Thermal conductivity, compressive strength and ultrasonic wave velocity of cementitious composite containing waste PET lightweight aggregate (WPLA)

Abstract

In this study, the influence of waste PET as lightweight aggregate (WPLA) replacement with conventional aggregate, on thermal conductivity, unit weight and compressive strength properties of concrete composite was investigated. For this purpose, five different mixtures were prepared (the control mixtures and four WPLA mixtures including 30%, 40%, 50%, and 60% waste PET aggregate by volume). Thermal conductivity (TC) coefficients of the specimens were measured with guarded hot plate apparatus according to TS ISO 8302 [1]. The thermal conductivity coefficient, unit weight and compressive strength of specimens decreased as the amount of WPLA increased in concrete. The minimum thermal conductivity value was 0.3924 W/m K, observed at 60% WPLA replacement. From this result, it was concluded that waste PET aggregates replacement with conventional aggregate in the mixture showed better insulation properties (i.e. lower thermal coefficient). Due to the low unit weight and thermal conductivity values of WPLA composites, there is a potential of using WPLA composites in construction applications.

Introduction

Unit weight, compressive strength and thermal insulation properties of concrete are related to each other. In general, while the unit weight of concrete increases, the strength of concrete increases. However, thermal insulation properties decrease, because of the increasing unit weight of concrete [2], [3]. Today, lightweight concrete is used in most engineering applications in order to eliminate the objectionable aspects which are mentioned above of normal concrete, and to give some positive properties. Thermal insulation properties of lightweight concrete are better than normal concrete because of the low unit weight. The thermal conductivity of lightweight concrete can take values of approximately 50% lower than the normal concrete [4].

Today, the lightweight concretes can be produced with various materials and construction methods. The most common method used in the production of lightweight concrete is using lightweight aggregate. Lightweight aggregate is an important material used in order to reduce the unit weight of both structural and non-structural concrete. Lightweight aggregates are generally used to reduce the unit weight of concrete by replacing the normal weight aggregates by partially or fully.

Unlike the well-known materials, waste plastic granules can be used as lightweight aggregate in the production of lightweight concrete. Polypropylene (PP), poly-ethylene (PE), poly-ethylene terephthalate (PET) and polystyrene (PS) are some of the waste plastic used as lightweight concrete aggregate. The PET bottles are ahead of the wastes with its high increasing speed of consumption. A large amount of natural aggregate is needed for production of concrete which is an indispensable material for the construction technology. For this reason, evaluation of waste PET aggregates in construction applications provides savings from the amount of natural aggregates.

In the recent years, a lot of experimental studies carried out on using waste PET bottles as aggregate in cement-based composites. In these studies, some physical and mechanical properties of the WPLA specimens are examined [5], [6], [7], [8], [9], [10], [11], [12]. However there was limited number of studies carried out on thermal conductivity of lightweight concrete made with waste PET aggregate [13], [14], [15]. In general, waste PET aggregate addition decreases unit weight of composites. Modulus of elasticity of the composites decreases and specimens show more ductile behavior with increasing PET aggregate amount. However, waste PET aggregate incorporation causes reduction at compressive strength, flexural-tensile strength and splitting tensile strength of cement-based composites. In addition, water absorption increases depending on increasing WPLA amount.

The most basic feature that used for classifying the thermal insulation materials is thermal conductivity (TC) coefficient. The TC coefficient is the amount of heat which passes through unit area in unit time when the temperature difference between the boundaries of the system is 1 °C. The materials which TC coefficient are less than 0.065 W/m K are defined as the thermal insulation materials, the other materials are defined as structural materials [1].

Mineralogical character of the aggregate quite influences the TC of concrete composites. The less conductive concrete can be produced by using aggregate with less TC; similarly more conductive concrete can be produced with more TC aggregates [16], [17]. The thermal conductivity coefficient of traditional concretes ranges between 0.8 and 1.65 W/m K [18]. Because of the low TC of PET aggregate (0.15 W/m K), the TC of lightweight composite with PET aggregate is expected to be low.

Marzouk et al. [13], have used waste PET lightweight aggregate (WPLA) partially or completely replacing the natural aggregate to produce concrete composites. The compressive strength of concrete composites they obtained was greater than 3.5 MPa. The TC of the specimens was found about 0.69 W/m K. They concluded that their product could be used as bearing insulator.

Yesilata et al. [14], have investigated that relative change in insulation property of the ordinary concrete due to adding polymeric based waste material. Waste PET bottle and automobile tire pieces were used in their study. Shredded waste PET bottles were in square and irregular shapes added into the fresh concrete. Thermal tests were performed with the dynamic adiabatic-box technique. Their experimental results showed that using proper proportions of waste PET and rubber particles reduced heat loss and improved insulation property of concrete composites. The degree of improvement in thermal insulation was found to vary with the amount of added waste materials and geometry of shredded-pieces.

Hannawi et al. [15], have examined the effect of different amounts and thickness of PET aggregates on the properties of mortars. Two different thicknesses (1 mm and 0.1 mm) of PET aggregates were used (PET1 and PET0.1). Increasing plastic content caused a considerable reduction in the thermal conductivity of the specimens. This reduction was about 64% for mixtures containing 50% PET1. For PET0.1, this reduction ratio reached about 88%. This trend could be attributed to the low thermal conductivity of PET aggregates (0.15 W/m K) in comparison to that of natural sand (2 W/m K). Moreover, the addition of waste plastic into cement matrix reduced the ultrasonic wave velocity which indicated a high level of sound insulation of the material.

The main aim of this study is to investigate the influence of the using waste PET aggregate on the thermal conductivity of lightweight concrete composites. For this purpose, guarded hot plate apparatus in accordance with TS ISO 8302 [1] was used for measuring the TC of specimens. The unit weight, compressive strength and ultrasonic pulse velocity of WPLA composites were also investigated in the study. In addition the relationships between these properties and TC coefficients of WPLA composites were evaluated.