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Microstructural features of interfaces in fibre cement composites
Microstructural features of interfaces in fibre cement composites C.L. PAGE This paper briefly reviews present knowledge of microstructural characteristics of interfaces in steel fibre-reinforced concrete and glass fibrereinforced cement. The influence of these features on the mechanism of debonding of steel fibres from a hardened cement matrix is discussed and this provides a basis for consideration of certain approaches towards the improvement of strength of steel fibre-reinforced concrete. Current ideas on the processes responsible for degradation of the strength and toughness of GRC during prolonged exposure to wet environments are examined with particular reference to the role of interface m icrostructure. Key words: composite materials; reinforced cement; steel fibres; glass fibres; fibre/matrix interface; microstructure; debonding; environmental effects Properties of fibre-reinforced cement composite materials are dependent on various characteristics of the fibres, the cement matrix and the fibre/matrix interfaces. In general, the nature and behaviour of the fibres and the matrix are reasonably well understood but those of the interfaces are known in considerably less detail. This has led to a substantial volume of research over the past few years, which has been aimed at characterization of the microstructure of interfaces in such materials. This paper presents a brief review of recent work in this field, with particular reference to studies concerned with steel fibre-reinforced concrete and glass fibre-reinforced cement systems. Ways in which microstructural information has promoted improved understanding of the role of the interface in relation to various features of mechanical behaviour and durability will be discussed and an attempt will be made to outline areas that appear to warrant further investigation. INTERFACES IN STEEL FIBRE-REINFORCED CONCRETE
The composition and morphology of interfaces formed between mild steel or stainless steel and Portland cement pastes, mortars or concretes have been examined in several studies involving the use of electron microscopy and X-ray analytical techniques. 1-s This work has led to general agreement that the layer of cement hydration products deposited in contact with steel, and extending to a depth of at least several tens of micrometers into the surrounding matrix, consitutes a zone of segregation. Within this layer, the composition, pore structure and development of morphology as curing progresses are distinct from those features of the bulk cement matrix. For reference to general aspects of microstructure in hardened cement pastes, comprehensive reviews are available 6, 7 and, in the present contribution, attention will be directed solely to characteristics of interracial layers. In the steel/ Portland cement matrix system the main features are as follows:
In intimate contact with the embedded surfaces, there is formed a dense layer consisting largely of hexagonal lamellar crystals of calcium hydroxide (portlandite). These crystals exhibit dendritic growth morphology of a sort which encourages a high incidence of plate-like particles to be deposited with c-axis roughly perpendicular to the steel surfaces, and they replicate the topography of the metal over an extensive part of the total contact area s - see Figs 1 and 2. The thickness of the interfacial portlandite varies from place to place over the steel surface and, at discontinuities, inclusions of other hydration products such as CSH gel are observeds - see Fig. 3. With increasing distance from the interface, the proportion of portlandite and its degree of preferred orientation diminish gradually, as the dense interfacial layer gives way to a transitional zone of relatively high porosity in comparison with the bulk of the matrix. By analogy with the findings of X-ray diffraction studies of interfaces formed between Portland cement pastes and other surfaces (such as quartz, limestone and polyethylene) it seems that oriented crystal growth may extend some 20 - 50 #m from the interface; the limiting distance apparently depends mainly on spatial restrictions related to the water/cement ratio of the matrix. 8, 9 Microhardness traverses have further indicated that the transitional zone of high porosity may extend beyond this level to depths of more than 100 pan. 4' 1o Mechanical properties of the interfaces in steel fibre-reinforced cement composites are dearly influenced by the microstructural features that have been described. Thus, measurements of tensile bond strength developed between steel and Portland cement mortars, have shown that the interfacial zone is significantly weaker than the bulk matrix and that its strength is affected by factors, such as water/ cement ratio and compaction, which determine the initial distribution of cement particles in the layer of water adjacent to the steel. It appears also that the'interfacial zone attains a relatively constant structure after only a few days of moist curing, since very little subsequent increase in tensile bond strength is found to take place. 11 This can be
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COMPOSITES. AP R IL 1982
age of oriented portlandite crystals. Theoretical values in the region of 1 GPa are estimated and the large discrepancy between these and measured bond strengths (which are less than 10 MPa) 11 is attributed to the presence of flaws within the interfacial zone. In view of the mixed adhesional and cohesional character of bond failure in steel fibre-reinforced cement composites, it seems clear that substantial improvements in bond strength depend on enhancing both the fibre/matrix adhesion and the cohesive strength of the surrounding matrix. Improvements in adhesion alone, which may be effected by the application of certain surface coatings to the fibres, give rise to bond failure of purely cohesive character and this change is reflected in modest but significant increases in bond and composite flexural strength. 14
Fig. 1 Interface between steel and hardened cement matrix, examined after fracture, shows characteristic dendritic morphology and preferred orientation of portlandite adhering to steel
Much greater strength gains may, however, be achieved by techniques which serve to increase both adhesion and cohesion. One effective, but fairly costly, approach is by polymer impregnation and it has been found, for example, that failure of steel fibre-reinforced concretes impregnated with methyl methacrylate involved extensive fibre yielding without slippage at the interfaces, is Another possible route might entail the use of fibre-reinforced concrete mixes incorporating ultra-t'me silica (ca 25 000 m 2 kg -1 specific surface) and superplasticizers, 16 with the objectives of reducing matrix porosity and transforming the readilycleaved portlandite crystals in the interfacial zones to CSH. In this context, it is of interest to note that even relatively coarse pozzolanic additives such as pulverized fuel ash, PFA, (ca 350 m 2 kg -1 specific surface) have been found to produce significant improvements in the long term values of tensile bond strength recorded for steel/mortar interfaces ~7 - see Fig. 4. Extensive production of ultra-fine silica as a by-product of ferro-aUoy manufacture has created considerable interest in the application of this material in the field of concrete technology. Continuing research into the potential benefits of its use in relation to steel fibre-reinforced cement composites would thus seem to be worthwhile.
Fig. 2 Surface of hardened cement matrix, exposed at region of adhesion failure, shows oriented portlandite layer bearing imprint of superficial scratch marks on steel with which it had been in contact
explained by the fact that precipitation of portlandite is expected to occur rapidly whilst the pore solution remains heavily supersaturated with respect to Ca 2÷ during the early stages of hydration of the C3 S component of the cement. 12 Bond failures of steel]mortar interfaces have been shown, by means of quantitative microscopy, to become increasingly adhesional in character as curing time increases, but some degree of cohesive failure of the matrix is normally observed even after prolonged storage, s This mixed failure mode has been explained by Tabor, is who concludes from a discussion of the relevant types of bonding that similar theoretical strengths may be associated with the rupture of an oxidized (passive) steel/portlandite interface and with the basal clear-
COMPOSITES. A P R I L 1982
Fig. 3 Surface of hardened cement matrix, exposed at region of adhesion failure, shows CSH gel deposited in intimate contact with steel at a discontinuity in the interracial portlandite
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Despite the considerable improvement in durability made possible by the development of alkali-resistant zirconosilicate glass fibres for use in GRC,al the gradual decline in various mechanical properties of such composites, made from first generation commercial fibres, under conditions of water storage or natural weathering is still a serious limitation. The origins of these problems have been extensively researched. 22, a2 It now appears to be established that the decline in composite flexural strength is directly linked to a reduction in strength of the embedded fibres, as recorded by 'strand-incement' tests, since the rates of decrease of both parameters show a common temperature dependence. ~
p 3 m
t~
•
15°/0 PFA
0
30%
v
I 7
PFA
I 14 Time (doys)
I 21
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Fig. 4 Development of tensile bond strength of steel/mortar interfaces at 22°C and 100% RH. Mortars contained proportions by weight of water: (cement + PFA): sand of 0.4 : 1.0 : 2.0 with variable PFA contents, expressed as percentages of the weight of (cement + PFA)
Analysis of the Arrhenius plots presented in Reference [33] indicates that, over the temperature range 20 - 80°C, the kinetics of the process responsible for the loss of strength are governed by a single dominant step with an activation energy of ca 90 kJ mole -1 . The somewhat confusing fact that strengths of individual fibres extracted from weathered GRC specimens do not show such a clear correlation with composite strength may be a reflection of the difficulty of removing short lengths of fibres that are representative of the weakest regions within the strands, a2
INTERFACES IN GLASS FIBRE-REINFORCED CEMENT
There have been a number of reports of research into the microstructure of interfaces in GRC~a-2s and, in the related field of model aggregate/cement interactions, extensive studies of the morphology of interfaces formed between glass slides and Portland cement pastes have been reported by the group at Perdue University. z6'27 The latter work has revealed the existence in intimate contact with the glass of a 'duplex layer' about 1 #In thick, consisting of portlandite, oriented with c-axis perpendicular to the glass, backed with mainly rod-shaped particles ofcsH. A typical example is illustrated in Fig. 5. 2s As hydration proceeds, this duplex film eventually becomes tied to the bulk cement paste by deposition of additional calcium hydroxide, ettringite pr other hydration products. 2 9 This is illustrated in Fig. 6"2 8 which shows the surface of a cement paste that had been separated from a glass slide to reveal a region where the duplex layer had become partly detached. The above observations demonstrate the high affinity of glass surfaces for portlandite and this has recently been confirmed by investigations of the relative tendencies of glass fibres and other materials to act as substrates for calcium hydroxide precipitation. 3° It has been suggested that, in GRC composites, the duplex layer may act as a barrier, limiting the corrosive interaction between the fibre surfaces and the alkaline pore solution present in the cement matrix. ~-° However, the continuity of such layers as may form at the exterior surfaces of the multi-t~ment strands in GRC is certainly not sufficient to prevent the solution from penetrating to the cores during long-term exposure in wet environments. This is clear from microstructural examinations of the inter-filament spaces, which indicate that prolonged storage in water leads to extensive precipitation of portlandite crystals in these regions, so that the interstices eventually become densely packed 19, 21,2a as shown in Fig. 7.
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Fig. 5 Interface between glass slide and hardened cement paste, examined after fracture, shows 'duplex layer' adhering to glass
Fig. 6 Surface of hardened cement paste that had been separated f r o m glass slide shows ' d u p l e x layer' and variety o f h y d r a t i o n products in underlying zone of high porosity
COMPOSITES. APRI L 1982
of the combined effects of increase in the fibre/matrix l~ond strength and decrease in the strength of the fibres themselves, the mode of failure of the composite gradually changes under these conditions to one involving little fibre pull-out. The toughness and impact resistance of the material therefore decline. A number of methods have been proposed for reducing the susceptibility of GRC to age-embrittlement when the material is exposed to water or natural weathering for long periods. These techniques appear to function generally by modifying the nature of the fibre/matrix interfaces
Fig. 7 G RC specimen, stored for 5 years in water before fracture, shows interstices within fibre bundle densely packed with portlandite. (Reference [21] )
The nature of the rate determining process involved in fibre strand-strength reduction remains unclear at present and it is therefore impossible to make any definite assessment of the influence of microstructural features of the interfacial zone. It has been suggested, however, that calcium hydroxide crystals may play some specific role s2 since lime slurries have been shown to be markedly more aggressive toward zircono-silicate glass fibres than are their supernatant solutions. ~ This view receives support from recently reported scanning electron microscope observations, which appear to indicate interactions between portlandite crystals and faults in the surfaces of alkali-resistant glass fibres. 23 Further elucidation of the role of the interface might be obtained from investigations of the effects of cement composition on the temperature dependence of strength loss, recorded by means of 'strand-in-cement' tests. In particular, it would be of interest to study the effect of using the pure cement mineral, Cs S, since this substance produces a high proportion of calcium hydroxide during hydration but its associated pore solution does not contain NaOH or KOH. The latter alkalis are always present in commercial Portland cement pastes and account for the fact that the pH values recorded for these materials are typically in excess of pH 13.5,1= whereas pure saturated Ca(OH)2 solutioia is ca pH 12.5. Changes in the impact resistance and fracture toughness of GRC composites stored in various environments have been shown to be closely associated with the microstructural character of the matrix zone in the vicinity of the fibre bundles. =1 Under conditions of dry storage, the interfaces retain a porous structure for indefinite periods and the interstices within the fibre strands remain largely devoid of hydration products. Failure of the composite is thus characterized by extensive fibre pull-out with subsidiary cracking of the weak surrounding matrix and these processes account for the maintenance of a high work of fracture. When the material is stored in a wet environment, however, the growth of the portlandite crystals gradually fills the voids around the fibres and a dense interfacial zone of the sort illustrated in Fig. 7 is developed. Because
COMPOSITES. AP R IL 1982
One such method involves the substitution of supersulphated cement for Portland cement and this has been found to show considerable promise under storage conditions that do not induce carbonation of ettringite, as this causes general weakening of the matrix. By means of scanning electron microscopy, it has been shown that extensive fibre pull-out is observed in GRC specimens fabricated from this type of cement and cured in a wet environment for several years. This has been attributed to the absence of portlandite from the hydrated product and to the development of interfacial zones which are less densely packed than those characteristic of Portland cement. =2' ~ Likewise, the use of certain pozzolanic materials as partial replacements for Portland cement in GRC has been found to produce similar beneficial effects owing to the reaction between the pozzolana and calcium hydroxide to form CHS.==,=s Other methods, which are being used commercially to improve durability of GRC, are based on polymer addition to the cement matrix 3s and on the application of various surface coatings or 'size' additives to the fibre yarn. s= The purpose of these treatments appears to be partly, at least, to form an impervious barrier around the fibre bundles so that gradual densification of the interstices is prevented. Very little information is yet available in the open literature concerning details of the mechanism of action of these surface modifications. It has, however, been pointed out that reaction with the Ca=* ions formed during the early stages of hydration is likely to be involved in the cases of several compounds that have recently been proposed as constituents of fibre coatings. 3= This is an area of active current research where important developments may be expected.
CONCLUSIONS
Research carried out during the last decade has yielded considerable information regarding the microstructural features of interfaces in steel fibre-reinforced concrete and glass fibre-reinforced cement. This has increased understanding of several aspects of the behaviour of these materials and a basis has thus been provided for attempts to modify the fibre/matrix interfaces in specific ways aimed at enhancing mechanical properties and durability. Several important questions remain unresolved, however, and it is hoped that this paper may have highlighted some of the areas where future research would be worthwhile.
ACKNOWLEDGEMENT
Sincere thanks are due to Dr A.J. Majumdar for drawing the author's attention to several areas of the literature related to this paper.
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REFERENCES 1 2
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Nicol, A. 'Apropos du comportement Fer-Ciment' Revue des Matdriaux de Construction et de Travaux Publics 537 (1960) pp 149-156; 538/9 (1960) pp 190-196 Moreau,M. 'Contribution~tl'~tude d'adhdrenee entre les constituants hydrates du eiment portland artfficiel et l'armature ertrob~e' Revue des Matdriaux de Construction et de Travaux Publics 678 (1973) pp 4-17 Sehnittstund, G.D. and Scott, J.IL 'Bonding between cement hydrates and steel' Construction Enging Res Lab Technical Manuscript M-176 (January 1976) 55 pp Pinchin, D.L and Tabor, D. 'Interfacial phenomena in steel fibre reinforced cement I: structure and strength of interfacial region', Cementand Concrete Research 8 No 1 (1978) pp 15-24 A! Kb~l_~f~M.N. and Page, C.L 'Steel/mortar interfaces: microstructural features and mode of failure' Cement and Concrete Research 9 No 2 (1979) pp 197-208 Williamson, R.B. 'Solidification of Portland cement' Progress in Mater Sci 15 No 3 (1972) pp 189-285 Diamond, S. 'Cement paste micxostructure - an overview at several levels' Proc Conf Hydraulic Cement Pastes: Their Structure and Properties, University of Sheffield, England, April 8-9, 1976 (Cement and Conczete Association, London) pp 2-30 Maso,J. 'The bond between aggregates and hydrated cement paste' Proc 7th Int Cong on Chemistry of Cement, Palls, 1980, Vol 1, Theme VII-1 (Editions Septima, Paris, 1980) pp 3-15 Grandet, J. and Ollivier, LP. 'Orientation of hydration products near aggregate surfaces' ibid. Vol III, Theme VII pp 63-68 Lyubimova, T.Yu. and Ptnus, E.R. 'Crystallisation structure in the contact zone between aggregate and cement in concrete' ColloidJ USSR 24 No 5 (1962) pp 491-498 Page,C.L., AI Khalaf, M.N. and Ritchie, A.G.IK 'Steel/mortar interfaces: mechanical characteristics and electrocapillarity' Cement and Concrete Research 8 No 4 (1978) pp 481-490 Longuet, P., Burglen, L. and Zelwer, A. 'La phase liquide du ciment hydratd' Revue des Mat~riaux de Construction et de TravauxPublics 676 (1973) pp 35-41 Tabor, D. 'Principles of adhesion-bonding in cement and concrete' Syrup.Adhesion Problems in the Recycling o f Concrete, Saint-Remy-les-Chevruse, near Paris, Nov 25-28, 1980, (NATO Advanced Research Institute) 24 papers to be published by Plenum Press, London AI Khalaf, M.N., Page, C.L. and Ritchie, A.G.B. 'Effects of fibre surface composition on mechanical properties of steel fibre reinforced mortars' Cement and Concrete Research 10 No 1 (1980) pp 71-77 Jamtozy, Z. and Sliwinski, L 'Properties of steel fibre reinforced concrete impregnated with methyl methac~ylate' lntJCement Composites 1 No 3 (1979) pp 117-124 Baehe,ILI-I. 'Cement-bound materials with extremely high strength and durability' Beton-Teknik No 8/03 (Aalborg Portland, PO Box 165 - DK 9100 Aalborg, Denmark, 1980) AI Khalaf, M.N. 'Bonding between metals and Portland cement' PhD Thesis (University of Strathclyde, June, 1976) Majumdar, A.J. 'The role of the interface in glass fibre reinforced cement' Cement and Concrete Research 4 No 2 (1974) pp 247-268 Jaras, A.C. and Litherland, ILL. 'Mic~ostructural features in
20 21 22 23 24 25 26 27
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glass fibre reinforced cement composites' Proc RILEM Symp, 1975, Fibre Reinforced Cement and Concrete (Edited by A. Neville, The Construction Press, London 1975) pp 327-334 Cohen, E.B. and Diamond, S. 'Validity of flexural strength reduction as an indication of alkali attack on glass in fibre reinforced cement composites' ibid, pp 315-325 Stucke, M.S. and Majumdar, A.J. 'Miczostructure of glass fibrereinforced cement composites' YMater Sci 11 (1976) pp 1019-1030 Majumdar, A.L and Laws, V. 'Fibre cement composites: research at BRE' Composites 10 No 1 (1979) pp 17-27 Mills,R.IL 'Age-embrittlement of glass-reinforced conczcte containing blasfurnace slag' Cement and Concrete Research 11 No 3 (1981) pp 421-428 Majumdar, A.J., Singh, B. and Evans, T.J. 'Glass fibrereinforced supersulphated cement' Composites 12 No 3 (1981) pp 177-183 Singh, Ilk and Majumdat, A.L 'Properties of grc containing inorganic fillers' Int J Cement Composites and Light-weight Concrete 3 No 2 (1981) pp 93-102 Hadley, D.W. 'The nature of the paste-aggregate interface' PhD Thesis (Purdue University, August, 1972) Barnes,B.D., Diamond, S. and Dolch, W.I- The contact zone between Portland cement paste and glass aggregate surfaces' Cement and Concrete Research 8 No 2 (1978) pp 233-244 Page,C.L. and Thompson, CJ. unpublished results Diamond, S., Ravina, D. and LoveU,L 'The occurrence of duplex films on fiyash surfaces' Cement and Concrete Research 10 No 2 (1980) pp 297-300 Mills,R.IL 'Preferential precipitation of calcium hydroxide on alkali-resistant glass fibres' Proc Mater Res Soc Syrup Advances in Cement-Matrix Composites (Boston, Mass, USA, November 17-18, 1980) pp 229-237 Majumdar, A.L and Ryder, LF. 'Glass fibre reinforcement of cement products' Glass Tech 9 No 3 (1968) pp 78-84 Majumdar, AJ. 'Some aspects of glass fibre reinforced cement research', Proc Mater Res Soc Syrup Advances in Cement Matrix Composites (Boston, Mass, USA, November 17-18, 1980) pp 37-57 Litherland, ILL, Oakley, D.R. and Proctor, B.A. 'The use of accelerated ageing procedures to predict the long term strength of grc composites ibid, pp 61-74 Proctor, B.A. and Yale, B. 'Glass fibres for cement reinforcement' Phil Trans Royal Soc London, A 294 (1980) pp 427-436 Bijen, J. 'Glassfibre reinforced cement: improvements by polymer addition' Proc Mater Res Soc Symp Advances in Cement Matrix Composites (Boston, Mass, USA, November 17-18, 1980) pp 239-249
AUTHOR The author is with Aston University's Department o f Construction and Environmental Health. Inquiries should be addressed to: Dr C.L. Page, Department of Construction and Environmental Health, University of Aston, Gosta Green, Birmingham B4 7ET, England.
COMPOSITES. APRIL 1982
The composition and morphology of interfaces formed between mild steel or stainless steel and Portland cement pastes, mortars or concretes have been examined in several studies involving the use of electron microscopy and X-ray analytical techniques. 1-s This work has led to general agreement that the layer of cement hydration products deposited in contact with steel, and extending to a depth of at least several tens of micrometers into the surrounding matrix, consitutes a zone of segregation. Within this layer, the composition, pore structure and development of morphology as curing progresses are distinct from those features of the bulk cement matrix. For reference to general aspects of microstructure in hardened cement pastes, comprehensive reviews are available 6, 7 and, in the present contribution, attention will be directed solely to characteristics of interracial layers. In the steel/ Portland cement matrix system the main features are as follows:
In intimate contact with the embedded surfaces, there is formed a dense layer consisting largely of hexagonal lamellar crystals of calcium hydroxide (portlandite). These crystals exhibit dendritic growth morphology of a sort which encourages a high incidence of plate-like particles to be deposited with c-axis roughly perpendicular to the steel surfaces, and they replicate the topography of the metal over an extensive part of the total contact area s - see Figs 1 and 2. The thickness of the interfacial portlandite varies from place to place over the steel surface and, at discontinuities, inclusions of other hydration products such as CSH gel are observeds - see Fig. 3. With increasing distance from the interface, the proportion of portlandite and its degree of preferred orientation diminish gradually, as the dense interfacial layer gives way to a transitional zone of relatively high porosity in comparison with the bulk of the matrix. By analogy with the findings of X-ray diffraction studies of interfaces formed between Portland cement pastes and other surfaces (such as quartz, limestone and polyethylene) it seems that oriented crystal growth may extend some 20 - 50 #m from the interface; the limiting distance apparently depends mainly on spatial restrictions related to the water/cement ratio of the matrix. 8, 9 Microhardness traverses have further indicated that the transitional zone of high porosity may extend beyond this level to depths of more than 100 pan. 4' 1o Mechanical properties of the interfaces in steel fibre-reinforced cement composites are dearly influenced by the microstructural features that have been described. Thus, measurements of tensile bond strength developed between steel and Portland cement mortars, have shown that the interfacial zone is significantly weaker than the bulk matrix and that its strength is affected by factors, such as water/ cement ratio and compaction, which determine the initial distribution of cement particles in the layer of water adjacent to the steel. It appears also that the'interfacial zone attains a relatively constant structure after only a few days of moist curing, since very little subsequent increase in tensile bond strength is found to take place. 11 This can be
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COMPOSITES. AP R IL 1982
age of oriented portlandite crystals. Theoretical values in the region of 1 GPa are estimated and the large discrepancy between these and measured bond strengths (which are less than 10 MPa) 11 is attributed to the presence of flaws within the interfacial zone. In view of the mixed adhesional and cohesional character of bond failure in steel fibre-reinforced cement composites, it seems clear that substantial improvements in bond strength depend on enhancing both the fibre/matrix adhesion and the cohesive strength of the surrounding matrix. Improvements in adhesion alone, which may be effected by the application of certain surface coatings to the fibres, give rise to bond failure of purely cohesive character and this change is reflected in modest but significant increases in bond and composite flexural strength. 14
Fig. 1 Interface between steel and hardened cement matrix, examined after fracture, shows characteristic dendritic morphology and preferred orientation of portlandite adhering to steel
Much greater strength gains may, however, be achieved by techniques which serve to increase both adhesion and cohesion. One effective, but fairly costly, approach is by polymer impregnation and it has been found, for example, that failure of steel fibre-reinforced concretes impregnated with methyl methacrylate involved extensive fibre yielding without slippage at the interfaces, is Another possible route might entail the use of fibre-reinforced concrete mixes incorporating ultra-t'me silica (ca 25 000 m 2 kg -1 specific surface) and superplasticizers, 16 with the objectives of reducing matrix porosity and transforming the readilycleaved portlandite crystals in the interfacial zones to CSH. In this context, it is of interest to note that even relatively coarse pozzolanic additives such as pulverized fuel ash, PFA, (ca 350 m 2 kg -1 specific surface) have been found to produce significant improvements in the long term values of tensile bond strength recorded for steel/mortar interfaces ~7 - see Fig. 4. Extensive production of ultra-fine silica as a by-product of ferro-aUoy manufacture has created considerable interest in the application of this material in the field of concrete technology. Continuing research into the potential benefits of its use in relation to steel fibre-reinforced cement composites would thus seem to be worthwhile.
Fig. 2 Surface of hardened cement matrix, exposed at region of adhesion failure, shows oriented portlandite layer bearing imprint of superficial scratch marks on steel with which it had been in contact
explained by the fact that precipitation of portlandite is expected to occur rapidly whilst the pore solution remains heavily supersaturated with respect to Ca 2÷ during the early stages of hydration of the C3 S component of the cement. 12 Bond failures of steel]mortar interfaces have been shown, by means of quantitative microscopy, to become increasingly adhesional in character as curing time increases, but some degree of cohesive failure of the matrix is normally observed even after prolonged storage, s This mixed failure mode has been explained by Tabor, is who concludes from a discussion of the relevant types of bonding that similar theoretical strengths may be associated with the rupture of an oxidized (passive) steel/portlandite interface and with the basal clear-
COMPOSITES. A P R I L 1982
Fig. 3 Surface of hardened cement matrix, exposed at region of adhesion failure, shows CSH gel deposited in intimate contact with steel at a discontinuity in the interracial portlandite
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Despite the considerable improvement in durability made possible by the development of alkali-resistant zirconosilicate glass fibres for use in GRC,al the gradual decline in various mechanical properties of such composites, made from first generation commercial fibres, under conditions of water storage or natural weathering is still a serious limitation. The origins of these problems have been extensively researched. 22, a2 It now appears to be established that the decline in composite flexural strength is directly linked to a reduction in strength of the embedded fibres, as recorded by 'strand-incement' tests, since the rates of decrease of both parameters show a common temperature dependence. ~
p 3 m
t~
•
15°/0 PFA
0
30%
v
I 7
PFA
I 14 Time (doys)
I 21
I 28
Fig. 4 Development of tensile bond strength of steel/mortar interfaces at 22°C and 100% RH. Mortars contained proportions by weight of water: (cement + PFA): sand of 0.4 : 1.0 : 2.0 with variable PFA contents, expressed as percentages of the weight of (cement + PFA)
Analysis of the Arrhenius plots presented in Reference [33] indicates that, over the temperature range 20 - 80°C, the kinetics of the process responsible for the loss of strength are governed by a single dominant step with an activation energy of ca 90 kJ mole -1 . The somewhat confusing fact that strengths of individual fibres extracted from weathered GRC specimens do not show such a clear correlation with composite strength may be a reflection of the difficulty of removing short lengths of fibres that are representative of the weakest regions within the strands, a2
INTERFACES IN GLASS FIBRE-REINFORCED CEMENT
There have been a number of reports of research into the microstructure of interfaces in GRC~a-2s and, in the related field of model aggregate/cement interactions, extensive studies of the morphology of interfaces formed between glass slides and Portland cement pastes have been reported by the group at Perdue University. z6'27 The latter work has revealed the existence in intimate contact with the glass of a 'duplex layer' about 1 #In thick, consisting of portlandite, oriented with c-axis perpendicular to the glass, backed with mainly rod-shaped particles ofcsH. A typical example is illustrated in Fig. 5. 2s As hydration proceeds, this duplex film eventually becomes tied to the bulk cement paste by deposition of additional calcium hydroxide, ettringite pr other hydration products. 2 9 This is illustrated in Fig. 6"2 8 which shows the surface of a cement paste that had been separated from a glass slide to reveal a region where the duplex layer had become partly detached. The above observations demonstrate the high affinity of glass surfaces for portlandite and this has recently been confirmed by investigations of the relative tendencies of glass fibres and other materials to act as substrates for calcium hydroxide precipitation. 3° It has been suggested that, in GRC composites, the duplex layer may act as a barrier, limiting the corrosive interaction between the fibre surfaces and the alkaline pore solution present in the cement matrix. ~-° However, the continuity of such layers as may form at the exterior surfaces of the multi-t~ment strands in GRC is certainly not sufficient to prevent the solution from penetrating to the cores during long-term exposure in wet environments. This is clear from microstructural examinations of the inter-filament spaces, which indicate that prolonged storage in water leads to extensive precipitation of portlandite crystals in these regions, so that the interstices eventually become densely packed 19, 21,2a as shown in Fig. 7.
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Fig. 5 Interface between glass slide and hardened cement paste, examined after fracture, shows 'duplex layer' adhering to glass
Fig. 6 Surface of hardened cement paste that had been separated f r o m glass slide shows ' d u p l e x layer' and variety o f h y d r a t i o n products in underlying zone of high porosity
COMPOSITES. APRI L 1982
of the combined effects of increase in the fibre/matrix l~ond strength and decrease in the strength of the fibres themselves, the mode of failure of the composite gradually changes under these conditions to one involving little fibre pull-out. The toughness and impact resistance of the material therefore decline. A number of methods have been proposed for reducing the susceptibility of GRC to age-embrittlement when the material is exposed to water or natural weathering for long periods. These techniques appear to function generally by modifying the nature of the fibre/matrix interfaces
Fig. 7 G RC specimen, stored for 5 years in water before fracture, shows interstices within fibre bundle densely packed with portlandite. (Reference [21] )
The nature of the rate determining process involved in fibre strand-strength reduction remains unclear at present and it is therefore impossible to make any definite assessment of the influence of microstructural features of the interfacial zone. It has been suggested, however, that calcium hydroxide crystals may play some specific role s2 since lime slurries have been shown to be markedly more aggressive toward zircono-silicate glass fibres than are their supernatant solutions. ~ This view receives support from recently reported scanning electron microscope observations, which appear to indicate interactions between portlandite crystals and faults in the surfaces of alkali-resistant glass fibres. 23 Further elucidation of the role of the interface might be obtained from investigations of the effects of cement composition on the temperature dependence of strength loss, recorded by means of 'strand-in-cement' tests. In particular, it would be of interest to study the effect of using the pure cement mineral, Cs S, since this substance produces a high proportion of calcium hydroxide during hydration but its associated pore solution does not contain NaOH or KOH. The latter alkalis are always present in commercial Portland cement pastes and account for the fact that the pH values recorded for these materials are typically in excess of pH 13.5,1= whereas pure saturated Ca(OH)2 solutioia is ca pH 12.5. Changes in the impact resistance and fracture toughness of GRC composites stored in various environments have been shown to be closely associated with the microstructural character of the matrix zone in the vicinity of the fibre bundles. =1 Under conditions of dry storage, the interfaces retain a porous structure for indefinite periods and the interstices within the fibre strands remain largely devoid of hydration products. Failure of the composite is thus characterized by extensive fibre pull-out with subsidiary cracking of the weak surrounding matrix and these processes account for the maintenance of a high work of fracture. When the material is stored in a wet environment, however, the growth of the portlandite crystals gradually fills the voids around the fibres and a dense interfacial zone of the sort illustrated in Fig. 7 is developed. Because
COMPOSITES. AP R IL 1982
One such method involves the substitution of supersulphated cement for Portland cement and this has been found to show considerable promise under storage conditions that do not induce carbonation of ettringite, as this causes general weakening of the matrix. By means of scanning electron microscopy, it has been shown that extensive fibre pull-out is observed in GRC specimens fabricated from this type of cement and cured in a wet environment for several years. This has been attributed to the absence of portlandite from the hydrated product and to the development of interfacial zones which are less densely packed than those characteristic of Portland cement. =2' ~ Likewise, the use of certain pozzolanic materials as partial replacements for Portland cement in GRC has been found to produce similar beneficial effects owing to the reaction between the pozzolana and calcium hydroxide to form CHS.==,=s Other methods, which are being used commercially to improve durability of GRC, are based on polymer addition to the cement matrix 3s and on the application of various surface coatings or 'size' additives to the fibre yarn. s= The purpose of these treatments appears to be partly, at least, to form an impervious barrier around the fibre bundles so that gradual densification of the interstices is prevented. Very little information is yet available in the open literature concerning details of the mechanism of action of these surface modifications. It has, however, been pointed out that reaction with the Ca=* ions formed during the early stages of hydration is likely to be involved in the cases of several compounds that have recently been proposed as constituents of fibre coatings. 3= This is an area of active current research where important developments may be expected.
CONCLUSIONS
Research carried out during the last decade has yielded considerable information regarding the microstructural features of interfaces in steel fibre-reinforced concrete and glass fibre-reinforced cement. This has increased understanding of several aspects of the behaviour of these materials and a basis has thus been provided for attempts to modify the fibre/matrix interfaces in specific ways aimed at enhancing mechanical properties and durability. Several important questions remain unresolved, however, and it is hoped that this paper may have highlighted some of the areas where future research would be worthwhile.
ACKNOWLEDGEMENT
Sincere thanks are due to Dr A.J. Majumdar for drawing the author's attention to several areas of the literature related to this paper.
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AUTHOR The author is with Aston University's Department o f Construction and Environmental Health. Inquiries should be addressed to: Dr C.L. Page, Department of Construction and Environmental Health, University of Aston, Gosta Green, Birmingham B4 7ET, England.
COMPOSITES. APRIL 1982