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Although research has shown a considerable influence of the pre-cracking phenomenon on steel-congested concrete members, only normal concrete (NC) has been considered in the literature. The intention in this paper is thus to study the effect of the pre-cracking phenomenon on the bond response of pre-cracked NC with different slump flow values and self-consolidating concrete (SCC). Initial crack widths ranging from 0.0 to 0.5 mm are studied. Results show that initial crack widths larger than 0.10 mm have a significant influence on bond properties, such that reduction factors greater than 30% and 50% are obtained for the maximum bond strength of concrete specimens exposed to initial crack widths of 0.2 mm and 0.4 mm, respectively. Results show that concrete mixtures with higher workability are less sensitive to the pre-cracking phenomenon as compared to NC mixtures. The average bond stress of steel rebar in the pre-cracked SCC is found to be similar to that of the NC with a slump flow of 200 mm, which is considerably better than for NC with a slump flow of 97 mm. Moreover, results show that 65.8, 80.6, 88.5 and 93.1% fracture energy reductions are obtained for crack widths of 0.20, 0.30, 0.40 and 0.50 mm, respectively, as compared to the small crack width of 0.15 mm.

The disposal of personal protective equipment (PPE) is a main concern of researchers. In recent years, the COVID-19 pandemic made this issue worse, so the production and use of large quantities of disposable gloves in recent years and the lack of a suitable solution for the disposal of these recycled materials are some of the consequences of this pandemic. To address this issue, the present study performed a comprehensive experimental program to determine the possibility of using recycled latex and vinyl gloves as recycled fibers within extrusion-based 3D printing concrete. Moreover, a graphene oxide (GO) nanomaterial was also used to compensate for some undesired properties of mixtures. Flow table, buildability, and mechanical tests were performed in this study. Results show that the synergic effect of recycled fibers and GO significantly improved the 3D printing characteristics of mortar. Although very promising results were obtained in this study, findings show that using a high content of recycled fibers reduces the concrete compressive strength. However, the addition of GO significantly compensates for this reduction.

This study intends to determine the effect of fiber type and aggregate content on hardened and durability properties of self-consolidating concrete. Steel, glass, and steel/glass hybrid fibers are tested in the present experimental program. Additionally, the effect of aggregate content is considered by using different packing factors and sand-to-all aggregate ratios. Moreover, normal and high strength concrete grades are prepared. Compressive, splitting tensile, flexural, and impact tests are conducted for measuring hardened properties of concrete mixtures. Absorption, desorption, acid attack, resistivity, potential, and chloride diffusion are also durability tests carried out in the present study. Results show that 1.0% and 0.05% are the optimal dosage of steel and glass fibers respectively. Results show that packing factor plays a major role in mechanical characteristics of fiber-reinforced self-consolidating concretes so that an optimal value of 1.12 and 1.14 is obtained for mechanical properties. Also, experimental results reveal that for 1.12 packing factor, the sand-to-all aggregate ratio needs to be equal to the value of 0.50, while 0.57 is achieved as an optimum value for packing factor of 1.14. Overall, results show that hybrid-reinforced self-consolidating concrete is promising for mechanical and durability performance as compared to other mixtures.

Improved autogenous healing capacity of concrete using superabsorbent polymers (SAPs) was used as an efficient approach for mitigating damage between steel rebar and self-consolidating conc...

Previous research on steel bar-concrete bond behaviour has been concentrated mostly on the intact concrete without considering initial cracks induced by transverse tensile loading called pre-cracking phenomenon. There is no accurate model for evaluating bond behaviour and development length of steel bar in pre-cracked concrete. This paper aims to characterise the bond-slip behaviour of steel bars in pre-cracked concrete by direct pull-out tests, and proposes a constitutive law as a function of the crack width. Results show that induced cracks, notably cracks wider than 0.15 mm, cause a significant reduction in maximum and residual bond strength. Also, results indicate that larger crack widths result in considerably lower dissipated energy by the bond mechanism. The results obtained from both the experimental tests and referenced database demonstrate that the pre-cracking phenomenon has a higher impact on the residual bond strength compared to the maximum bond strength. Unlike existing equations, the proposed model accurately considers cracking effects on the steel bar-concrete bond properties and shows a satisfactory fit with the experimental database. A predictive equation is also proposed for calculation of the development length in pre-cracked concrete, which is more conservative and prudent compared to existing regulations in design codes.

Although extensive bond models have been developed for use in the numerical simulation of uncracked reinforced concrete, no simplified method exists providing satisfactory accuracy and effi...

The addition of nanomaterials affects the workability of cementitious composites by reducing the free water available within the mixtures. In order to address this issue, the present study intends to study the addition of ground granulated blast furnace slag (GGBS) in cementitious materials containing graphene (G) and graphene oxide (GO). An experimental program is considered to measure fresh properties, compressive strength, and service life of cementitious composites. Dosages of 0.03% and 0.06% (by weight of cement) of G and GO are tested in cement pastes and mortars. GGBS in three different dosages of 15%, 30%, and 45% is investigated. A constant water/cement ratio of 0.35 is used in all mixtures. Results show that GGBS improves the yield stress and plastic viscosity of G- and GO-modified cementitious composites. Moreover, GGBS dosages of 30% and 45% compensate the reduced fluidity of 0.03 wt% of G- and GO-modified cementitious composites, respectively. The thixotropy of the composite paste containing GO decreases with the addition of GGBS. Moreover, comparable and slightly improved compressive strengths are obtained for mixtures containing GGBS. The results from the nomogram show a promising trend for the service life of mixtures containing G, GO, and GGBS.

This work discusses the input parameters of discrete element method (DEM) for defining contact element between concrete and rebar. A comprehensive study is performed from a numerical database built in this study. A targeted method of discrete element model considering key parameters of bond strength between concrete and steel bar is developed for reinforced concrete structures so that DEM is customized for defining bond-slip effect between rebar and surrounding concrete. Accuracy of hypothesis is validated by an experimental program. Results show that the targeted method of discrete element model is more applicable and precise than embedded element method in which the perfect bond with no slip is dominant between the materials.

This paper intends to study the effect of air-entraining admixture (AE) on self-healing method at rebar-concrete interface using superabsorbent polymer (SAP). AE with a constant dosage of 0.83 kg/m3, and 0.25% and 1.0% SAP dosages are considered. Two types of superabsorbent polymer with different chemical compositions and particle sizes are considered for the experimental tests. Pull-out test results of mixtures containing AE admixture are compared with those in non-AE concrete. Scanning electron microscopy/energy dispersive X‐ray spectrometry (SEM/EDS) along with microscopic analysis is performed to study the healing products at crack surface and SAP macro voids around the rebar. Overall, results indicate that AE admixture has a considerable impact on the performance of the self-healing method at the rebar-concrete interface especially for higher dosage of SAP (1.0%). This can be attributed to the internal voids networks around the rebar generated by AE admixture, which can ease the water transfer between SAP macro voids to participate in healing cracks after wet-dry cycles. SEM analysis shows that stalactites, healing products at the external surface of crack, are composed of a large amount of calcium and oxygen.

This study intends to determine the effects of incorporating superabsorbent polymers (SAP) within concrete on the bond properties of steel reinforcing bars (rebar) embedded in uncracked and pre-cracked concrete. An experimental program is conducted to check the performance of SAP, as a healing agent inside the concrete, in mitigating internal damage at the rebar-concrete interface due to the pre-cracking phenomena. Two types of SAP with different particle sizes (0.15 and 0.50 mm) and chemistries are considered in the experimental program. Pull-out test results show improved bond properties of steel rebars embedded in uncracked and healed concrete containing lower dosages of SAP. However, concrete containing a high dosage of SAP shows lower bond strength, compared to normal concrete, due to the presence of macro voids. A considerable healing effect is observed for the initial bond-slip curve portion, the bond strength, and the energy absorbed by the bond mechanism, within the cracks of pre-cracked SAP-modified concrete subjected to wet-dry cycles. This study shows that SAP can significantly increase the autogenous healing performance of concrete at rebar-concrete interfacial damage sites.

The interaction between the steel bar and concrete is one of the most important issues that control the efficiency of the composite behavior. Accurate evaluation of the interaction between the reinforcement and the surrounding concrete can lead to more reliable finite element models. The implementation of a modified steel bar model in embedded elements to consider the bond-slip phenomenon is presented in this study. The procedure includes the addition of equivalent bond strain to the strain of the steel bar. This leads to a decrease in the effective stiffness of steel bar in the layered model. Validation of the modified steel bar model with existing experimental results demonstrates that the model is capable to consider the bond effects in embedded elements for use in analysis of reinforced concrete structure. A comprehensive parametric study is accomplished to obtain the influence of parameters such as concrete and steel properties, bar diameter, reinforcement ratio and confinement conditions on modification factors. Results revealed the significant effect of bond-slip on total behavior of the member.