International Concrete Abstracts Portal

This study investigates the impact of under-sulfated cement combined with high-calcium fly ash and lignosulfonate-based admixtures in ready mixed concrete, leading to rapid stiffening and delayed setting. Using an on-board slump-monitoring system (SMS) installed on a ready mixed concrete truck, significant increases in water demand were recorded to maintain target slumps, with mixtures showing minimal slump response to water additions. Laboratory tests, including isothermal calorimetry and mortar trials, confirmed the under-sulfated cement’s inadequate sulfate levels as the cause. Optimal sulfate addition was determined through calorimetry, and adjustments with gypsum effectively remedied rapid stiffening and delayed setting. This research demonstrates that an SMS can detect undesirable combinations of cement, fly ash, and admixtures in concrete, allowing real-time corrections. It underscores the importance of optimized sulfate levels in cement, particularly when using high-calcium fly ash combined with some high-range water reducers, to achieve desired concrete performance under varying field conditions.

This study investigates the feasibility of using a hybrid combination of scrap tire recycled steel fiber (RSF) and manufactured steel fibers (MSF) in concrete pavement overlay applications. A total of five concrete mixtures with different combinations of MSF and RSF, along with a reference concrete mixture, were studied to evaluate fresh and mechanical properties. The experimental findings demonstrate that the concretes incorporating a hybrid combination of RSF with hooked-end MSF exhibit similar or higher splitting tensile strength, flexural strength, and residual flexural strength compared to that of concretes containing only hooked-end MSF, straight MSF, or RSF. This enhanced mechanical performance can be ascribed to the multiscale fiber reinforcement effect that controls different scales (micro to macro) of cracking, thereby providing higher resistance to crack propagation. The concretes containing only RSF show lower splitting tensile strength, flexural strength, and residual flexural strength compared to concrete solely reinforced with straight MSF or other steel fiber-reinforced concrete (SFRC) mixtures due to the presence of various impurities in the RSF such as thick steel wires, residual rubber, and tire textiles. Interestingly, blending RSF with hooked-end MSF overcomes these limitations, enhancing tensile strength, flexural strength, and residual flexural strength, while significantly reducing costs and promoting sustainability. Last, the findings from the pavement overlay design suggest that using a hybrid combination of RSF with hooked-end MSF can reduce the design thickness of bonded concrete overlays by 50% compared to plain concrete without fiber reinforcement, making it a practical and efficient solution.

This research investigates the impact of using washed waste fines (WWF), a by-product from ready mixed concrete (RMC) plants, as a partial replacement for natural sand in concrete. Cylindrical (100 x 200 mm) and cubic (50 x 50 x 50 mm) mortar specimens were created with 20% WWF substitution. Hardened properties, such as compressive strength, tensile strength, and ultrasonic pulse velocity (UPV), and durability parameters, such as chloride migration coefficient and carbonation coefficient, were evaluated. The study also examined the microstructure of concrete using a scanning electron microscope (SEM). Results showed that incorporating WWF enhanced both the hardened and durability properties of concrete, increasing compressive strength by 25% compared to the control case. Additionally, WWF decreased the non-steady-state chloride migration and carbonation coefficients, indicating improved durability. SEM analysis revealed a denser microstructure, and WWF incorporation reduced the permeable porosity and absorption capacity of concrete.

A capsule phase-change material (CPCM) was synthesizedusing n-tetradecane as the core, expanded graphite as the shell,and ethyl cellulose as the coating material through a controlledassembly process. The results demonstrate that the infiltration ofn-tetradecane significantly enhances the density of the expandedgraphite, while the ethyl cellulose coating effectively preventsthe desorption and leakage of the liquid phase-change materialduring phase transitions. As a result, the CPCM exhibits a compactstructure, chemical stability, and excellent thermal stability. Theincorporation of this CPCM into cement-based materials endowsthe material with an autonomous heat-release capability attemperatures below 5°C. When the CPCM content reaches 20%,the thermal conductivity of the cementitious matrix increases by24.66%. Moreover, the CPCM significantly improves the freezing- and-thawing resistance of the cement-based materials, reducingthe compressive strength loss by 96% and the flexural strengthloss by 65% after freezing-and-thawing cycles. This CPCM fundamentally enhances the frost resistance of cement-based materials, addressing the issue of freezing-and-thawing damage in concrete structures in cold regions.

This paper presents the torsional behavior of hollow reinforced concrete beams strengthened with carbon fiber-reinforced polymer (CFRP) U-wraps. Test parameters involve variable wall thickness in the section and the width and spacing of the externally bonded CFRP sheets. An experimental program is conducted with 27 beams (three unstrengthened and 24 strengthened) to examine their capacities, shear flows, and force distributions when incorporating a ratio of 0.27 to 0.46 between the areas of the hollow and gross cross sections. The stiffness and capacity of the test beams are dominated by the wall thickness, and the effectiveness of CFRP strengthening becomes pronounced as the void of the beams decreases. The presence of CFRP redistributes internal shear forces in the cross section, which is facilitated by narrowing the spacing of the U-wraps. The effective zone of CFRP retrofit is positioned near the outer boundary of the strengthened section. Regarding crack control, multiple discrete U-wraps with narrow spacings outperform wide U-wraps with enlarged spacings. While the location of a shear-flow path is dependent upon the wall thickness, the width of the U-wraps controls the effective shear-flow area of the beams. The size of the void is related to the stress levels of internal reinforcing components, including yield characteristics. Transverse stirrups are the principal load-bearing element for the unstrengthened beams; however, the reliance of the stirrups is reduced for the strengthened beams because the U-wraps take over portions of the torsional resistance. Through a machine learning approach combined with stochastic simulations, design recommendations are proposed.