dcyphr | An experimental study on the shear behaviour of recycled concrete aggregate incorporating recycled tyre waste


Shear behavior is how a material reacts to a stress or strain that occurs due to a force parallel to a surface. The researchers mixed recycled concrete aggregate (RCA) and fine or coarse crumb rubber. The researchers conducted shear tests with mixtures of RCA and rubber in different proportions. They find that adding rubber increases cohesion of the mixture. But if you add more than 0.5%, it decreases the cohesion. They try to create a model to predict the shear strength of the mixture.


The researchers aim to analyze the shear behavior of different concrete and rubber mixtures.


More than half of the world’s waste comes from construction and demolition. This is a major environmental concern. Recycled concrete aggregates (RCA) are the resulting pieces when concrete pavements are demolished. Recycling these pieces prevents waste addition and preserve resources from the quarry. A lot of waste also comes from tires. But, crumb rubber from tires are rarely recycled and difficult to break down. As a result, rubber waste contributes to carbon dioxide emission, chemical contamination in water, and more. 

Pavements undergo three main stresses: compression, tension, and shear stress or strain. These three stresses can result in the breakdown of pavement (rutting). These stresses gradually wear down the pavement until the pavement no longer functions. Shear stresses are dependent on the friction between the particles in the pavement. Increased moisture can also increase shear stresses and risk of rutting. 

RCA can handle high amounts of compression forces but not tension. Rubber can withstand high amounts of tension. Therefore, RCA is often mixed with rubber, so the mixture can withstand both compression and tensile forces. However, few studies have looked at the shear behavior of a concrete and rubber mixture.

Results and Discussion

In a sample of only RCA or RCA and rubber, the mixture had the maximum shear stress at a horizontal displacement of 20 mm. For a horizontal displacement of more than 20 mm, the shear stress remained about the same. This is because small particles likely filled any open spaces in the mixture at this much displacement. This keeps the shear stress around the same. The interlocking of rubber and concrete particles at high displacement meant that the mixture had dilatant behavior. Dilatant behavior is when the mixture gets thicker or more viscous due to the high amounts of shear stress. With increasing normal stress, shear thickness and shear stress increased. With increasing horizontal displacement, vertical displacement decreased. This means the mixture was experiencing more compression. (Figure 5).

As the amount of time the mixture is subject to a shear force, the more shear stress the mixture experiences up to a maximum (Figure 6). The different RCA mixtures with either 0.5%, 1%, or 2% rubber can also withstand high shear stress with increased friction angles. Because the mixtures can withstand a lot of friction, they meet the requirements for use in pavements and roads (Figure 7). 

Rubber can withstand high amounts of tension. So, it helps increase the shear strength when combined with concrete. However, if the researchers put more than 0.5% of rubber into the mixture, the shear strength decreases. The higher amounts of rubber will no longer just fill in the space but will undermine the shear strength. Coarse rubber increases strength more than fine rubber (Figure 8).

The researchers also developed a model to predict how effective a proportion of rubber would be in increasing the shear strength of concrete. The model indicates that the shear strength is dependent on normal stress, rubber size, and rubber proportion. The researchers confirmed that the model stood true with their own data.


The researchers mixed different proportions of RCA and crumb rubber. They conducted a large direct shear test (LDST) on the different mixtures.


Recycled concrete aggregate (RCA) mixed with either fine or crumb rubber increased the shear strength of just concrete alone. RCA with 0.5% fine crumb rubber had the most shear strength. Other mixtures of RCA and rubber also met the standards for use in pavements and other applications. The researchers also created a model to predict the shear strength of a rubber and concrete mixture.