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Chapter forty-two

Fibre-reinforced

cements

42.1 Asbestos cement42.2 Glass-reinforced cement (GRC)42.3 Natural bres in cement42.4 Polymer bre-reinforced cement42.5 References

42.1 Asbestos cementAsbestos cement is familiar to all engineers as theubiquitous roong and cladding material whichhas had a very low cost and excellent durabilityduring the past 100 years. A reason for its successhas been the great durability of asbestos breswhich are shown in Figure 42.1. The bresshown in this micrograph have been exposed toweathering for more than 10 years but the sub-micron bres within the bre bundle show nosign of deterioration. Other studies have shownthat the strengths of the bre bundles varybetween 400MPa and 1400MPa irrespective of exposure up to seven years.

However, due to the well-publicised healthhazards associated with asbestos bres, which areknown to be carcinogens, there has been a rapid

decrease since 1980 in the UK in sales of asbestoscement sheeting products. Stringent safety pre-cautions are essential not only during the manu-facturing process but also to prevent inhalation of dust during cutting and drilling such sheets onsite.

Another signicant problem is that the materialis brittle and the impact strength is notoriouslylow so that there are a number of deaths in theUK every year as a result of people falling

through roofs when not using the required crawl-ing boards.

The net result of these adverse factors has been

to virtually eliminate sales of asbestos cementproducts in Europe and the USA but neverthelessthere is still more asbestos cement on structuresthroughout the world than all other types of brecement combined and therefore a detaileddescription of its properties is warranted herein.

42.1.1 Mix design and manufactureThe proportion by weight of asbestos bre is nor-mally between 9 to 12 per cent for at or corru-gated sheet, 11 to 14 per cent for pressure pipesand 20 to 30 per cent for re-resistant boards,and the binder is normally a Portland cement.Fillers such as nely ground silica at about 40 percent by weight may also be included in auto-claved processes where the temperature mayreach 180C. Fibre volume, stress direction andproduct density all have an effect on propertiesand hence the properties depend to a certainextent on the manufacturer.

The most widely used method of manufactureof asbestos cement was developed from paper-making principles in about 1900 and is known asthe Hatschek process. A slurry or suspension of asbestos bre and cement in water at about 6 percent by weight of solids is continuously agitatedand allowed to lter out on a ne screen cylinder.The ltration rate is critical and coarser cementthan normal (typically with a specic surface areaof 280m 2 /kg compared with the normal value of

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320m 2 /kg) is used to minimise ltration losses.Referring to Figure 42.2, in very much simpliedterms, the Hatschek machine operates as follows.A dilute slurry pours into the vat and drainsthrough a porous sieve cylinder depositing thesolid contents as a layer on the surface of thesieve. The water passes through the sieve surface

and returns to the vat via the backwater circuit.The sieve cylinder rotates and the layer rises outof the vat. A continuous felt runs in a loop fromthe sieve cylinder to an accumulation roll. Surfacetension forces the layer to transfer from the topof the sieve cylinder to the underside of the felt.The movement of the felt transfers the layer fromthe sieve to the accumulation roll and on the wayit is vacuum dewatered.

Typical outputs are 1 tonne per hour per metre

width of vat. Felt speeds range from 40 to 70metres per minute. A typical three-vat machinewill make a 6mm thick sheet in six to nine revo-lutions and will make one sheet every 20 seconds.

Manufacturing costs are therefore very low.Due to the process, the bres are essentially dis-persed in two directions with the predominantalignment of the bres in the direction of rotationof the sieve drum. The stacks of products mayreach 60C and autoclaving is sometimes used toreduce the shrinkage of the products.

FIGURE 42.1 Asbestos bre bundle in cement pasteafter natural weathering for more than 10 years.

FIG URE 42.2 Hatschek manufacturing process forasbestos cement sheeting.

0

Strain ( 10 6 )

S t r e s s

( M P a

)

26

24

20

16

12

8

4

600 1200 1800 2200

FIGURE 42.3 Tensile stressstrain curve forasbestos cement.

42.1.2 PropertiesAsbestos cement is the only bre composite forwhich there are International Standards require-ments for certain properties. These are generallyexpressed in terms of minimum bending strength,density, impermeability and frost resistance. Forinstance, the minimum bending strength generallyvaries between 15MPa and 23MPa when testedunder dened conditions and depending onwhether the sheet is semi- or fully compressed.

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Also, various loading requirements are denedfor corrugated sheets such as snow loads up to1.5kPa, and point loads to simulate men workingon a roof. Water absorption should not exceed20 to 30 per cent of the dry weight depending onthe type of product.

A typical tensile stressstrain curve for a com-mercial product is shown in Figure 42.3 wherethe failure strain is about 2000 10 6. No crackswere visible before failure which may be con-trolled by crack suppression as described inSection 41.2. A modulus of elasticity of about20GPa in tension and compression and amodulus of rupture well in excess of 30MPa havecombined to provide probably the most success-ful example of all time of a bre-reinforced com-

posite both in terms of tonnage and protability.

42.1.3 DurabilityAsbestos cement is known to be very durableunder natural weathering conditions and littledeterioration in exural properties takes placedue to weathering, although the material becomesprogressively more brittle.

However, it has been shown that asbestosbres in cement sheets at ages of 2, 16 and 58years do suffer a certain amount of corrosionwhich is compensated for, in terms of compositestrength, by an increase in bond between the breand the cement. The corrosion of the bre is pro-moted by the penetration of airborne carbondioxide which causes carbonation at the surfaceof the bre. Also, certain magnesium hydroxidesand carbonates may be formed as reaction prod-ucts. The natural variability in bre strengthtends to mask any measured changes in bre

strength due solely to weathering.

42.1.4 UsesCorrugated roong and cladding for agriculturaland industrial buildings has formed by far thelargest application, and the ability to be mouldedinto complex shapes has enabled a wide range of accessories to be produced for roong applica-tions. Flat sheet has been used for diagonal tiles

to replace natural slate. Asbestos cement pressurepipes have been used for many years for convey-ing mains water, sewage, sea water, slurries andindustrial liquors. Diameters range from 50mmto 900mm with working pressures from0.75MPa to 1.25MPa. It is the extraordinaryability of the asbestos bres to suppress crackingin the matrix which has made these watertightapplications possible and no other bre cementhas been able to emulate its performance. Also,non-pressure uid containers such as rainwatergoods, conduits, troughs, tanks and ue pipeshave accounted for a large proportion of theminor applications of asbestos cement.

42.2 Glass-reinforced cement (GRC)

Glass-reinforced cement is normally made withalkali-resistant glass bre bundles combined witha matrix consisting of Ordinary Portland cementplus inorganic llers. E-Glass bres have beenused with a polymer modied cement matrix toprotect the glass against attack by the alkalis inthe cement. The material described in this sectionrelates to zirconia-based alkali-resistant breswhich are normally produced in the form of strands consisting of 204 laments each of between 14 and 20 microns in diameter. Up to 64strands may be wound together as a roving whichis cut during the making of GRC into strands12mm to 38mm long. Pre-chopped strands maybe supplied in lengths as short as 3mm. The indi-vidual laments are bonded together in the strandby an organic size which determines the physicalnature of the end product. A photograph of astrand embedded in cement is shown in Figure42.4.

The presence of zirconia (ZrO 2) in the glassimparts resistance to the alkalis in the cementbecause the Zr B O bonds, in contrast to theSiB O bonds, are only slightly attacked by theOH ions thus improving the stability of the glassnetwork.

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42.2.1 ManufactureSpraying and premixing are the main productionprocesses for glass-reinforced cement.

In spray processes the cement/sand mortarslurry is fed to the spray gun and is broken intodroplets by compressed air. Glass bre roving is

fed to a chopper on the spray head which chopsthe bre into 25mm to 40mm lengths and injectsthem into the mortar spray to be deposited on themould. The slurry mortar is typically 1:1sand:cement ratio with a W/C ratio of 0.33.Admixtures are used to increase the workabilityand the resulting composite contains typically 5per cent by weight of glass bre.

The composite may be sprayed manually or bymachine and some processes use dewatering by

vacuum to result in an end product with a freeW/C ratio of about 0.28 which has greaterdensity and strength that the non-dewateredproduct.

Premixing processes , as the name implies,involve the blending together of cement, sand,water, admixtures and chopped strands in amixer before placing in the mould. The bres areadded at the end of the mixing process at a lowerspeed to avoid damage. Typical mixes contain asand:cement ratio of 2:3, a water:cement ratio of

FIGURE 42.4 Glass bre strand in cement paste.

about 0.35, workability aids and, typically, 3 percent by weight of 12mm long bres. Having beenmixed, the composite may be cast by pumpingwith or without vibration and also by spraying.Pressing combined with dewatering may also beused with premixes.

Prebagged formulations containing between0.5 per cent and 2.5 per cent by weight of glassbre have also been developed for renderingbrickwork or blockwork.

As with all cement-based products, it is import-ant to moist cure glass-reinforced cement prod-ucts for as long as possible to ensure that amaximum amount of hydration and hencestrength gain has occurred before loading.

42.2.2 PropertiesThe tensile stressstrain curves for glass-rein-forced cement will typically follow either of curves of type A or B in Figure 41.1. Curve typeA is representative of the lower bre volumesused in the premix composite where a singlecrack and no increase in the post-cracking stressis expected. Curve B is representative of the earlyage sprayed composite with more than the critical

bre volume at that age. This results in multiplecracking and a high failure strain as shown inFigure 42.5.

Fibre length, volume and orientation also affectthe performance of the composite at 28 days inuniaxial tension (Ali et al. , 1975). The shape of the curves is approximated by the theoreticalapproach described in Section 41.1 and it can beseen that strength and strain to failure are bothincreased by increases in bre length and volume.

Nominal exural tensile strengths may varybetween 15MPa and 50MPa and follow the rela-tionship between tension and exure shown inFigure 41.11.

However, although the mechanical propertiesare good at early ages, the strength and toughnessof some GRC formulations may change with timeand hence design stresses are conservative.Typical design stresses quoted from trade liter-ature are shown in Table 42.1. In this table, thedifference in performance between sprayed and



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take account of the non-alignment, we multiplythe value of fu by 0.27 (the efciency factor forstress) and nd that a total bre volume fractionof more than 3.5 per cent at 28 days, and morethan 7.4 per cent at 10 years is required to main-tain ductility. Typical total bre volumes areabout 4 per cent and it is known that the energyabsorbed to failure, found from the area underthe measured stressstrain curve, reduces fromabout 120kJ/m 3 at 28 days to less than 5 kJ/m 3 at5 years. This change may therefore be explainedby the increase in critical bre volume with time.

An understanding of these potential problemsresulted in research to eliminate calcium hydrox-ide growth by including a metakaolin syntheticpozzolana (described in Chapter 15) to react withthe free lime in a controlled way to prevent limebuild up within the bre bundle. The metakaolinparticles also appear not to migrate into the brebundle. Results from these modied matricesusing accelerated tests have indicated that greatlyimproved durability and long-term toughness of GRC composites should now be possible usingabout 25 per cent cement replacement bymetakaolin. Thus, long-term stable properties formetakaolin-based matrices approximate to curve

A in Figure 42.6 rather than the embrittled curvetype B. In contrast, silica fume with a particle sizeof less than 0.1 micron, although reacting withthe lime, may penetrate to individual lamentsand result in the formation of hard calcium sili-cate hydrates which are as damaging to the bresas lime deposition.

However, a note of caution should be maderegarding the long-term predictive ability of high-temperature accelerated tests because, as with

many durability problems with cement-basedmaterials, total condence can only be gained byreal time trials in natural weathering conditionsover many years.

42.2.4 UsesCladding panels are a major eld of applicationfor glass-reinforced cement. Due to crackingcaused by restrained warping in early applica-tions particular attention should be paid to

FIGURE 42.6 Effect of natural weathering on thetoughness of glass-reinforced cement.

thermal and moisture movements and to xingdetails in large, double-skinned sandwich panelsof this type of construction. Light colouring of the panels is preferred because this helps toreduce thermal stresses particularly where there isan insulating core. There is increasing emphasis

on the use of single-skin cladding panels attachedto prefabricated steel frames by exible anchors.This is known as a GRC Stud Frame system. Thexings are designed to allow unrestrained thermalor moisture movements of the glass-reinforcedcement skin which may be 6m long by storeyheight.

An important use where the early age strengthand toughness are benecial is in permanentformwork for bridge decks, the advantage being

that no temporary support works are requiredand a dense, high-quality cover is provided to thereinforcement.

The greater efciency ensured by using contin-uous glass bre rovings in the main stress direc-tion has been utilised in a process for producingcorrugated sheeting. The corrugated sheets startas a continuous at sheet made from two layers,each 3.25mm thick. Short strands as cross rein-forcement are immersed in the matrix on theupper/underside and this leads to an ideal sand-



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wich structure. The lengthwise reinforcement ismostly made up of one-directional inlaid glassbre rovings, which are sandwiched between thetwo layers that make up the sheet. The at sheetsare then corrugated on formers before curing.

There are many other uses such as cable ducts,agricultural products, sewer linings, culverts,sound barriers, drainage channels, re-resistantboards, septic tanks, roong slates and mortarrenders for dry block wall construction. Sometrials have been made using glass bre tendonsprotected with polyester resin as prestressingtendons in prestressed concrete structures.

42.3 Natural bres in cement

The use of natural cellulose or vegetable bres incement or mortar products is common in bothdeveloped and developing countries and thesubject has been reviewed in detail by Swamy(1988) and Bentur (1990).

42.3.1 Wood bre products

Manufacture

In developed countries the bulk usage is for woodcellulose bres from trees. The wood is mechani-cally and chemically pulped to separate the indi-vidual bres which may be between 1mm and3mm long and up to 45 microns in width. Hard-woods and softwoods are used and the elasticmodulus of individual bres may vary between18GPa and 80GPa with strengths between350MPa and 1000MPa depending on the angleof cellulose chains in the cell wall. Cellulose bres

produced from timber have several advantageswhen used in thin cement or autoclaved calciumsilicate sheets. The bres are low cost comparedwith most man-made bres, they are a renewableresource, there is considerable experience in theuse of such bres in existing plant for asbestoscement, and they have an adequate tensilestrength for cement reinforcement. However, cel-lulose is sensitive to humidity changes and theelastic modulus of the bres reduces when wet so

that the properties of the composite may varyconsiderably from dry to wet. The bres are usedin volumes up to 10 per cent in conjunction withpolymer bres in asbestos-free products in whichmanufacturing procedures are very similar to theHatschek process already described for asbestoscement.

Properties

When suitably pre-treated by rening, woodbres used in conjunction with polyvinyl alcoholbres in a matrix of Portland cement and llerscan provide a tough and durable bre cement.This composite is suitable for the commercialproduction of corrugated sheeting and pressed

tiles on traditional slurry dewatered systems suchas the Hatschek machine ( Figure 42.2) . The vari-ation in composite properties wet to dry andpressed to unpressed is shown in typical tensilestressstrain curves for commercial composites inFigure 42.7.

Uses

In Australia, cellulose bres have completelyreplaced asbestos bres in at sheeting productsmade from an autoclaved calcium silicate. Auto-claved systems are said to have the advantageover air-cured hydrated cement binders in thatthere is greater dimensional stability in relation tomoisture and temperature movements. Alsobecause of the absence of free alkalinity theboards can be more easily decorated. In the UK,at sheet for internal and external applicationshas been available for a number of years, pro-duced from cellulose bres in an autoclaved

calcium silicate matrix. A wide range of proper-ties is available in cellulose bre boards, typicalvalues being an elastic modulus of 12GPa withthe tensile strength varying between 6MPa and20MPa and the modulus of rupture between15MPa and 30MPa depending on whether thecomposite is wet or dry and on the bre volume.Typical tensile stressstrain curves in the dry andwet states for these materials are shown in Figure42.8.



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Wood cellulose bres in concrete

Wood cellulose bres are not suitable for use inbulk concrete applications because of difcultiesin mixing and compaction and their use is there-fore limited to automated factory processes. Insome of these processes wood chips or akes atup to 20 per cent are mixed with cement and nesand to make a variety of compressed wood chipboards or particle boards. These are generally

used internally and have low exural tensilestrength, often below 1MPa. They are not strictlybre-reinforced cements.

42.3.2 Vegetable bre productsThe use of vegetable stem bres in developingcountries is generally aimed at producing cheapbut labour-intensive, locally constructed cement-

FIGURE 42 .7 Effect of moisture condition on the tensile properties of a cement composite containing cellulosebres and articial bres pressed, unpressed.

FIGURE 42 .8 Effect of moisture condition on tensile properties of autoclaved calcium silicate containing cellu-lose bres.



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based roof sheeting often of corrugated or foldedplate design. Long bres which are indigenous tothe locality are used, such as akwara, banana,bamboo, coir, elephant grass, ax, henequen,ute, malva, musamba, palm, plantan, pineapple

leaf, sisal, sugar cane and water reed. Lengths of bres may be up to 1m or more and are handplaced in a matrix of sand and cement. Corru-gated sheets of up to 2m by 1m in size of 610mm thickness and tiles may be producedwith bres in preferential directions.

Manufacture and applications

Composites made from vegetable stem bres aremanufactured by simple hand lay-up processes

which are potentially suitable for low-costhousing applications.In these applications the bre content is usually

less than 5 per cent when applying mixing tech-nologies, but it may be greater when using thetechnologies of hand lay-up of long bre rovings.Hand laying involves the application of a thinmortar layer on a mould, followed by alternatelayers of bres and mortar matrix. The bres canbe rolled into the matrix or worked into it manu-ally, and the process may involve some vibration.In the mixing technique, there is a limit to thecontent and length of bres that can be incorpo-rated, since as with any other bres workability isreduced. However, many of the natural brecomposites are intended for the production of thin components such as corrugated sheets andshingles, and for these applications there is arequirement for both plasticity and fresh strengththat will permit the shaping of the product. Forsuch purposes, reduced ow properties are not as

detrimental as in the case of conventional con-crete. In these components, which have a typicalthickness of about 10mm, the matrix is a cementmortar, and the mix with bres, or with hand-laid bres, is spread on a mould surface and thenshaped. Corrugation can be achieved by pressingbetween two corrugated sheets.

Properties

The cracking stress and strength of the compos-ites are not greatly increased compared with theunreinforced matrix (i.e. about 1MPa to 3MPa)but the bres enable the sheets to be formed in

the fresh state and handled and transported in thehardened state. Considerable toughness isachieved in the short term.

Bamboo, when split into strips and woven intomeshes, has been used as reinforcement for avariety of applications from roads and structuresto water tanks. Tensile strengths of the bre arecommonly in excess of 100MPa with elasticmoduli between 10 and 25GPa. Toughening andpost-cracking performance are the most import-ant characteristics and optimum bre volumesbetween 1.5 per cent and 3 per cent have beenquoted.

Durability

The high alkalinity of the pore water preventsmicrobiological decay in the bres but, to setagainst this, the calcium hydroxide penetrates thebre to mineralise or petrify it. The high alkalin-ity can cause severe reduction in bre strength

but where carbonation has penetrated, this rateof reduction of strength is reduced. However,natural stem bres are not expected to give thecomposite a long lifetime, although short cellu-lose bres as used in alternatives to asbestoscement products have been shown to be moredurable than natural stem bres.

42.4 Polymer bre-reinforcedcementThe inclusion of polymer bres into cement-basedproducts is potentially a very large world-widemarket. For instance, about 90 countries haveproduced asbestos cement for cladding, roongor pipes and about 3.5 million tonnes of asbestosbre have been used annually in the asbestoscement and building products industries, givingabout 28 million tonnes of products. This marketwill either be lost to other products or man-made



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bres will be substituted for the asbestos bres. Itis clear therefore that there is a potential marketand considerable inroads have been made sincethe early 1980s by polymer bres into this indus-try.

Although a wide variety of polymers has beenused on a trial basis in cement-based materials,only a few have been commercially successful.Polypropylene and polyvinyl alcohol have beenthe most used although polyethylene pulp is alsoused in some thin sheet products.

42.4.1 Chopped polypropylene lmsChopped polypropylene lms have been used atbre volumes of 3 per cent to 5 per cent toproduce alternative products to asbestos cementwith some modications being required to thetraditional machinery.

The polypropylene in this case was speciallystretched and heat treated to give elastic moduliof 9 to 18GPa with tensile strengths from 500 to700MPa and ultimate strain of 5 to 8 per cent.Various surface treatments to improve wetting of the lms and increase their bond were carried outbefore splitting the tape and chopping into

lengths between 6 and 24mm to give bres with abasically rectangular cross-section but withfrayed edges.

42.4.2 Continuous opened polypropylenenetworksLayers of networks of continuous polypropylenelms, as shown in Figure 42.9, in combinationwith continuous glass bre rovings have been

used commercially in ne grained cement-basedmaterials to produce alternative products toasbestos cement. The advantage of this system isthat the full bre strength of both bres is usedbecause there is no pull-out and excellentmechanical bonding in the polypropylene isachieved by virtue of the uneven micro- andmacro-slits in the lms and the many ne hairsproduced in the production process. Also it hasbeen found that there is a synergistic interactionbetween the bre types so the performance of

FIGURE 42.9 Polypropylene networks.

each bre is enhanced by the presence of theother bre.

Manufacture

The continuous networks are laid up in twodirections in twelve-layer packs. Three or four

packs are fed simultaneously to a machine whichimpregnates them with cement slurry in sequence,producing a wet, at sheet with good two-dimensional strength. This is then corrugated in avacuum corrugator. Both chopped and continu-ous glass bres can be included in the process.

Properties

All the requirements for roong sheet can be met

and additionally, toughness values of 1000kJ/m3

are possible. The failure strain remains in excessof 5 per cent after weathering, provided that thecritical bre volume is exceeded at the appropri-ate age.

Durability

Polypropylene bres show excellent resistance toalkalis and in a cement matrix have been shownto have good resistance to a variety of weathering



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conditions up to 18 years of real time exposure(Figure 42.10). Equally important is that thebond strength has been shown to be time stableup to 18 years (Hannant, 1999), giving con-dence in the long-term toughness of polypropy-lene bre composites.

42.4.3 Polyvinyl alcohol bres (PVA)High-strength and stiffness PVA bres are usedwidely as an asbestos replacement in asbestoscement products. However, taken by themselvesin a cement slurry, they offer little retention of the cement grains and hence must be used in con-unction with cellulose pulp to keep the cement in

the system as water is sucked out by vacuum. Thebres are treated on the surface to enhance theircompatibility with the matrix, the quantity of bres being typically 3 per cent by volume. Flex-ural strengths of the sheeting are adequate tomeet the requirements of the appropriate Euro-pean standards. Alkali resistance has been statedto be excellent and the bres can survive expo-sure to temperatures of 150C without loss instrength.

42.4.4 Polyethylene pulpPolyethylene pulp made from short bres hasmainly been used as a cement retention anddrainage aid as a substitute for asbestos bres inHatschek type process for the manufacture of thin sheet products. Up to 12 per cent by volumehas been used and at this level improvements inexural strength and ductility have also beenobserved. Because the bres do not swell in thepresence of water, the durability of the productsis said to be improved in comparison with similarsystems using cellulose bres.

42.4.5 Continuous networks of high-modulus polyethylene bresHighly orientated polyethylene bres may be pro-duced by gel spinning or high draw ratios andbres have been produced with the elasticmodulus of glass and the strength of steel. Com-mercial brillated tapes with initial elastic moduliof about 30GPa have been used in thin cementsheets in a similar fashion to polypropylene nets.

Durability in alkalis is expected to be good butthe lms which have been available have suffered

0

Time (years)

F i b r e s

t r e n g

t h ( M P a

)

300

5

250

200

150

100

50

010 15 20

Natural weathering

Laboratory air

Under water

95% confidence limits

FIG URE 42.10 Durability of polypropylene networks in cement.



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from high creep strain in comparison withpolypropylene.

42.5 ReferencesAli, M.A., Majumdar, A.J. and Singh, B. (1975) Pro-

perties of glass bre cement the effect of brelength and content. J. Materials Science , 10,173240.

Bentur, A. and Mindess, S. (1990) Fibre Reinforced Cementitious Composites , Elsevier Applied Science,London and New York.

Hannant, D.J. (1999) The effects of age up to 18 yearsunder various exposure conditions on the tensileproperties of polypropylene bre reinforced cementcomposites. Materials and Structures, RILEM , 32,March, 838.

Oakley, D.R. and Proctor, B.A. (1975) Tensilestressstrain behaviour of glass bre reinforcedcement composites. In Fibre Reinforced Cement and Concrete , Construction Press Ltd, Lancaster, UK,pp. 34759.

Swamy, R.N. (ed.) (1988) Natural Fibre Reinforced Cement and Concrete , Vol. 5, Concrete Technologyand Design, Blackie, Glasgow.

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