CRi). The thermodynamic modelling also revealed that pozzolana contents below 30% primarily promoted pozzolanic reactions, enhancing performance compared to the control mix. Based on these results, 20–30% clay pozzolana replacement is recommended to ensure reliable performance, while higher levels (>30%) require further durability evaluation for long-term use.

This study employs machine learning (ML) to predict ultrasonic pulse velocity (UPV) based on the mixture composition and curing conditions of concrete. A data set was compiled using 1495 experimental tests. Extreme gradient boosting (XGBoost) and support vector regression (SVR) were applied to predict UPV in both direct and surface transmissions. The Monte Carlo approach was used to assess model performance under input fluctuations. Feature- importance analyses, including the SHapley Additive exPlanation (SHAP), were conducted to evaluate the influence of input variables on wave propagation velocity in concrete. Based on the results, XGBoost outperformed SVR in predicting both direct and surface UPV. The accuracy of the XGBoost model was reflected in average R2 values of 0.8724 and 0.9088 for direct and surface UPV, respectively. For the SVR algorithm, R2 values were 0.8362 and 0.8465 for direct and surface UPV, respectively. In contrast, linear regression exhibited poor performance, with average R2 values of 0.6856 and 0.6801 for direct and surface UPV. Among the input features, curing pressure had the greatest impact on UPV, followed by cement content. Water content and concrete age also demonstrated high importance. In contrast, sulfite in fine aggregates and the type of coarse aggregates were the least influential variables. Overall, the findings indicate that ML approaches can reliably predict UPV in healthy concrete, offering a useful step toward more precise health monitoring through the detection of UPV deviations caused by potential damage.

This study comprehensively investigates the development of ambient-cured self-compacting geopolymer concrete (SCGC) based on the chemical composition of binder and alkaline activator. Five factors of the chemical composition of binder and alkaline activator, each with four levels, are used to evaluate and optimise the workability and compressive strength of the high-strength SCGC. The designed SCGC mixes provided sufficient workability properties and compressive strength between 28 MPa [4061 psi] and 70.3 MPa [10196 psi]. It was found that the SCGC mix with a binder content of 600 kg/m³ [37.4 lb/ft³], a CaO/(SiO₂+Al₂O₃) mass ratio of 0.55, a Na2O/binder mass ratio of 0.11, a SiO₂/Na₂O mass ratio of 1.2, and Na₂O/H₂O mass ratio of 0.35 was the optimum mix, which achived slump flow of 770 mm [30.3 in.], 28-day compressive strength of 70.3 MPa [10196 psi], and final setting time of 80 min. The CaO/(SiO₂+Al2O₃) ratio in binders, binder content, and Na₂O/binder mass ratio have been found to be the most influential factors on the workability and compressive strength of ambient-cured SCGC. Microstructure analysis of SCGC mixes showed that the increase in the CaO/(SiO₂+Al₂O₃) ratio promoted the formation of calcium-aluminate-silicate-hydrate (C-A-S-H) gels and enhanced the compressive strength by filling voids and creating a compact and dense microstructure.