Use of recycled construction and demolition wastes as aggregates in pre cast block works

Abstract

As landfill space in Hong Kong is running out, the high waste generation rate in the territory becomes a major concern. In 2004, there was about 17,500 tonnes of waste required to be landfilled per day and about 40% of this waste was construction and demolition (C&D) waste. To reduce the waste quantity, government has implemented a charging scheme on C&D waste at the end of 2005. The purpose of this policy is to encourage contractors to reduce, reuse and recycle C&D waste. In fact, reuse of C&D waste is very common in other countries. C&D waste can be used as an aggregate replacement in concrete, sub base and masonry products. Although the high water absorption and porosity of recycled aggregates derived from C&D waste would affect the quality of concrete, there have been many studies which showed that the use of recycled aggregate in nonstructural concrete is feasible. Recycled aggregates can also be used in structural concrete by applying special mixing curing and casting methods. In masonry applications, although a number of studies have shown that it is feasible to produce paving and partition blocks by using recycled aggregates, there is a need to further understand the factors affecting the engineering properties of the masonry and paving products produced in order to optimize the production. This study aims to develop a set of scientific design principle and guidelines to optimize the performance of pre-cast masonry and paving products produced with recycled aggregates. The parameters studied included the effect of aggregates to cement (A/C) ratio, grading of the aggregates and aggregate properties on the properties of the blocks. The study was conducted by carrying out a series of experiments using different A/C ratios, types and grading to understand their effects on the properties of the blocks. The study results showed that strength of the blocks prepared with different aggregates increased with the decrease in A/C ratio, and water absorption and abrasive resistance of the blocks were also improved when the A/C ratio decreased. The study results also showed the grading of the aggregates significantly affected the performance of the blocks. Models based on ideal grading curve, fineness modulus and packing density were used in the study to optimize the grading requirements of aggregates for masonry and paving block applications. To determine the effect of aggregate types and properties, different types of aggregates (recycled crushed glass, recycled crushed aggregate and natural crushed aggregate) were used in the experiments. The result indicated that the 28th day compressive strength would increase when the recycled aggregate was replaced by natural crushed aggregate but decrease when the recycled aggregate was replaced by recycled crushed glass. Although the use of recycled crushed glass would reduce the strength, it could nevertheless compensate the high water absorption of the recycled aggregate and thus reduced the overall water absorption of the blocks making a better product. However, alkali silica reaction (ASR) became a concern when glass was used in the cementitious system. A series of accelerated mortar bar tests were carried out to determined the level of ASR expansion. It was found that by using appropriate mix proportioning and addition of PFA, the expansion caused by ASR reaction could be effectively suppressed. With the assistance of a local block manufacturer, the optimized mix proportions developed in the laboratory was used at a few plant trials. The plant trials results confirmed that it was feasible to produce quality pre-cast masonry and paving blocks (meet Grade A standard) by using recycled aggregates.