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Principles and practice of grc. A review
Principles and practice of grc. A review B.A. PROCTOR
The theory of fibre reinforcement is reviewed and requirements for effective reinforcement identified. These requirements are then related to the stress/strain and ageing behaviour of typical sprayed grc to identify relevant and important factors. Finally an attempt has been made to explain certain design, production and control practices in the light of reinforcement theory and material properties.
Glass fibre-reinforced cement (grc)* is an important new cement-based material which is finding increasing use in the construction industry. It is also a fibre-reinforced composite material which isoftenreplacing traditional materials and moving into fields of use where experience in the concepts and understanding of fibre reinforcement is somewhat restricted. This, therefore, seemed an appropriate time and place to review, quite simply, the basic concepts and requirements of fibre reinforcement, to relate these to the load bearing behaviour of grc and to use both types of information to justify and explain some of the production, control and design practices required for the safe and efficient use of grc - which is also in the end the economic use.
FUNDAMENTALS OF FIBRE REINFORCEMENT
If we consider a very simple composite material containing (say) glass fibres running in one direction right through the length of the sample; and if we further assume that fibres and matrix are bonded together so that, when a load is applied, they stretch equally; then it is easy to show (eg Reference 1) that the stress in the composite, Oc, is divided between fibre and matrix according to oc
=
(EfVf+EmVm)e
c
which are important in making effective use of fibres as reinforcement. These are that the fibre modulus, Ef, is large compared with the matrix stiffness, Em, and that the fibre content, Vf, is as high as practicable. In the context of grc it is important to note that this is the fibre content by volume and not (directly) by weight as is usually measured. This point will be returned to later in the paper. In grc, as in many practical composites, the fibres are usually incorporated in discrete lengths and load has to be transferred into the fibre from the surrounding matrix material. Fig. 1 (Reference 1) shows in a diagrammatic way how a relatively stiff fibre modifies the deformation of the matrix, setting up shear stresses (r) in the surrounding matrix and at the fibre/matrix interface, so that the direct tensile stress carried by the fibre (of), rises rapidly as one moves away from the end. Simplified treatments due to Outwater 2 and Kelly 3 which assume a constant value of shear stress at the fibre/matrix interface
(1)
where
Ef, Em =
gf, Vm = ~C
=
Young's Modulus of fibre and matrix respectively volume fraction of fibre and matrix in the composite strain in the composite
Equation 1 is one form of the Law of Mixtures - a fundamental equation governing the behaviour of fibrereinforced composites. Although this equation must be modified to take account of various factors in practical composites (such as fibres of finite length lying in more than one direction) it does already indicate two factors
l--
crf
'
T:7;,e
o'f
/ rT/l 0
t
x~ 'l,z.~ L o-f for frictional or yeilding load transfer ( ie constant r )
* The data and behaviour descriptions given in this paper refer to grc made with Pilkington Cem-FI L alkali-resistant glass fibre. Pilkington Brothers Ltd, G R C Research and Testing Department, Ormskirk, Lancs
44
Fig. 1 Diagrammatic representation o f the load transfer mechanism, showing h o w a relatively stiff fibre modifies the d e f o r m a t i o n o f the matrix
COMPOSITES. JANUARY 1978
~ a
Strain
• Fibre content (by volume) must be adequate (Vf) • There must be a good fibre/matrix bond (z) • Fibre length must be sufficient re
• Fibres must be relatively long and thin (high. aspect ratio) Region 5
STRESS~STRAIN BEHA VlOUR OF GRC
b
Glass fibre-reinforced plastics are familiar materials in which the fibres are very much stiffer than the plastics matrix thus fulfilling the first requirement in the previous section. The strain-to-failure of the matrix is similar to that of the fibre and when the composite is put under load the stress inthe fibre rises much more rapidly than that in the lower modulus matrix (Fig. 2a). Since fairly high fibre contents are used most of the load is carried by the fibres and the reinforcement is effective.
Strain
Fig. 2 Idealized stress/strain curves for fibre-reinforced materials; (a) in grp and (b) in grc
The role of glass fibres in reinforcing cement is somewhat different from this. The stiffness of the fibres is now only about three times that of the matrix and, even more significantly the failure strain of the cement matrix is only a fraction of that of the fibre. Consequently the idealised tensile stress/strain curve is as shown in Fig. 2b (Reference 4), consisting essentially of three separate regions - although the extent and distinctness of these regions may vary considerably depending on the particular composite, its fibre content, matrix type, age and even manner of testing.
limited by either friction or yielding of the matrix give equations of the form 7rr~of =
(2)
27rrf~-x
where rf = radius of fibre af = tensile strength in fibre at distance x from fibre end T bond strength (in shear) between fibre and matrix Equation 2 draws attention to three more factors which are important if effective use is to be made of the reinforcing fibres. These are a good fibre/matrix bond (r), a sufficient length of fibre (x) and a high ratio of perimeter to crosssectional area (2rrrf)/(rrrf 2) - often called aspect ratio and achieved by using long, thin, fibres.
In Region 1 matrix and fibres act together more or less elastically, the stiffness of the composite and the sharing of load between fibres and matrix being governed by the Law of Mixtures as indicated in the previous section. Various factors (including cost) limit the quantity of fibre incorporated in cement composite to no more than 5 to 10%. Because of this relatively low fibre content and the small stiffness ratio (Ef/Em) the overall degree of reinforcement in Region 1 is small. That is, most of the load is still carried by the matrix and the stiffness of the composite is
Thus, fundamental considerations of composite behaviour lead to the following rules for effective fibre reinforcement. • Fibres must be significantly stiffer than the matrix material (Ef ~>Em)
16
~
/ rnachine Longitudinal spray-dewatesarnl:le~ ~fard
14
/ / J
12
/ . mActual a t ~ ~curves for good this range, qualitybut havsgrb°gen
E zlO
a) ,- 8 (/3
50100
( 1%strain ) I
__
I0 0 0 0
S~roin (/z~) Fig. 3
Observed tensile stress/strain curves for grc materials
COMPOSITES. J A N U A R Y 1978
45
essentially that of the matrix material. However, unreinforced cement pastes and mortars generally contain many voids together with cracks and microcracks due to differential shrinkage, aggregate boundaries, etc. These propogate under an applied stress, join together, and cause fracture at very variable stress levels so that such materials are not normally regarded as capable of bearing tensile or flexural loads. Each void or crack produces a localized region of much reduced stiffness and in grc this load can be transferred to nearby fibres which play an important role in restraining early crack growth; this prevents very low stress matrix failure, and hence raises the average matrix failure stress, s The fibres also distribute shrinkage stress more uniformly. Romualdi 6 and Aveston 4 have argued on theoretical grounds that the presence of reinforcing fibres actually raises the strength or strain to failure of a cement or concrete matrix. There has been some dispute about this in the literature (eg Reference 7) and the effect is difficult to show clearly with the relatively low fibre contents used in practice. In normal sprayed grc, failure of the matrix - ie the end of Region 1 - probably occurs at stress levels at or above those which would be attained in tests on small samples of well cured, good quality matrix material - and considerably above the average for larger pieces of the equivalent unreinforced matrix, if these could indeed be made and cured at all. At the end of Region 1 (Fig. 2b) the matrix cracks and all the load at the cracked section is thrown onto the bridging glass fibres - provided that there are enough of them to bear it! (see insert in Fig. 2b). Moving away from the crack face, load is transferred back from the fibres to the matrix by shear forces at the fibre/cement interface until the stress in the matrix has again risen to such a level that a new crack is formed. This process continues until the material is traversed by an array of very fine cracks which are quite often difficult to see. This forms Region 2 of the idealized stress/strain curve in Fig. 2b - a region of considerable extension and energy absorption providing toughness for the composite. As more and more fine cracks are formed there comes a stage at which it is no longer possible to transfer sufficient load back into the matrix, in the short distance between existing cracks, to raise the stress in the cement to breaking point. Region 2 is then complete and all further load is carded entirely by the bridging fibres, accompanied by opening of the existing cracks as the fibres stretch and perhaps begin to slip in the cement. 4'8 This gives Region 3 of Fig. lb leading to final failure which is governed by the strength of the fibres and/or the fibre/cement bond strength.
spray-dewatered board. Regions of multiple cracking (Region 2) and crack-opening (Region 3) follow, leading to failure at a strain of about 1%. Thus at this stage in its life, grc has an extremely high toughness when compared with concrete or asbestos cement, together with a degree of pseudo-ductility conferred by the fine multiple-cracking which gives it an ability to relieve stress concentrations to a considerable extent. The pseudo-ductile behaviour of Regions 2 and 3 in the tensile test, leads to a redistribution of stress across the section of a sample in a bending test, together with a movement of the neutral axis towards the compression face of the specimen. Thus the nominal bending strength or Modulus of Rupture (MOR) as calculated by simple elastic bending theory is about 2½ times the tensile strength. A feature common, to a lesser extent, with most cementitious materials which have (in detail) somewhat non-linear tensile stress/strain curves. The stress level at which noticeable departure from linearity occurs (Limit of Proportionality, LOP) is also considerably higher than the tensile Bend-Over Point. This relationship between tensile and bending behaviour is illustrated in Fig. 4a for a simple strip samples of grc sheet of rectangular section. In more complex material samples, such as sandwich structures, the stress distribution may more nearly approach the tensile condition even in bending, and bending strengths will be only slightly greater than tensile. AGEING BEHA VIOUR OF GRC
The tensile and bending characteristics illustrated in Figs 3 and 4a are those of good quality sprayed grc sheet after an adequate cure. The material is tough and impact resistant. 50 ,--
~
N E z 40
4(
curve
3£
~, 3 O
//"~dOR o
~ 2o
2C
Although this is a very idealized theoretical model for the stress/strain behaviour of grc, detailed studies of the tensile behaviour of typical sprayed material 8'9 show that it conforms quite closely to theory. Fig. 3 shows actual tensile curves for both spray-dewatered and direct-spray material and the three regions of behaviour can be deafly identified. In the first Region behaviour is matrix dominated with an elastic modulus equal to that expected from cement, there is then a sharp Bend-Over Point (BOP) into Region 2 at stress levels which are rather high for the tensile strength of cement paste or cement mortars indicating some fibre influence: a view further confirmed by the change in the BOP value with orientation (and hence effective fibre content) on testing longitudinal and transverse samples from the same
46
5C
~LOP
UT
curve
-=-BOP and UT5
I0
I
a
0.5 Strain (%)
1.0
0 b
0.5 Strain (%)
Fig. 4 Typical stress/strain curves in tension and bending (a) at 28 days (ie on completion of cure) and (b) after t w o years water immersion o r 415 years U K weathering
COMPOSITES. JANUARY 1978
Present durability data now extending to 5/6 years, indicate very little change from this initial behaviour pattern in dry storage conditions with long term predictions indicating relatively small falls in strength and LOP/BOP values 1° whilst toughness and Fig. 4a characteristics are retained. However in wet conditions there are changes in both strength and fracture characteristics over a period of years. The nature of these changes is indicated by a comparison of Fig. 4b with Fig. 4a. After about 1-2 years immersion in water at ambient temperature, or about 5-6 years exposure to the English climate, the stress/strain curves in tension and bending have become those shown in Fig. 4b: the failure strain, impact strength, MOR and UTS have reduced noticeably whilst LOP and BOP have increased a little. The grc has become essentially a brittle material. The oldest grc samples available to us were 'laid down' for long term weathering and test programmes by the Building Research Establishment (BRE) in 1968/69, they were tested at intervals up to 5 years and are due for further testing at 10 years in 1978/79. In 1973/74 certain examples were selected from these materials and the test results analysed in detail by a joint BRE/Pilkington Working Party to give guidance on the expected long term strengths of grcJ ° These predictions are summarized in Table 1. Since that time very considerable experience of accelerated testing has been accumulated by Pilkington and has led to an ability to predict glass fibre strength values in grc out to much longer periods. The results of the work indicate that the strength of good quality grc in UK weather at 20 years, and even after much longer periods, should lie within the range of values given in Table 1. THE PRACTICE OF GRC
It is now useful to consider what conclusions can be drawn from a knowledge of the fundamentals of fibre-reinforceTable 1. Estimated strength properties of spray-dewatered OPC/GRC (5% wt glass fibre) after 20 years exposure to given conditions
ment and/or the basic mechanical behaviour of grc, and to see how these conclusions relate to practical considerations in producing and using good quality composites. Perhaps the most obvious factor is that the long term strength of grc in wet or natural weathering conditions is considerably lower than the strength of the materials as made. Thus design stresses and working loads must be chosen so as to take account of the lower property values given in Table 1. This has always been Pilkington practice with normal design stress values of 6 MN/m 2 (in bending) and 3 MN/m z (in direct tension) chosen so that losses in grc strength with ageing have been allowed for from the start, n Another important consequence of this ageing behaviour relates to the interpretation of component test results. Components are frequently tested on completion of cure in an unaged state and it is essential that an adequate margin of safety in the ratio of failure load to design load is obtained. It has always been Pilkington practice to look for a factor of at least times 5,in components which are expected to have long working lives. The fact that the bending strength of grc is considerably higher than the tensile strength (Fig. 4) must be taken into account when choosing design stresses. A thick sandwich panel, deep section box beam or channel, when subject to overall bending loads, will have the grc skins working in almost uniform direct tension whilst a thin skin or solid section will be subject to a bending stress state. This explains the need for the two different design stress values quoted above. It has been shown that the fibre content is a vital factor in achieving true reinforcement, high ultimate strengths and the valuable early life toughness of grc. It also assists in distributing shrinkage stresses, maintaining a high LOP and preventing shrinkage cracking. Finally the retention of composite strength after long term ageing is directly related to fibre content (Fig. 5). In all these properties it is the fibre content by volume that is important. This is determined by two practical measurements, that of fibre content by weight and that of composite density. Volume
Condition Property
Dry air (40% RH at 20°C)
Under water (18-20°C)
UK weather 50
a) Bending MOR (MN/m 2) LOP (MN/m 2)
26-34
20-25
12-23
8-10
16-18
12-18
3o
b) Tensile UTS (MN/m 2) BOP (MN/m 2)
12-15
8-11
4-10
2O
7- 8
8-11
4-10
Young's modulus (GN/m 2)
E 40 J~
volume)
I0 (by volume)
20-25
Izod impact strength (Nmm/mm 2) 14-20
28-34
25-32
I
,
i
I
,;
2'o 'o
I
6o
I
,oo
Years
4-- 7
COMPOSITES . JANUARY 1978
2- 6
Fig. 5 Predicted fibre strength contribution to the flexural strength of grc for d i f f e r e n t volume fractions of glass fibres
47
Table 2. The effect of grc Density on Fibre Content by Volume (Vf) for Given Weight Contents of Cem-FIL Fibre (Wf)
components are installed or first loaded. Thus LOP must be monitored as part o f quality control procedures and an adequate value must be ensured as soon as the cure schedule is completed.X~' 12
Glass content % by weight (Wf)
Grc density Pc (g/cm 3)
Glass content % by volume (Vf)
5.0
1.6 1.8 2.0 2.2
3.0 3.4 3.7 4.1
5.5
1.6 1.8 2.0 2.2
3.3 3.7 4.1 4.5
6.0
1.6 1.8 2.0 2.2
3.6 4.0 4.5 4.9
Fibre-reinforced composite theory leads to requirements of adequate fibre content, good fibre/matrix bonding, sufficient length of the chopped fibre strands, and uniform fibre distribution. These parameters may be assessed by the normal quality control measurements o f fibre content, density, LOP and MOR; they may be achieved by correct choice o f raw materials, good spraying practice, and good curing. A study of grc properties emphasizes the need for the correct choice of design stress in relation to environment, working life and type of stress situation in the component. Load testing of newly made components must also allow a sufficient margin o f failure over working stress to take account of material strength changes on ageing.
content o f fibre (Vf) is related to weight content (Wf) by the equation
vf =
P~x
wf
(3)
Pf where Pc and pf are the densities o f composite and fibre. Some typical values are given in Table 2 and it is easy to see how a low composite density can lead to a low volume content o f fibre even if the fibre weight content is high. This effect is even more pronounced with deliberately lightweight grcs, it emphasizes the importance o f specifying b o t h Wf and Pc in order to achieve an adequate Vf, o f measuring b o t h Wf and Pc in quality control procedures 12 and o f maintaining a good degree o f compaction. For reinforcement to be uniform and for material strength to be the same at all points, the proper quantity o f glass fibre must also be distributed uniformly throughout the grc. These factors are checked and controlled by carrying out 'paper face' and 'trowel face' measurements o f MOR in quality control tests, and by checking strengths o f samples cut in two perpendicular directions from a sprayed board) 2 Fibres must be long enough to be gripped by the cement, that means they must not be chopped too short nor broken down too much in any mixing process. To carry load there must also be a reasonable bond between fibre and m a t r i x various factors contribute to this. Well compacted, void free, high density mixes with low water/cement ratios contribute to good bonding. Adequate curing is also necessary to develop the bond and make use o f the fibre that has been incorporated. These same factors also contribute to good LOP values which must be high enough to exceed design stress values (with a safety factor) immediately after cure otherwise cracking will occur when
48
CONCLUSIONS
REFERENCES 1
Proctor, B.A. 'Fibre reinforced composite materials' Faraday Special Discussions o f the Chemical Society No 2
2 3 4 5
(1972) pp 63-76 Outwater, J.O.O. Modern Plastics (1956) 33, p 156 Kelly, A. Chapter 5 in "Strong solids' 2nd edition (Clarendon Press, Oxford, 1973) (Note. Also a useful general introduction to fundamental Composites concepts) Aveston, J., Cooper, G.A. and Kelly, A. 'Single and multiple fracture' in The Properties o f Fibre Composites (IPC Science and Technology Press, 1971) pp 15-26 Nair, N.G. 'Mechanics of Glass Fibre Reinforced Cement' in Rilem Symposium 1975, 'Fibre reinforced cement and concrete' edited by A. Neville (The Construction Press Ltd)
6 7 8
pp 81-93 Romauldi, J.P. and Bat.son, G.B. 'Mechanics of crack arrest in concrete' Proc ASCE in Journal of Engng Mechanics Division 89 No EM3 (June, 1963) Allen, H.G. 'The stiffness and strength of two glass-fibre reinforced cement laminates' J Composite Mater 5 (April 1972) pp 194-207 Oakley, D.R. and Proctor, B.A. 'Tensile stress-strain behaviour of glass-fibre reinforced cement composites' in Rilem Symposium 1975, 'Fibre Reinforced Cement and Concrete' edited by A. Neville (The Construction Press Ltd)
9
10 11 12
pp 347-359 Proctor, B.A., Oakley, D.R. and Wiechers, W. 'Tensile stress/ strain characteristics of glass-fibre reinforced cement' in "Composites - Standard, Testing and Design" (IPC Science and Technology Press, 1974) pp 106-107 'A study of the property of Cem-FIL/OPC Composite' (Report of a Joint BRE/PB Working Group) BRE Current Paper CP 38/76 Design Guide: Glassfibre Reinforced Cement (Pilkington Brothers Ltd) (on issue to all Licencees and available on request to Cem-FIL GRC Specifiers and Designers) Quality Control Test Booklet - Application data for use with Cem-FIL Fibre (Pilkington Brothers Ltd, 1976) (on issue to all Licencees and available on request to Cem-FIL GRC Specifiers and Designers)
COMPOSITES. JANUARY 1978
The theory of fibre reinforcement is reviewed and requirements for effective reinforcement identified. These requirements are then related to the stress/strain and ageing behaviour of typical sprayed grc to identify relevant and important factors. Finally an attempt has been made to explain certain design, production and control practices in the light of reinforcement theory and material properties.
Glass fibre-reinforced cement (grc)* is an important new cement-based material which is finding increasing use in the construction industry. It is also a fibre-reinforced composite material which isoftenreplacing traditional materials and moving into fields of use where experience in the concepts and understanding of fibre reinforcement is somewhat restricted. This, therefore, seemed an appropriate time and place to review, quite simply, the basic concepts and requirements of fibre reinforcement, to relate these to the load bearing behaviour of grc and to use both types of information to justify and explain some of the production, control and design practices required for the safe and efficient use of grc - which is also in the end the economic use.
FUNDAMENTALS OF FIBRE REINFORCEMENT
If we consider a very simple composite material containing (say) glass fibres running in one direction right through the length of the sample; and if we further assume that fibres and matrix are bonded together so that, when a load is applied, they stretch equally; then it is easy to show (eg Reference 1) that the stress in the composite, Oc, is divided between fibre and matrix according to oc
=
(EfVf+EmVm)e
c
which are important in making effective use of fibres as reinforcement. These are that the fibre modulus, Ef, is large compared with the matrix stiffness, Em, and that the fibre content, Vf, is as high as practicable. In the context of grc it is important to note that this is the fibre content by volume and not (directly) by weight as is usually measured. This point will be returned to later in the paper. In grc, as in many practical composites, the fibres are usually incorporated in discrete lengths and load has to be transferred into the fibre from the surrounding matrix material. Fig. 1 (Reference 1) shows in a diagrammatic way how a relatively stiff fibre modifies the deformation of the matrix, setting up shear stresses (r) in the surrounding matrix and at the fibre/matrix interface, so that the direct tensile stress carried by the fibre (of), rises rapidly as one moves away from the end. Simplified treatments due to Outwater 2 and Kelly 3 which assume a constant value of shear stress at the fibre/matrix interface
(1)
where
Ef, Em =
gf, Vm = ~C
=
Young's Modulus of fibre and matrix respectively volume fraction of fibre and matrix in the composite strain in the composite
Equation 1 is one form of the Law of Mixtures - a fundamental equation governing the behaviour of fibrereinforced composites. Although this equation must be modified to take account of various factors in practical composites (such as fibres of finite length lying in more than one direction) it does already indicate two factors
l--
crf
'
T:7;,e
o'f
/ rT/l 0
t
x~ 'l,z.~ L o-f for frictional or yeilding load transfer ( ie constant r )
* The data and behaviour descriptions given in this paper refer to grc made with Pilkington Cem-FI L alkali-resistant glass fibre. Pilkington Brothers Ltd, G R C Research and Testing Department, Ormskirk, Lancs
44
Fig. 1 Diagrammatic representation o f the load transfer mechanism, showing h o w a relatively stiff fibre modifies the d e f o r m a t i o n o f the matrix
COMPOSITES. JANUARY 1978
~ a
Strain
• Fibre content (by volume) must be adequate (Vf) • There must be a good fibre/matrix bond (z) • Fibre length must be sufficient re
• Fibres must be relatively long and thin (high. aspect ratio) Region 5
STRESS~STRAIN BEHA VlOUR OF GRC
b
Glass fibre-reinforced plastics are familiar materials in which the fibres are very much stiffer than the plastics matrix thus fulfilling the first requirement in the previous section. The strain-to-failure of the matrix is similar to that of the fibre and when the composite is put under load the stress inthe fibre rises much more rapidly than that in the lower modulus matrix (Fig. 2a). Since fairly high fibre contents are used most of the load is carried by the fibres and the reinforcement is effective.
Strain
Fig. 2 Idealized stress/strain curves for fibre-reinforced materials; (a) in grp and (b) in grc
The role of glass fibres in reinforcing cement is somewhat different from this. The stiffness of the fibres is now only about three times that of the matrix and, even more significantly the failure strain of the cement matrix is only a fraction of that of the fibre. Consequently the idealised tensile stress/strain curve is as shown in Fig. 2b (Reference 4), consisting essentially of three separate regions - although the extent and distinctness of these regions may vary considerably depending on the particular composite, its fibre content, matrix type, age and even manner of testing.
limited by either friction or yielding of the matrix give equations of the form 7rr~of =
(2)
27rrf~-x
where rf = radius of fibre af = tensile strength in fibre at distance x from fibre end T bond strength (in shear) between fibre and matrix Equation 2 draws attention to three more factors which are important if effective use is to be made of the reinforcing fibres. These are a good fibre/matrix bond (r), a sufficient length of fibre (x) and a high ratio of perimeter to crosssectional area (2rrrf)/(rrrf 2) - often called aspect ratio and achieved by using long, thin, fibres.
In Region 1 matrix and fibres act together more or less elastically, the stiffness of the composite and the sharing of load between fibres and matrix being governed by the Law of Mixtures as indicated in the previous section. Various factors (including cost) limit the quantity of fibre incorporated in cement composite to no more than 5 to 10%. Because of this relatively low fibre content and the small stiffness ratio (Ef/Em) the overall degree of reinforcement in Region 1 is small. That is, most of the load is still carried by the matrix and the stiffness of the composite is
Thus, fundamental considerations of composite behaviour lead to the following rules for effective fibre reinforcement. • Fibres must be significantly stiffer than the matrix material (Ef ~>Em)
16
~
/ rnachine Longitudinal spray-dewatesarnl:le~ ~fard
14
/ / J
12
/ . mActual a t ~ ~curves for good this range, qualitybut havsgrb°gen
E zlO
a) ,- 8 (/3
50100
( 1%strain ) I
__
I0 0 0 0
S~roin (/z~) Fig. 3
Observed tensile stress/strain curves for grc materials
COMPOSITES. J A N U A R Y 1978
45
essentially that of the matrix material. However, unreinforced cement pastes and mortars generally contain many voids together with cracks and microcracks due to differential shrinkage, aggregate boundaries, etc. These propogate under an applied stress, join together, and cause fracture at very variable stress levels so that such materials are not normally regarded as capable of bearing tensile or flexural loads. Each void or crack produces a localized region of much reduced stiffness and in grc this load can be transferred to nearby fibres which play an important role in restraining early crack growth; this prevents very low stress matrix failure, and hence raises the average matrix failure stress, s The fibres also distribute shrinkage stress more uniformly. Romualdi 6 and Aveston 4 have argued on theoretical grounds that the presence of reinforcing fibres actually raises the strength or strain to failure of a cement or concrete matrix. There has been some dispute about this in the literature (eg Reference 7) and the effect is difficult to show clearly with the relatively low fibre contents used in practice. In normal sprayed grc, failure of the matrix - ie the end of Region 1 - probably occurs at stress levels at or above those which would be attained in tests on small samples of well cured, good quality matrix material - and considerably above the average for larger pieces of the equivalent unreinforced matrix, if these could indeed be made and cured at all. At the end of Region 1 (Fig. 2b) the matrix cracks and all the load at the cracked section is thrown onto the bridging glass fibres - provided that there are enough of them to bear it! (see insert in Fig. 2b). Moving away from the crack face, load is transferred back from the fibres to the matrix by shear forces at the fibre/cement interface until the stress in the matrix has again risen to such a level that a new crack is formed. This process continues until the material is traversed by an array of very fine cracks which are quite often difficult to see. This forms Region 2 of the idealized stress/strain curve in Fig. 2b - a region of considerable extension and energy absorption providing toughness for the composite. As more and more fine cracks are formed there comes a stage at which it is no longer possible to transfer sufficient load back into the matrix, in the short distance between existing cracks, to raise the stress in the cement to breaking point. Region 2 is then complete and all further load is carded entirely by the bridging fibres, accompanied by opening of the existing cracks as the fibres stretch and perhaps begin to slip in the cement. 4'8 This gives Region 3 of Fig. lb leading to final failure which is governed by the strength of the fibres and/or the fibre/cement bond strength.
spray-dewatered board. Regions of multiple cracking (Region 2) and crack-opening (Region 3) follow, leading to failure at a strain of about 1%. Thus at this stage in its life, grc has an extremely high toughness when compared with concrete or asbestos cement, together with a degree of pseudo-ductility conferred by the fine multiple-cracking which gives it an ability to relieve stress concentrations to a considerable extent. The pseudo-ductile behaviour of Regions 2 and 3 in the tensile test, leads to a redistribution of stress across the section of a sample in a bending test, together with a movement of the neutral axis towards the compression face of the specimen. Thus the nominal bending strength or Modulus of Rupture (MOR) as calculated by simple elastic bending theory is about 2½ times the tensile strength. A feature common, to a lesser extent, with most cementitious materials which have (in detail) somewhat non-linear tensile stress/strain curves. The stress level at which noticeable departure from linearity occurs (Limit of Proportionality, LOP) is also considerably higher than the tensile Bend-Over Point. This relationship between tensile and bending behaviour is illustrated in Fig. 4a for a simple strip samples of grc sheet of rectangular section. In more complex material samples, such as sandwich structures, the stress distribution may more nearly approach the tensile condition even in bending, and bending strengths will be only slightly greater than tensile. AGEING BEHA VIOUR OF GRC
The tensile and bending characteristics illustrated in Figs 3 and 4a are those of good quality sprayed grc sheet after an adequate cure. The material is tough and impact resistant. 50 ,--
~
N E z 40
4(
curve
3£
~, 3 O
//"~dOR o
~ 2o
2C
Although this is a very idealized theoretical model for the stress/strain behaviour of grc, detailed studies of the tensile behaviour of typical sprayed material 8'9 show that it conforms quite closely to theory. Fig. 3 shows actual tensile curves for both spray-dewatered and direct-spray material and the three regions of behaviour can be deafly identified. In the first Region behaviour is matrix dominated with an elastic modulus equal to that expected from cement, there is then a sharp Bend-Over Point (BOP) into Region 2 at stress levels which are rather high for the tensile strength of cement paste or cement mortars indicating some fibre influence: a view further confirmed by the change in the BOP value with orientation (and hence effective fibre content) on testing longitudinal and transverse samples from the same
46
5C
~LOP
UT
curve
-=-BOP and UT5
I0
I
a
0.5 Strain (%)
1.0
0 b
0.5 Strain (%)
Fig. 4 Typical stress/strain curves in tension and bending (a) at 28 days (ie on completion of cure) and (b) after t w o years water immersion o r 415 years U K weathering
COMPOSITES. JANUARY 1978
Present durability data now extending to 5/6 years, indicate very little change from this initial behaviour pattern in dry storage conditions with long term predictions indicating relatively small falls in strength and LOP/BOP values 1° whilst toughness and Fig. 4a characteristics are retained. However in wet conditions there are changes in both strength and fracture characteristics over a period of years. The nature of these changes is indicated by a comparison of Fig. 4b with Fig. 4a. After about 1-2 years immersion in water at ambient temperature, or about 5-6 years exposure to the English climate, the stress/strain curves in tension and bending have become those shown in Fig. 4b: the failure strain, impact strength, MOR and UTS have reduced noticeably whilst LOP and BOP have increased a little. The grc has become essentially a brittle material. The oldest grc samples available to us were 'laid down' for long term weathering and test programmes by the Building Research Establishment (BRE) in 1968/69, they were tested at intervals up to 5 years and are due for further testing at 10 years in 1978/79. In 1973/74 certain examples were selected from these materials and the test results analysed in detail by a joint BRE/Pilkington Working Party to give guidance on the expected long term strengths of grcJ ° These predictions are summarized in Table 1. Since that time very considerable experience of accelerated testing has been accumulated by Pilkington and has led to an ability to predict glass fibre strength values in grc out to much longer periods. The results of the work indicate that the strength of good quality grc in UK weather at 20 years, and even after much longer periods, should lie within the range of values given in Table 1. THE PRACTICE OF GRC
It is now useful to consider what conclusions can be drawn from a knowledge of the fundamentals of fibre-reinforceTable 1. Estimated strength properties of spray-dewatered OPC/GRC (5% wt glass fibre) after 20 years exposure to given conditions
ment and/or the basic mechanical behaviour of grc, and to see how these conclusions relate to practical considerations in producing and using good quality composites. Perhaps the most obvious factor is that the long term strength of grc in wet or natural weathering conditions is considerably lower than the strength of the materials as made. Thus design stresses and working loads must be chosen so as to take account of the lower property values given in Table 1. This has always been Pilkington practice with normal design stress values of 6 MN/m 2 (in bending) and 3 MN/m z (in direct tension) chosen so that losses in grc strength with ageing have been allowed for from the start, n Another important consequence of this ageing behaviour relates to the interpretation of component test results. Components are frequently tested on completion of cure in an unaged state and it is essential that an adequate margin of safety in the ratio of failure load to design load is obtained. It has always been Pilkington practice to look for a factor of at least times 5,in components which are expected to have long working lives. The fact that the bending strength of grc is considerably higher than the tensile strength (Fig. 4) must be taken into account when choosing design stresses. A thick sandwich panel, deep section box beam or channel, when subject to overall bending loads, will have the grc skins working in almost uniform direct tension whilst a thin skin or solid section will be subject to a bending stress state. This explains the need for the two different design stress values quoted above. It has been shown that the fibre content is a vital factor in achieving true reinforcement, high ultimate strengths and the valuable early life toughness of grc. It also assists in distributing shrinkage stresses, maintaining a high LOP and preventing shrinkage cracking. Finally the retention of composite strength after long term ageing is directly related to fibre content (Fig. 5). In all these properties it is the fibre content by volume that is important. This is determined by two practical measurements, that of fibre content by weight and that of composite density. Volume
Condition Property
Dry air (40% RH at 20°C)
Under water (18-20°C)
UK weather 50
a) Bending MOR (MN/m 2) LOP (MN/m 2)
26-34
20-25
12-23
8-10
16-18
12-18
3o
b) Tensile UTS (MN/m 2) BOP (MN/m 2)
12-15
8-11
4-10
2O
7- 8
8-11
4-10
Young's modulus (GN/m 2)
E 40 J~
volume)
I0 (by volume)
20-25
Izod impact strength (Nmm/mm 2) 14-20
28-34
25-32
I
,
i
I
,;
2'o 'o
I
6o
I
,oo
Years
4-- 7
COMPOSITES . JANUARY 1978
2- 6
Fig. 5 Predicted fibre strength contribution to the flexural strength of grc for d i f f e r e n t volume fractions of glass fibres
47
Table 2. The effect of grc Density on Fibre Content by Volume (Vf) for Given Weight Contents of Cem-FIL Fibre (Wf)
components are installed or first loaded. Thus LOP must be monitored as part o f quality control procedures and an adequate value must be ensured as soon as the cure schedule is completed.X~' 12
Glass content % by weight (Wf)
Grc density Pc (g/cm 3)
Glass content % by volume (Vf)
5.0
1.6 1.8 2.0 2.2
3.0 3.4 3.7 4.1
5.5
1.6 1.8 2.0 2.2
3.3 3.7 4.1 4.5
6.0
1.6 1.8 2.0 2.2
3.6 4.0 4.5 4.9
Fibre-reinforced composite theory leads to requirements of adequate fibre content, good fibre/matrix bonding, sufficient length of the chopped fibre strands, and uniform fibre distribution. These parameters may be assessed by the normal quality control measurements o f fibre content, density, LOP and MOR; they may be achieved by correct choice o f raw materials, good spraying practice, and good curing. A study of grc properties emphasizes the need for the correct choice of design stress in relation to environment, working life and type of stress situation in the component. Load testing of newly made components must also allow a sufficient margin o f failure over working stress to take account of material strength changes on ageing.
content o f fibre (Vf) is related to weight content (Wf) by the equation
vf =
P~x
wf
(3)
Pf where Pc and pf are the densities o f composite and fibre. Some typical values are given in Table 2 and it is easy to see how a low composite density can lead to a low volume content o f fibre even if the fibre weight content is high. This effect is even more pronounced with deliberately lightweight grcs, it emphasizes the importance o f specifying b o t h Wf and Pc in order to achieve an adequate Vf, o f measuring b o t h Wf and Pc in quality control procedures 12 and o f maintaining a good degree o f compaction. For reinforcement to be uniform and for material strength to be the same at all points, the proper quantity o f glass fibre must also be distributed uniformly throughout the grc. These factors are checked and controlled by carrying out 'paper face' and 'trowel face' measurements o f MOR in quality control tests, and by checking strengths o f samples cut in two perpendicular directions from a sprayed board) 2 Fibres must be long enough to be gripped by the cement, that means they must not be chopped too short nor broken down too much in any mixing process. To carry load there must also be a reasonable bond between fibre and m a t r i x various factors contribute to this. Well compacted, void free, high density mixes with low water/cement ratios contribute to good bonding. Adequate curing is also necessary to develop the bond and make use o f the fibre that has been incorporated. These same factors also contribute to good LOP values which must be high enough to exceed design stress values (with a safety factor) immediately after cure otherwise cracking will occur when
48
CONCLUSIONS
REFERENCES 1
Proctor, B.A. 'Fibre reinforced composite materials' Faraday Special Discussions o f the Chemical Society No 2
2 3 4 5
(1972) pp 63-76 Outwater, J.O.O. Modern Plastics (1956) 33, p 156 Kelly, A. Chapter 5 in "Strong solids' 2nd edition (Clarendon Press, Oxford, 1973) (Note. Also a useful general introduction to fundamental Composites concepts) Aveston, J., Cooper, G.A. and Kelly, A. 'Single and multiple fracture' in The Properties o f Fibre Composites (IPC Science and Technology Press, 1971) pp 15-26 Nair, N.G. 'Mechanics of Glass Fibre Reinforced Cement' in Rilem Symposium 1975, 'Fibre reinforced cement and concrete' edited by A. Neville (The Construction Press Ltd)
6 7 8
pp 81-93 Romauldi, J.P. and Bat.son, G.B. 'Mechanics of crack arrest in concrete' Proc ASCE in Journal of Engng Mechanics Division 89 No EM3 (June, 1963) Allen, H.G. 'The stiffness and strength of two glass-fibre reinforced cement laminates' J Composite Mater 5 (April 1972) pp 194-207 Oakley, D.R. and Proctor, B.A. 'Tensile stress-strain behaviour of glass-fibre reinforced cement composites' in Rilem Symposium 1975, 'Fibre Reinforced Cement and Concrete' edited by A. Neville (The Construction Press Ltd)
9
10 11 12
pp 347-359 Proctor, B.A., Oakley, D.R. and Wiechers, W. 'Tensile stress/ strain characteristics of glass-fibre reinforced cement' in "Composites - Standard, Testing and Design" (IPC Science and Technology Press, 1974) pp 106-107 'A study of the property of Cem-FIL/OPC Composite' (Report of a Joint BRE/PB Working Group) BRE Current Paper CP 38/76 Design Guide: Glassfibre Reinforced Cement (Pilkington Brothers Ltd) (on issue to all Licencees and available on request to Cem-FIL GRC Specifiers and Designers) Quality Control Test Booklet - Application data for use with Cem-FIL Fibre (Pilkington Brothers Ltd, 1976) (on issue to all Licencees and available on request to Cem-FIL GRC Specifiers and Designers)
COMPOSITES. JANUARY 1978