Mechanical properties of brick aggregate concrete

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Abstract

This paper presents the results of a research program carried out at University of Aveiro, Portugal to evaluate the properties of concrete made with crushed bricks replacing natural aggregates. Two types of brick were investigated. The bricks were crushed in order to obtain a usable aggregate. The properties investigated were the workability and the density of fresh concrete, and the compressive strength, tensile splitting strength, modulus of elasticity and stress–strain behaviour of hardened concrete. Replacement ratios of natural aggregates by 15% and 30% were investigated as well as water/cement ratios of 0.45 and 0.5. Strength indexes were used to assess the effectiveness of aggregate replacement. The results of concrete produced with recycled aggregates were compared with a reference concrete produced with natural limestone aggregates currently used in Portugal. Observed results indicate that ceramic residuals could be used as partial replacement of natural aggregates in concrete without reduction of concrete properties for 15% replacement and with reductions up to 20% for 30% replacement. The type and the manufacturing process of bricks seem to influence the properties of the resulting concrete. The properties and aesthetics of concrete with bricks indicate the possibility of using this type of concrete in precast applications.

Introduction

The sustainable construction concept was introduced due to the growing concern about the future of our planet because construction industry is a huge consumer of natural resources and, simultaneously, waste producer. Concrete industry, in particular, is one of the biggest natural resource consumer as a consequence of being one the most used construction materials. Concrete is a material that is often seen as a potential place for wastes, because of its composite nature (a binder, water and aggregates) and because it is widely used, which means that if a waste could be used in concrete, then certainly large quantities of it can be recycled. Since aggregates in concrete comprise about 60% to 75% of the total volume of concrete any reduction in natural aggregates consumption will have significant impacts in the environment. Environmental constrains of stone pits, such as noise, dust, vibrations, considerable impact on the countryside, besides the consumption of a non-renewable material tend to considerably limit their exploitation. Consequently, alternative materials such as construction and demolition waste (CDW) as well as other industries by-products are increasingly being tested and used as environmental sustainable natural aggregates substitutes, see for instance [1], [2], [3], [4], [5], [6], [7].

Ceramic materials are largely used in Portugal, both as bricks and tiles. Consequently, big quantities of wastes are produced simultaneously by the brick and tile manufacturers and by the construction industry. Most of the wasted of the manufacturers is already incorporated as raw material for new ceramic materials. Nevertheless, part of these wastes and those produced by the construction industry are placed in landfills. The studies in this field showed that some problems arise when using bricks from CDW because of its contamination with mortar, timber and concrete [8]. Akhtaruzzaman and Hasnat [9] carried out some research using well-burned brick as coarse aggregate in concrete where they found that it was possible to achieve concrete of high strength using crushed brick as the coarse aggregate. Brito et al. [10] reported strength ranging from 22% to 45%, respectively for 33% and 100% of natural aggregate replacement. Debieb and Kenai [11], using both coarse and fine crushed bricks reported a decrease in strength ranging from 20% to 30% depending of the degree of substitution. Khatib [12] and Poon and Chan [13] also reported a decrease of strength when using crushed bricks as fine aggregate substitute. Khaloo [14] and Hansen [15] found that there is an increase in tensile strength if crushed brick aggregates are used in concrete compared to concrete made with natural aggregates.

Regardless of all limitations that may exist, the utilization of recycled aggregates in concrete will certainly be mandatory in a near future.

The aim of this paper is the assessment of the properties of concrete which use crushed bricks as natural coarse aggregate partial substitute. To achieve this, fresh and hardened state properties of concrete are measured to ensure that fundamental parameters needed in design are evaluated (such as slump, compressive and tensile strength, stress–strain relation and modulus of elasticity).

Section snippets

Materials

All the materials used in this study were commonly available in the central region of Portugal. The cement used was ordinary Type II 32.5 Portland cement complying with NP EN 197-1:2001. Natural sand and calcareous coarse aggregates were used. Coarse aggregates were divided into two groups; one in the range 5–10 mm (NA-1) and the other in the range 10–20 mm (NA-2). Sieve analysis of all types of aggregates is shown in Table 1.

Recycled aggregates were ceramic bricks obtained as construction waste,

Concrete mix proportions

The main objective of the experimental program was the assessment of concrete properties made with waste clay brick aggregates produced from crushed bricks. Two series of tests were carried out, one with a water/cement ratio (W/C) of 0.5 and other with W/C of 0.45. The water absorbed by aggregates was not included for the calculation of W/C. This methodology ensures that the workability of fresh concrete could be kept constant for each W/C. Water saturation of aggregates was achieved by placing

Concrete workability

Workability of concrete is measured with the slump test, according to NP EN 12350-2:2002. The average slump for W/C = 0.45 was 5 cm and for W/C = 0.5 was 15 cm, as shown in Fig. 3. Apart from the amount of natural aggregate replaced, the workability was kept approximately constant for each W/C, confirming the efficiency of the adopted mixing procedure in maintaining the workability of concrete.

Density

Density of fresh concrete was determined using NP EN 12350-6:2002. Results exhibit a clear and obvious

Conclusions

The experimental results clearly show that crushed bricks can be used as natural aggregates substitutes in percentages up to 15% without strength reduction. For 30% of natural aggregate substitution, there is a reduction of concrete properties (up to 20%, depending on the type of brick). The use of strength indexes allows a better understanding of the effect of aggregate replacement. The uses of strength indexes allowed a better comparison between results.

The stress–strain relations are very

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