IntroductionConcrete is a basic construction material in the construction industry particularly in the fields of civil engineering all over the world. Most of concrete is used in infrastructure development, defense installations and in high rise buildings.
It is manufactured with cement, aggregate and water forming the essential elements. Of this mixture aggregate forms the largest proportion. The amount of waste arising from demolition of buildings and highway constructions constitute the largest fraction of waste streams in many developed counties. Increased consumption of natural aggregates by the concrete industry is continuing to increase year after year and is expected to reach highs of 10 billion tones by the year 2010. This consumption rate of natural aggregate creates severe environmental implications and consequences. These environmental consequences can no longer be sustained and therefore there is increased pressure on the concrete industry all over the world to seek alternatives to overcome these challenges.The continued exploitation of natural aggregates has also been resisted by environmentalists across the globe. This is because this exploitation depletes the quality of basic aggregates and also to create global environmental awareness and protection.
Large proportion of the wastes which would otherwise be useful materials is disposed of to landfills. However, the landfill space is diminishing tremendously and therefore there are growing concerns to reduce such wastes. These environmental concerns have led to the idea of using recycled aggregate concrete. As a result of this development, many industries currently recycle concrete wastes into secondary construction products such as aggregates.These materials are then used in constructing roads, drainage and low cost housing constructions. Nevertheless, there are outstanding issues that need to be taken into consideration before employing recycled aggregates in high level uses. This paper therefore seeks to examine the effects of recycled concrete aggregate on the resulting mixture (Keun-Hyeok, Heon-Soo, & Ashraf, 2008).
Recycled Concrete Aggregate (RCA) and its characteristicsRecycled aggregates are derived from construction and demolition debris from buildings and crushed to create Recycled Concrete Aggregate (RCA).Properties of RCARecycled concrete differs from natural concretes because they are angular in nature and their surfaces are rough. The rough and angular textual property of recycled concrete implies that they have more water absorption rate in order to yield a workable concrete. They also have a low compact weight as compared to natural aggregates producing a higher yield in terms of available volume for a certain weight making it attractive in construction. Specific gravity of recycled concrete is also low. The water absorption rate and specific gravity of recycled aggregates depends on the amount of cement used in the mixture.
As a result, as the proportion of cement is increased, the water absorption rate of the recycled concrete aggregate increase and its specific gravity reduces (Rahal, 2007).Slump and workability With increased usage of recycled concrete aggregate on the fresh concrete mixture, the value of slump reduces. Several factors according to studies have been shown to have contributed to this slump loss.
The slump of recycled aggregates concrete is affected by the condition of the moisture of the aggregate. The slump is also affected by the shape and the particle distribution within the aggregate and the volume of the recycled aggregate used as these affects the water absorption rate of the fresh aggregate concrete. Therefore, the slump is controlled by grading, recycled aggregate volume, and the admixture ration of water. The slump of the fresh recycled aggregate concrete decrease linearly with time. The relative slump versus the elapsed time of the fresh concrete produces an index which computes the workability loss of the resulting concrete. The gradient of the slump loss increases linearly with the increase in water absorption rate and the volume of the recycled aggregate used thereby reducing the overall workability of the resulting concrete. Recycled aggregates dipped in water continue to absorb moisture for a limited period of time with decreasing absorption rate as elapsed time increases. The overall implication of the volume of the recycle aggregate used is that it affects workability loss more that it affects the slump (Poon, Kou, & Lam, 2007).
BleedingThe amount of bleeding of the fresh aggregate concrete decreases with increase in water absorption capacity of the recycled aggregate. The amount of bleeding is also dependent on the amount of recycled coarse used. Increase in recycled coarse causes the bleeding of the concrete to decrease because the bleeding water become absorbed by the cement particles on the surface of the fresh aggregate. The bleeding tendency of the recycled aggregate depends on the characteristics of the cement used, the water content and the addition of other refined materials (Poon, Kou, & Lam, 2007).
Compressive strengthThe relative compressive strength of a fresh concrete increases as the elapsed time increase. This implies that it is much more favorable to develop a recycled concrete with long term strength than natural concretes. This is brought about by the cementing activity of cement particles around the surface of fresh aggregate concrete.
The water absorption capability of the recycled aggregate concrete ensures that there is constant availability of water to in order for reaction with the cement to occur and thereby curing the mixture. However, the compressive strength of the fresh recycled aggregate decreases with the increase in water absorption capability of the aggregate. The compressive strength of the recycled concrete remains at optimal levels with reduced water absorption capability of the concrete. As the water absorption capability of the concrete increases, the compressive strength begins to drop.
This weak compressive strength of the recycled concrete could be as a result of inadequate hydration and the poor interface zone of the different elements of the concrete because of large amounts of cement particles present within the recycled aggregates surface. The incoherent surface of the fresh aggregate concrete is vulnerable to small cracks between the cement particle layer and the recycled concrete aggregate layer thereby reducing the compressive strength of the concrete (Keun-Hyeok, Heon-Soo, & Ashraf, 2008).Modulus of elasticity The modulus of elasticity of the recycled concrete aggregate decreases with increase in the water absorption capacity of the concrete. Therefore a lower elastic modulus of the recycled concrete is achieved with high water absorption capabilities of the concrete. The pressure exerted during while crushing the waste concrete is particularly responsible for reduced strength and stiffness of the concrete thereby reducing the elastic modulus of the resulting fresh recycled aggregate concrete (Rahal, 2007).
Shrinkage strainDuring the early stages of recycled concrete aggregate formation, the shrinkage strains are. This is because of the high water absorption capability of the concrete in the initial stages. The quality and the volumes of recycled aggregate used also affect the shrinkage strain. The shrinkage strain of the recycled concrete increases as its water absorption capability increases. The stiffness and volume of the recycled concrete aggregate plays a significant role in the final shrinkage of the resulting recycled concrete. Consequently, recycled concretes with low stiffness values causes shrinkages to become more prevalent in the concrete in the long run (Hansen, & Almudaiheem, 1987).Conclusion As discussed in the above sections, a reinforced concrete can be produced form concrete wastes.
However, because of the cement particles on the surface of the recycled concrete aggregates, their resulting densities are lower than those of natural aggregates. Also their water absorption rate is higher as compared to the natural aggregates. The water absorption capacity and density of the recycled concrete aggregate should therefore be carefully be determined before using them in the production of concrete. This is important since it would eliminate variations in the properties of the resulting hardened concrete as well as producing fresh recycled concrete aggregate of sufficient workability, cohesiveness and stability. Recycling construction and demolition wastes may be uneconomical and impractical at times and therefore faces external influence such as regulations.
However, the concrete industry needs to evaluate the effectiveness of concrete aggregates in order to derive economic benefits as well as achieve environmental benefits.Reference:Keun-Hyeok, Y., Heon-Soo, C., & Ashraf, A.F. (2008).
Influence of Type and ReplacementLevel of Recycled Aggregates on Concrete Properties. Retrieved March 9th, 2009 http://findarticles.com/p/articles/mi_qa5360/is_200805/ai_n25501273Hansen, W., & Almudaiheem, J. A. (1987).Ultimate Drying Shrinkage of Concrete-Influenceof Major Parameters. ACI Materials Journal, 84,217-223.
Poon, C.S., Kou, S. C., & Lam, L. (2007).
Influence of Recycled Aggregate on Slump andBleeding of Fresh Concrete,” Materials and Structures, 40, 981-988.Rahal, K. (2007). Mechanical Properties of Concrete with Recycled Coarse AggregateBuilding and Environment. 42, 407-415.