This paper presents experimental investigations to compare the effect of three environmental solutions on the degradation of tensile performance of fiber reinforced polymer (FRP) bars. Three kinds of FRP bars, basalt FRP (BFRP), carbon FRP (CFRP), and E-glass FRP (GFRP), were tested in tension after conditioning in tap water, artificial seawater, and alkaline solutions at room temperature for different exposure periods of 0, 45, 90, 135, and 180 days. The test results showed that the changes of the surface appearance of GFRP bars were more obvious that those of BFRP and CFRP bars after 180 days of exposure to three environmental solutions. The tensile failure mode and load-displacement curves of all tested FRP bars exhibited similar characters regardless of the immersion solution, the exposure period, or type of FRP bars. By comparing the residual mechanical properties of three kinds of FRP bars tested in this study, it has been found that the chemical attack of harsh environments simulated by immersion in seawater solution and alkaline solution resulted in a significant damage to BFRP bars. The effect of the seawater solution on the strength loss of CFRP bars was more obvious than those of the other two solutions. While the alkaline solution caused the greatest strength damage on GFRP bars. These findings were in accordance with the properties of material susceptibility to aggressive solutions and the moisture absorption of the matrix. No significant degradation of the elastic modulus of the three kinds of FRP bars had been found after exposure to aggressive solutions because their glass transition temperatures were kept unchanged in the experiment. Finally, based on the Arrhenius theory, the time-dependent tensile strength retention of all tested FRP bars exposed to the three aggressive solutions were fitted with experimental data obtained in this study. They were also compared with various test results collected from previous studies. The comparison results indicated that our experimental data were in good agreement with the predictions and most of the existing results.

In order to investigate the effect of transverse cracks on chloride penetration and steel corrosion in concrete beams, two series of cracked specimens, Series A, with sustained flexural cracks, and Series B, with precracks, were designed and tested under a cyclic drying-wetting condition of NaCl solution. Profiles of free chloride concentration were obtained from the cracked beams in Series A after 30 and 60 weeks; the corroded rebars were removed from the precracked beams in Series B after nearly 4 years. Experimental and analytical results showed that the depth of the surface convection zone in the cracked concrete varied from 10 to 20 mm, which was greater than that in the sound concrete, and increased with increasing crack width. Around the steel bar located in the inner concrete, beyond the convection zone, chloride content was mainly influenced by concrete type when crack width was less than 0.2 mm, and then by crack width. For precracked concrete, pitting corrosion in the steel was obviously seen where cracks were located, and the scope of localized corrosion expanded when crack width increased. The effect of transverse cracks on steel corrosion depends on the coupled actions of cover depth and crack parameters. For the same specimens, a great difference in the effect of crack width ($\sim 0.2\mathrm{mm}$ ) on steel corrosion was observed when crack spacing decreased from 250 to 150 mm, especially for the steel bar with a lower cover depth of 20 mm.

Nano-SiO₂ (NS) is widely used in cement-based materials due to its excellent physical properties. To study the influence of NS content on a cement paste and the interfacial transition zone (ITZ), cement paste samples containing nano content ranging from 0 to 2% (by weight of cement) were prepared, and digital image correlation (DIC) technology was applied to test the mechanical properties. Finally, the optimal NS content was obtained with statistical analysis. The mini-slump cone test showed that, with the help of superplasticizer and ultrasonic treatment, the flowability decreased continuously, as the NS content increased. The DIC experimental results showed that NS could effectively improve the mechanical properties of the cement paste and the ITZ. Specifically, at the content level of 1%, the elastic modulus of cement paste and ITZ was 20.95 GPa and 3.20 GPa, respectively. When compared to that without nanomaterials, the increased amplitude was 73.50% and 90.50%, respectively. However, with the further increase in NS content, the mechanical properties decreased, which was mainly caused by the agglomeration of nanomaterials. Additionally, the NS content did not exhibit a significant effect on the thickness of the ITZ, and its value was maintained at 76.91–91.38 μm. SEM confirmed that NS would enhance the microstructure of both cement paste and ITZ.

The coupled effect of transverse cracking and cyclic drying–wetting conditions on chloride penetration in concrete is explored in this paper. The transport mechanisms of chloride in cracked concrete subjected to drying–wetting cycles are described and simplified. An experimental investigation was conducted for sustained cracked concrete beams and the results are reported here. It is found that the depth interval of the convection region in cracked concrete varies from approximately 10 to 20 mm, which is slightly greater than that found in sound concrete. Also, it increases with the increase in crack width and decreases with the increase in fly ash content. At the cracked sections, the speed of chloride penetration is increased by the presence of cracks. It is shown that the modified diffusion model is still practicable for describing the chloride distribution in transversely cracked concrete when the crack width is within a range close to 0·30 mm. Finally, a regressive relationship between equivalent chloride diffusion coefficient, D_Cl^eq, and crack width, w_cr, is proposed with a quadratic function of crack effect.

Corrosion-induced concrete cover cracking is an important indication of durability limit state for marine reinforced concrete structures. In this paper, two analytical models predicting the time from corrosion initiation to cover cracking and their main differences were introduced. Based on an accelerated corrosion test, two models’ applicability and variability were compared and discussed with experimental data. Considering the random nature of influencing factors, a probabilistic model was developed by using Monte Carlo simulation technique. The results showed that the cracking time could be modelled by the Weibull distribution. Finally, the probability analytical technology was applied to a marine reinforced concrete pier with four different durability design levels. It is found that both the mean and 90% confidence interval of the cracking time increase when the durability design specifications change from low level to extreme level, which indicates that the accurate prediction of cracking time with a deterministic model will become more difficult accordingly.

Using industrial by-products to replace cement is an important way to reduce carbon emissions in the cement industry. The purpose of this article is to understand the effect of quartz powder on the properties of cement-based materials. Experimental studies were conducted on the macroscopic and microscopic properties of cement-based materials mixed with quartz powder to evaluate their feasibility as a replacement for cement. The substitution rates of quartz powder were 0% (Qu0), 7.5% (Qu7.5), and 15% (Qu15). The test time was from 1 day to 28 days, and the main results are as follows: In the early stage of the hydration reaction, as the amount of quartz powder substitution increases, the cumulative hydration heat increases. This is mainly because the nucleation effect of quartz powder accelerates the hydration reaction of cement. In the later stage of the hydration reaction, as the amount of quartz powder substitution increases, the cumulative heat of hydration decreases. This is mainly due to the diluting effect of quartz powder. For Qu0, Qu7.5, and Qu15, the decrease in compressive strength after 1 day is not obvious. The decrease in compressive strength at 28 days is more obvious. Overall, there are exponential relationships between the UPV measurement or surface resistivity results and the compressive strength measurement results at 1, 3, 7, and 28 days. The XRD test results show that the main products of the reaction are AFt, CH, Hc, and Mc. From Day 1 to Day 28, the content of Mc becomes evident. The test results for TG showed that, as the amount of quartz powder substitution increases, the mass loss decreases. For different specimens of Qu0, Qu7.5, and Qu15 at different test times (3 and 28 days), there is an exponential function relationship between chemically bound water and strength. A numerical hydration model is proposed for cement–quartz binary blends. The parameters of the hydration model are determined based on the hydration heat normalized by the cement mass. Moreover, the hydration heat at 28 days is calculated using the proposed model. The strength development of all specimens and all test ages can be expressed as an exponential function of hydration heat.