The focus of the study is to determine the optimum length of micro (average diameter less than 0.3 mm) and macro (average diameter greater than or equal to 0.3 mm) hemp fibers subjected to tensile loading in a cement paste mixture. Optimizing the length of the fibers to carry tensile loading for concrete members is important to minimize the waste of hemp material and to provide the best performance. This study evaluated three water/cement (w/c) ratios; 0.66, 0.49, 0.42 (f’c= 17.2, 24.1, 27.6 MPa respectively – f’c = 2500, 3500, 4000 psi respectively). Because of the high cost of cement, the replacement of cement with fly ash was also part of the program to determine if the addition of fly ash would have a negative impact on the performance of the hemp fibers. The results show that micro and macro hemp fibers bonded to the cement matrix and carry higher tensile loads at higher w/c ratios. Statistical analysis (regression modeling) shows that the optimum length for macro hemp fibers is 30 mm (1.18 in.) and 20 mm (0.79 in.) for micro-fibers.

External bonding with fiber-reinforced polymer (FRP) offers a potential solution to mitigate the detrimental effects caused by load impact and corrosion, which can weaken the bond strength of reinforced concrete structures. However, existing models need to be improved in addressing the FRP confinement mechanism and failure modes. As a solution, the proposed model employs stress intensity factor (SIF)-based criteria to determine the internal pressure exerted on the steel-concrete interface during various stages of comprehensive concrete cracking. Critical parameters are evaluated using weight function theory and a finite element model. A bond-slip model is introduced for the FRP-concrete interface and reasonable assumptions on failure plane characteristics. The internal pressure model employed demonstrates that FRP confinement has the ability to generate dual peaks in stress distribution and modify their magnitude as the confinement level increases. The proposed predictive model demonstrates superior performance in failure modes, test methods, and wrap methods for assessing bond strength with FRP confinement. The accuracy of this model is indicated by an integral absolute error (IAE) of 9.6% based on 125 experimental data, surpassing the performance of the other three existing models. Moreover, a new confinement parameter is introduced and validated, showing an upper bound of 0.44 for enhancing FRP bond strength. Additionally, a general expression validating the bond strength model with FRP confinement is established, allowing for the prediction of bond length.

This work investigated the effects of substrate surface moisture condition and texture on ultra-high-performance concrete overlay bond strength. This investigation was performed in three parts that studied extreme substrate moisture conditions, partially dried substrate moisture conditions, and surface texture. These studies investigated the effects of substrate surface moisture conditions, from dry to a surface with a thin layer of free moisture, and surface textures that provided various aggregate exposure conditions on overlay bond strength. Direct tension pull-off tests were conducted to assess overlay bond strength. Results for specimens with exposed fine aggregate surface textures showed that visibly moist substrate surfaces facilitated development of excellent bond strengths, and adequate bond was achieved for conditions with a thin layer of free moisture. For specimens with saturated surface-dry conditions, acceptable bond was achieved with a slightly exposed fine aggregate texture and increasing bond strength was observed with increasing aggregate exposure.

Shotcrete (sprayed concrete) differs from ordinary cast concrete through the application technique and the addition of set accelerators that promote immediate stiffening. The bond strength development between shotcrete and rock is an important property that depends on the texture of the rock, the type of accelerator, and application technique. This investigation focuses on the development of the microstructure in the interfacial transition zone (ITZ) and the strength of the bond at the shotcrete-hard rock boundary. The results show that the bond strength is related to the hydration process—that is, the strength gain of the shotcrete—and remains low before the acceleration period of the cement hydration. With a scanning electron microscope (SEM), it is possible to observe changes over time for the early development of the interfacial zone, both before and after proper cement hydration. Results from tests with wet-sprayed concrete on granite rock are presented. The test method—using both bond strength and the SEM to investigate the development of the microstructure at the ITZ—is interesting, but has to be more broadly examined. Different mixtures, accelerators, and rock types have to be used.

At low temperatures, the development of concrete strength is slow and can seriously hinder construction progress. The traditional early-strength components cannot meet the requirements of green and high-performance concrete. In addition, research on the early-strength accelerators at low temperature is paltry, and the effect of early strength was limited, but the mechanism of early strength at low temperature remains unclear. Calcium bromide (CaBr2) was used as a new kind of early-strength component, and its effects on mortar strength, cement paste setting time, and early hydration characteristics were evaluated. The incorporation of CaBr2 shortened the setting time of cement pastes and accelerated the strength development of specimens of all ages (the 28-day strength continued to increase dramatically). The compressive strength of mixed mortars increased between 18 and 376%, and the mortar strengths after 3 days met or even exceeded that of the contrast sample, cured at 20°C (293.15 K). The presence of CaBr2 decreased the solubility of Ca(OH)2, so it reached saturation and precipitated more easily. CaBr2 also significantly increased the dissolution and hydration rates of C3S by shortening hydration induction and advancing acceleration. Furthermore, the maximum heat-release rate and cumulative heat release could be increased by CaBr2, which shortens the nucleation and crystal growth (NG) stage and the phase-boundary reaction (I) stage. Large amounts of Ca(OH)2 formed after only 12 hours, and new products, such as bromine-containing calcium-silicate-hydrate (C-S-H) gels and hydrated calcium bromoaluminate (Ca4Al2O6Br2∙10H2O), were also generated. These products piled up and bonded, resulting in a denser microstructure.