Shear strength at the interface of old and new concrete: Foaming agent content and freeze-thaw cycles impact

This comprehensive study investigates the relationship between the number of foaming agents and the increase in shear strength of concrete under various parameters, including cement grades (C300, C350, C400), water-to-cement ratios (0.4, 0.45, 0.5), and curing times (3, 7, 14, 21, and 28 days), and the shear strength at the interface of old and new concrete under successive freeze-thaw cycles. This research analyzes how increasing the concentration of foaming agent (0 to 0.45 wt%) affects the mechanical properties of concrete, with special emphasis on the trend of decreasing shear strength. After 3, 7, 14, 21 and 28 days of curing, 300 consecutive freeze-thaw cycles were used in this study to investigate the effect of these cycles. The temperature of the samples decreased from 4 to -18°C and increased from -18 to 4 °C during the freeze-thaw cycles. This was done alternately and at a 4-hour interval for each cycle. After three hours of freezing, the samples were immersed in water for one hour to defrost. The results show that with increasing foaming agent content, a continuous decrease in shear strength occurs, and the rate of decrease is significantly affected by cement grade, water-to-cement ratio, and curing time. The shear stress increases with increasing weight percentage of foaming agents and decreases with increasing melting and freezing cycles. Higher grade cement (C400) shows greater resistance to strength loss compared to lower grades (C300), especially at longer curing periods. The water-cement ratio plays an important role, with higher ratios (0.5) accelerating strength loss due to increased porosity. The study shows that early stages of curing (3−7 days) experience the most rapid strength loss, while longer curing (28 days) partially mitigates this effect through continued hydration. A nonlinear relationship is observed between foaming agent content and strength loss, with critical thresholds identified at 0.25−0.35% foaming agent, beyond which the strength loss becomes greater. In the concrete sample with a curing period of 7 days and a foam consumption of 0.45 and zero, the shear strength after applying the temperature cycle decreased by 81.63% and 1.36% for different water-cement ratios and cement grades, respectively. In the concrete sample with a curing period of 28 days and a foam consumption of 0.45 and zero, the shear strength after applying the temperature cycle decreased by 82.48% and 7.09% for different water-cement ratios and cement grades, respectively. Water-cement ratios of 0.45 and 0.5 are associated with the highest and lowest percentage reduction in shear strength per weight percent of foaming agent, respectively. These findings provide valuable insights for optimizing foam concrete mixtures in applications that require a balance between lightweight properties and structural integrity. Practical implications for mix design, durability considerations, and performance-based specifications are discussed, which will help improve material selection for insulation, non-load-bearing foam concrete, and semi-structural foam concrete applications.

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