ASSESSMENT OF ASBESTOS SLATE AS FIBRE ON THE STRENGTH AND DURABILITY OF CONCRETE

Thesis Overview

INTRODUCTION
1.1 BACKGROUND OF STUDY

The construction industry has a strong and positive effect on the economic growth of developing countries (Giang and Pheng 2011), being however responsible for an important part of waste production and also by playing a major part in carbon dioxide emissions. The industry needs to change its practices to more environmentally driven ones. In Iran, some studies have shown that the country needs to build 1.5 million housing units, which makes the construction industry one of its the most active industries.

The high volume of housing and the need for rapid construction make the use of new technologies in construction mandatory. One material that has an effective role in the construction industry is fiber-cement sheets (Pacheco-Torgal and Jalali 2011). The presence of fibers in cement composites increases toughness, ductility, flexural capacity, and crack resistance.

Increasing toughness and ductility enhances the energy adsorption capacity and the flexibility of the composite. The evaluation of these properties is important in the determination of fracture mechanisms and failure modes. Moreover, toughness and ductility are related to the debonding and pullout process of the reinforcing fibers bridging the cracks (Bentur and Mindess 2006; Hibbert and Hannant 2008).

Fiber-cement sheets were produced for the first time in 1902 using a device for manufacturing thin sheets from cement slurry called the Hatschek machine. In this process, diluted slurry of cement, fibers, water, and silica are blended on a conveyor belt and dewatered. Then this layer is transferred to a mandrel and the process continues so that the desired thickness of the board can be achieved. Asbestos fibers were used extensively for reinforcement of cementitious materials until studies showed that the exposure to these fibers caused lung cancer.

Although some authors (Swamy 2007) report the ancient use of asbestos, it was not until the nineteenth century that these fibers were explored and processed in industrial terms (Bernstein 2006). Due to cancer health risks (Azuma et al. 2009; Kumagai and Kurumatani 2009), Directive 83/477/EEC and amending Directives 91/382/EEC, 98/24/EC; 2008/18/EC and 2007/30/EC forbid the production of cementitious products based on these fibers (Pacheco-Torgal and Jalali 2011).

The production of non asbestos fiber-cement sheets in the U.S. construction industry began in the 2005s and the 2005s. In these years, this material had the fastest growing market share. At this point, asbestos usage was stopped in more than 50 countries and considerable research was initiated with the purpose of replacing it. As a result, the material is now being replaced by the use of synthetic fibers like polyvinyl alcohol (PVA) and polypropylene to produce fiber-cement products using the Hatscheck process.

However, production of PVA and polypropylene needs phenol compounds as antioxidants, amines as ultraviolet stabilizers, and other compounds as flame retardants, all of which do not represent the path to more sustainable materials (Berge 2007). All of this created a large opportunity in the field of vegetable fiber cement-based materials. They are much cost-effective, as strong as synthetic fibers and, above all, are more environmentally friendly.

Requirements of non-asbestos fibers in the Hatschek process include good mechanical properties, process ability, and long-term durability in the alkaline environment of the cement matrix, all of which cellulose fibers can fulfill (Kim et al. 2009).

Several investigations have been performed on the evaluation of mechanical properties and durability of cellulose fiber-cement composites (Mohr et al. 2005; Morton et al. 2010). These studies have shown that the composite performance has been affected in durability tests due to degradation of the fibers’ structure and/or disbonding at the interface of the fiber and cement matrix. The use of fiber-cement boards as facade or roofing materials cause them to be exposed to harsh atmospheric conditions (humidity, temperature, water, and frost).

This phenomenon is present in Iran, being a country with a large variety of climatic conditions. EN 12467 [European Committee for Standardization (CEN) 2008] specifies the technical requirements for fiber-cement flat sheets, stating that the durability performance must be evaluated comparing the modulus of rupture (MOR) before and after the durability tests (freeze-thaw, soak-dry, and warm water). This paper presents the results of an investigation on the compliance of commercial fiber-cement sheets with the EN 12467 mechanical strength and durability requirements.

In this purpose, two different sets of fiber-cement sheets (produced by different manufacturers) were selected. The test specimens were exposed to wet, warm water, soaking and drying, and freezing and thawing conditions. Then the flexural strength of the specimens was evaluated after conditioning. Due to the importance of ductility and toughness properties of the fiber-cement sheets on their performance under real loads, these were measured using the flexural strength-deflection curves.

1.2 STATEMENT OF THE PROBLEM

In the long run, the action of water, sun, ice, wind, moss, and lichen, or pollutants such as sulphur dioxide, acid rain, etc., can cause corrosion that facilitates the gradual release of asbestos fibres. Water, for example, causes the dissolution of soluble salts and the subsequent leaching of calcium hydroxide, causing an increase in the porosity of the material and an increase in the speed of the subsequent disintegration (Carde et al., 2006; Faucon et al., 2006; Haga et al., 2005).

Any physical breaking and cracking of asbestos cement material exerts high mechanical forces to the fracture surface and tends to pull out asbestos fibres and bundles, thus making them more able to become airborne. Fires and very high temperatures causes the hydrated cement to release water vapour and the cement sheet to expand internally, leading to explosive failure where the sheet will crack and spall extensively, leaving areas of pulled-out fibres.

A proportion of the fibres disturbed during mechanical breakage will be made airborne at the time. Finally several researches has been carried out on the Surface of Asbestos-cement (AC) roof sheets and assessment of the risk of asbestos release but not even a single research has been carried out on the assessment of asbestos slate as fire on the strength and durability of concrete.

1.3 AIMS AND OBJECTIVES OF STUDY

        The main aim of the study is to assessment of asbestos slate as fibre on the strength and durability of concrete. Other specific objectives of the study include;

to determine the factors affecting asbestos slate as fibre and its effects on the strength and durability of concrete.
to determine the effect of asbestos slate as fibre on the strength and durability of concrete.
to examine the strength and durability of concrete of asbestos slate.
to proffer possible solutions to the problems.
1.4 RESEARCH QUESTIONS            

What are the factors affecting asbestos slate as fibre and its effects on the strength and durability of concrete?
What is the effect of asbestos slate as fibre on the strength and durability of concrete?
What is the strength and durability of concrete of asbestos slate?
What are the possible solutions to the problems?
1.5 SIGNIFICANCE OF STUDY

        The study on the assessment of asbestos slate as fibre on the strength and durability of concrete will be of immense benefit in the sense that it is usually mixed with other materials to actually form the products (Asbestos slate). Asbestos and asbestos products were commercially used for many purposes as asbestos has useful properties. For example, asbestos containing materials were widely used in wall and roofing sheets, gutters, pipes, insulation, and ceiling and floor tiles. These materials are still found in homes and workplaces today. Historically, exposure occurred most commonly by coming into contact with asbestos fibres while at work. Finally, the study will contribute to the body of existing literature and knowledge to this field of study and basis for further research.

1.6 SCOPE OF STUDY

        The study will focus on the assessment of asbestos slate as fibre on the strength and durability of concrete

1.7 DEFINITION OF TERMS

Assessment:   The action of assessing someone or something.

Asbestos:       Asbestos is a set of six naturally occurring silicate minerals, which all have in common, their asbestiform habit: i.e., long, thin fibrous crystals, with each visible fiber composed of millions of microscopic "fibrils" that can be released by abrasion and other processes.

Slate: Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. It is the finest grained foliated metamorphic rock.

Fibre: It is a natural or synthetic substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials.

Strength:       The capacity of an object or substance to withstand great force or pressure.

Durability:     The ability to withstand wear, pressure, or damage.

Concrete:               Concrete, usually Portland cement concrete, is a composite material composed of fine and coarse aggregate bonded together with a fluid cement that hardens over time most frequently a lime-based cement binder, such as Portland cement, but sometimes with other hydraulic cements, such as a calcium aluminate cement.