Vegetable Plants and their Fibres as Building Materials

Proceedings of the Second International RILEM Symposium

H.S. Sobral

• Publisher: Routledge
• Year: 2004
• eBook ISBN:
  9781134990955
• Topic: Architecture
• Subtopic: Architecture Methods & Materials

This book examines the state-of-the-art on plants and fibres as building materials for low cost construction, emphasizing their use, properties, fabrication, new procedures and future developments. It makes available research results on new techniques for fibre reinforcement and their use in concrete, stabilized clay and other matrices. Procedures for making vegetable fibres and wood-based building materials in developing countries are also analysed.

PART ONE
INTRODUCTION AND BACKGROUND

1
VEGETABLE FIBRE REINFORCED CEMENT COMPOSITES—A FALSE DREAM OR A POTENTIAL REALITY?

R.N.SWAMY

Department of Mechanical and Process Engineering, University of Sheffield, England

Abstract

The use of natural fibres in concrete matrices poses a special challenge to science and technology. Their use can save energy, conserve scarce resources and protect environment whilst alleviating the housing problem and enhancing a country’s infrastructure. The inherent weaknesses of the fibres such as low modulus, lack of adequate interfacial bond and long term stability can be overcome by microstructural studies and micromechanics, but ultimately, it is their long term engineering performance that needs to be ensured. Natual fibre cement composites can contribute significantly to enhance the quality of life and of living.

Keywords: Natural Fibres, Cement Composites, Durability, Cement Matrix, Low Modulus, Microstructure, Engineering Performance, Housing.

1 Introduction

The development and understanding of building materials has generally received much less attention in the last few decades compared to sophisticated analysis and design procedures. The latter is much more exciting, and often considered intellectual, whereas the former is labelled as mundane and experimental. Probably one area where the building materials technology has been much neglected is in the realm of housing. The human habitat has become an almost intractable world problem. It is only when one realises that housing is as much a problem for the developed societies as for the developing countries of the Third World, and that some six hundred million houses need to be built before the end of the century if every family in the world is to have a roof over its head, that the enormity of the problem, and of the challenge to engineers and architects, dawns on all of us.

Natural organic fibres have a very important and unique role in the contribution they can make to alleviate the housing problem. They not only occur in luxurious abundance in many parts of the world, but they also can directly lead to energy savings, conservation of the world’s more scarce resources and protect the environment. The fact that one of the most easily and readily replenishable earth’s resources can be used to solve, at least in part, one of the most acute forms of human misery, is just as challenging not only to the basic human instinct of fellow feeling but also to the science and skills of developed technologies. Bamboo, for example, is one of the fastest-growing, highest-yielding and easily renewable of natural resources, and yet our ability to use them in durable construction is far from reality. There is also the other point about these materials— Nature will continue to be bountiful to mankind so long as we do not wantonly consume, misuse or destroy the vast and rich resources that it so generously provides. Natural and vegetable plants and fibres have thus a unique irreplaceable role in the ecological cycle, and their natural abundance, plentiful supply, relative cheapness and swift replenishability are the strongest arguments to utilise them in the construction industry.

Cement and concrete matrices reinforced with short, discrete or long single/bundles of fibres present exciting and challenging new construction materials. The major role of the fibres is in delaying and controlling the tensile cracking of the matrix. This controlled multiple cracking reduces deformations at all stress levels, and imparts a well-defined post-cracking and post-yield behaviour. The fracture toughness, ductility and energy absorption capacity of the composite are then substantially improved. These technical benefits can be utilised both in semi-structural elements such as thin sheets and cladding panels as well as in load bearing members.

Cement composites reinforced with steel, glass and polypropylene fibres have seen extensive development and a wide range of practical applications, Swamy et al (1986). There is currently an enormous body of data available on these composites, and yet, natural fibre reinforced concrete composites have not enjoyed the same sort of development and applications they deserve, in spite of the fact that there is also sufficient research and practical experience in the use of natural fibres in concrete, Gram (1983), Gram et al (1984), Gram et al (1986), IPT (1987), Mwamila (1984), RILEM (1986), Swamy (1984a, 1988b). By comparison with many natural fibres, and vegetable plants, composites incorporating wood fibre and cellulose fibres are well advanced in the manufacture of both autoclaved and air-cured products and flat sheets, corrugated roofing, moulded products and non-pressure pipes are produced by industrialized production processes, Akers et al (1989), Coutts (1988), Fordos (1986). Their use at village-level small scale industries, on the other hand, is much less developed. This paper is primarily addressed to the latter: its aim is to make a critical evaluation of the present status of the use of natural fibres in cement and concrete, mainly for housing units, the problems to be tackled, and point the way forward to make natural fibre cement composites economic and durable construction materials.

2 The problems

Although natural fibres exist in abundance and are readily available at low cost, they have many inherent weaknesses such as low elastic modulus, high water absorption, susceptibility to fungal and insect attack, lack of durability in an alkaline environment and variability of properties amongst fibres of the same type. It is not surprising therefore that natural fibres have not always been the ready or automatic choice as a reinforcing medium in cement matrices despite widespread interest, numerous research efforts and many trial applications.

A major factor contributing to this slow development is the lack of precise scientific information on the structure and properties of natural fibres, their compatibility with the various matrices and the properties of the composites themselves. In published literature, a wide range of values exist for any given property both within a given class of fibre and across classes of fibres. The reasons for this are not difficult to find. Fibres can be derived from leaf, stem or wood, and both fibre diameter and length are dependent on age of growth. Abacas, for example, occurs in as many as 200 varieties, with reported fibre lengths varying from 5.0–9.0mm, and fibre widths of 12.4–21.5mm. Bamboos, on the other hand, can be subdivided into four families and an estimated fifty genera: further, over 1250 individual species of bamboo have been identified so far. Another factor making studies on composites difficult and complex is that natural fibres are hollow; the central lumen changes in size with age, and may sometimes collapse, altering the cross-sectional area of the fibre within a given fibre type.

There has also been a mistaken and misguided belief that chopped material fibre composites have similar properties of strength and durability as asbestos cement sheets. The destructive effects of the alkaline pore solution on organic fibres is well established. The net effect of these chemical degradation processes is that with time the fibres will cease to exist and provide no reinforcing property. In the short term the fibres provide cohesion and workability to the cement matrix, and help to arrest plastic shrinkage and initial drying shrinkage cracks. In the long term, prolonged reinforcing action betwen the matrix and the fibre can only be ensured by carefully designed procedures calculated to enhance long term stability of the fibre in the cement matrix.

Many plant fibres contain hemicellulose, starch, sugar, tannins, certain phenols and lignins, all of which are known to inhibit the normal setting and strength development properties of the cement matrix. Apart from increasing the handling time, the water soluble extractives also prevent the composite from attaining its full strength and durability characteristics. The decomposition of the fibre is much faster in an alkaline medium than in water, and the rate and degree of decomposition will depend upon the nature of the fibre. The degree of polymerisation, and the location or accessibility to the hemicellulose and lignin will both affect adversely setting time and strength development, and these need to be controlled. In practice, both setting time and strength loss can be controlled by low water-binder ratios, and a compatible accelerator can also make good further the loss of strength.

2.1 Engineering aspects

The study of the microstructure of the fibre cement matrix and their interaction and their interface has a very important role in the development of these fibre composites. However, microstructural investigations cannot and should not be isolated from engineeringperformance. There are several lessons to be learnt from previous developments, where emphasis and confidence in one at the expense of the other has led to many practical difficulties and economic implications unforeseen by microstructural or micromechanics studies. The effects of thermal moisture gradients, of fixings and external stresses, and of the production processes on anisotropic properties, in addition to the effects of realistic exposure conditions are sufficiently well established to warn producers and users alike of the need to link changes in microstructural characteristics to behaviour at engineering level, Swamy (1986).

2.2 Size effects

In many laboratory studies, size effects are ignored. Size effects on strength are well known, strength increasing as the size of the specimen is reduced. This is particularly relevant when composites are judged from flexural strength tests. For example increases in strength of up to 50% or more can be observed when the size of specimen is reduced from 1000×300×10mm to 500×100×10mm, and strength increases of 100% or more may be observed when the size is reduced to 150×50×10mm, the usual size adopted in many studies. The size of specimen will also affect its mode of failure, shear failures and delamination occurring instead of the usual flexural failure.

The differences in strength and mode of failure will generally be accentuated if the fibres fail to provide any reinforcing effect. A larger product like a sheet is thus more likely to give trouble, crack and fail suddenly than a geometrically smaller product like a tile. This partly explains why a high incidence of cracking and considerable performance deficiencies have been reported by users of natural fibre reinforced sheets and tiles, limiting useful life of these products to no more than 2 to 4 years. Extensive surveys have shown clearly that it is not the technology or the product that failed but the inherent destructive effect on the fibres of the alkalinity of the cement, particularly in tropical environments, and the lack of adequate standards of roof construction and installation that were responsible for these failures. In addition, there has been a general lack of appreciation of the role of the natural fibre in the cement matrix, and the need to relate the geometrical shape of the product, and how it is fixed, to the inherent decomposition of the fibre and lack of quality control which inevitably occurs in less developed technologies.

3 Cement matrix—the key to composite stability

The main concern with natural fibres is one of long term strength and stability. Marked embrittlement of sisal fibres, for example, embedded in thin roofing sheets exposed to a tropical climate has been observed within a period of months. The embrittlement is known to be caused directly by the alkaline environment of the cement system. There is also evidence, on the other hand, that natural fibres remain practically intact in carbonated concrete. Several methods are currently available for waterproofing and for prophylactic treatment against deterioration. Protective impregnation of the fibres with various chemical treatments have, however, been unable to prevent the chemical decomposition of the fibre components completely, and have resulted in poorer composite properties due to impairment of fibre tensile strength and the effects on fibre matrix interfacial bond properties. While further research can probably develop other protection methods that are simple, effective and economic, it is very unlikely that such methods result in viable technologies at an acceptable cost to developing countries.

Since fibre embrittlement is caused primarily by the high alkalinity of the cement system, the solution to the deterioration of the natural fibre lies in the matrix itself. There are several simple techniques readily available to reduce the alkalinity of the cement matrix. Cements other than portland cement such as aluminous cements and cements with negligible alkali contents can now be manufactured, but these methods may not offer a ready solution ...

Table of contents

• COVER PAGE
• TITLE PAGE
• COPYRIGHT PAGE
• PREFACE
• INTRODUCTION
• PART ONE: INTRODUCTION AND BACKGROUND
• PART TWO: SURVEY OF PRESENT SITUATION IN RELATION TO FIBRES
• PART THREE: PROPERTIES OF VEGETABLE FIBRE COMPOSITE MATERIALS
• PART FOUR: BUILDING COMPONENTS WITH VEGETABLE FIBRE COMPOSITE MATERIALS
• PART FIVE: BUILDING COMPONENTS MADE WITH WOOD
• PART SIX: BUILDING WITH CULMS AND STICKS
• PART SEVEN: RECYCLING OF AGRICULTURAL WASTE AND RELATED TOPICS