Effect of coarse aggregate concentration on bond strength and bond-slip behavior between reinforcing steel and low and mid-strength concrete

Abstract

Concrete is currently the most consumed building material in the world. The 20 th century saw a significant increase in the use of concrete in construction due to its high quality, speed, and ease of implementation. Reinforced concrete is one of the most commonly used load-bearing systems in buildings. However, despite the development of high-performance concrete, medium, and low-strength concrete is still widely used in ordinary residential buildings due to various factors. These factors include the use of substandard materials, insufficient details, lack of implementation based on clear scientific bases, and bad construction practices, such as pouring concrete on site without giving importance to increasing the water/cement (W/C) ratio or using aggregates with an inappropriate gradation. Such practices are responsible for most failures in reinforced concrete structures. While most modern buildings use deformed rods to improve the bonding between the rod and concrete, many older structures still rely on smooth rods.The increasing need to evaluate existing construction means that there is a constant need for information about its performance. The research on plain rebars was discontinued because they were not used in the first place when producing ribbed rebars, and ordinary bars have been surpassed in progress in understanding and behavior since the 1960s with the advent of ribbed rebars. The bond between concrete and reinforcement is an important factor in the evaluation of reinforced concrete structures. With the widespread use of reinforced concrete structures, it has become essential to understand the bonding properties between concrete and steel. For a structural element consisting of concrete and reinforcement to act as reinforced concrete, the bars must be clamped to the concrete.This interlocking is affected by many variables, such as the tensile strength of the concrete, the bond strength between reinforcement and concrete, and the concrete compressive strength. Other factors include the concrete reinforcement interface properties, geometric properties of the reinforcement, reinforcement production technique, reinforcement diameter, corrosion, embedment length, concrete confinement, concrete cover thickness, and the type and size of aggregate used. Therefore, the characteristic properties of concrete have a great effect on the bond between concrete and reinforcement. In areas that are located on the active seismic belt, in order to understand the behavior and performance of existing buildings, The worst implementation scenarios in addition to using two types of rebars that resemble the existing case were simulated. In the presented work, investigating the stress-strain properties and bond behavior of reinforced concrete with low and medium strength at different coarse aggregate concentrations was aimed. To achieve this, concrete mixtures with 3 different W/C ratios (0.6, 0.9, and 1.2) and 4 different coarse aggregate concentrations (0%, 20%, 40%, and 60%) were produced. The volume of aggregate and cement paste was kept constant in all mixtures. Pull-out tests were carried out to examine the bond properties between concrete and reinforcement. For this purpose, 12mm nominal diameter ribbed and plain steel rebar were used. The results showed that the compressive strength increased up to a certain coarse aggregate concentration and then decreased, particularly in low-strength classes, with this trend decreasing as the W/C ratio decreased. The contribution of coarse aggregate concentration to compressive strength became more evident with a decrease in the W/C ratio. For instance, in concrete groups with W/C ratios of 1.2, 0.9, and 0.6, the strength increases percentages of concrete with a 40% coarse aggregate ratio compared to a 0% coarse aggregate ratio were 13.8%, 28.8%, and 70.2%, respectively. The study also found that the modulus of elasticity values increased with the increasing coarse aggregate ratio, and the slope of the post-peak region of the stress-strain curves became steeper. The bond strength of the concrete-reinforcement interface is affected by the W/C ratio. A decrease in the W/C ratio results in a denser concrete structure with reduced porosity, which positively affects the adherence between concrete and reinforcement. The bond strength values of mixtures with a W/C ratio of 0.60 were found to be higher than those with other dosages for both types of rebars. In summary, the type of rebar, water-cement ratio, and concentration of coarse aggregate affected the bond strength and bond-slip behavior between the concrete and reinforcement. Understanding these factors is crucial in designing and constructing safe and durable concrete structures.