- Email: [email protected]
Imparting fracture resistance to cement mortar by intermittent interlaminar bonding
CEMENT and CONCRETE RESEARCH. Vol. 12, pp. 661-663, 1982. Printed in the USA 0008-8846/82/050661-03503.00/0 Copyright (c) 1982 Pergamon Press, Ltd.
NOTE IMPARTING FRACTURE RESISTANCE TO CEMENT MORTAR BY INTERMITTENT INTERLAMINAR BONDING
¥.W. Mai, M. Hakeem and B. Cotterell Department of Mechanical Engineering, University of Sydney, Sydney. N.S.W. 2006 Australia.
(Communicated by D.M. Roy) (Received March 16, 1982)
It is well-known that cement mortars do not have sufficient tensile strength and fracture toughness to render them useful for any engineering applications. Fibres are therefore added to provide improvements in both of these propertles. For building products used in Australia, asbestos fibres have been traditionally used but for many reasons they are gradually giving way to cellulose fibres, particularly for non-structural applications. Although reasonable bend strengths (~ 20 MPa) may be obtained in these cellulose fibre-cement composites their fracture toughnesses are very low (~ 500 J/m 2) [I]. In a recent paper [2] we have used epoxy resins in an endeavour to improve these mechanical properties by pre-mixing the cement-silica slurry with the cellulose fibres and then adding the epoxy resins to the mixture. However, the improvements in strength and toughness are only marginal [2]. In the present note we report a method whereby we can impart considerable toughness to the cement-si±ica mortar so that if this is to be used Jointly with the cellulose fibres a material with a reasonable strength and high toughness can be manufactured. The basic principle of the method is to have many layers of perforated papers or some other suitable material stacked up within the cement-matrix so that good bonds are maintained through the perforations and poor bonds exist in regions where the paper separates the matrix. When loads are applied in bending, due to the differential interlaminar stresses across the sheet thickness, delamlnation in the weak regions occurs giving rise to a sink of fracture energy abaorptlon. However, sufficient regions of good bond through the perforations are required to give a reasonable bend strength. For the control samples equal amounts of cement and silica with a water cement ratio of 0.8 were pre-m/xed in a planetary mixer and poured into a steel mould meaauring 190 mm square. This was then hand-trowelled to a 661
662
Vol. Y.W.
Mai,
12, No.
et al.
fiat surface and followed by hand v i b r a t i o n s to get rid of the trapped air bubbles. For the cement m o r t a r s w i t h intermittent interlaminar bonds, copier papers which were punched w i t h circular holes of 6 mm diameter were hand laid in the mould over its entire surface area as the cement slurry was g r a d u a l l y added. The p e r f o r a t e d papers were kept approximately equidistant apart. For this p r e l i m i n a r y w o r k the proportion of perforations per paper was ~11% and the v o l u m e fraction of the papers varied between 3% to 7%. The cast cement pads were then compressed at about 6 MPa and water cured for 24 hours at r o o m temperature. These castings were further a u t o c l a v e d for 8 hours at 175°C and o v e n - d r i e d for 24 hours at ll6°C. Rectangular strips a p p r o x i m a t e l y 30 x 120 mm 2 were cut from these castings for the modulus of rupture and f r a c t u r e toughness tests. The thickness of the specimens varied from 12 m m to 20 mm. The fracture toughness specimens were m a c h i n e d with notches to about half the beam width. All tests were conducted with the Instron u n i v e r s a l testing machine. The bend strength (o b ) was c a l c u l a t e d from the m a x i m u m load using the elastic beam theory and the specific fracture r e s i s t a n c e (R) was obtained by dividing the work area under the l o a d - d i s p l a c e m e n t d i a g r a m of the fracture test with the ligament area [2]. A summary of these e x p e r i m e n t a l results is given in Table i. Note that two different types of cements were used and these are designated as coarse and fine cements.
TABLE
Condition
i.
MECHANICAL
P R O P E R T I E S OF CONTROL AND MODIFIED C E M E N T MORTARS
Volume f r a c t i o n of papers
(vf) Control (coarse
Modulus of rupture (0 b)
Specific fracture resistance
(re'a)
R(j/m 2 )
7.7 _+ 0.6
20
8.5 ± 0.5
63.5
-+ i
cement)
Control (fine cement)
-+ 0.5
Paper modified (coarse cement)
0.045
3.7+-0.1
405 ± 15
Paper modified (fine cement)
0.037
4.5 _+ 0.6
265 ± 15
0.073
5.7 i O.6
584 ± 60
It is clear from these results that the modified cement mortars have much larger fracture r e s i s t a n c e s compared with the controls. However, the large increase (200% to 800%) of fracture work is at the expense of a 30% to 50% reduction in bend strength. This strength loss may not be a d e s i r a b l e result but it can be c o m p e n s a t e d for by the addition of som~ c e l l u l o s e fibres so that a r e a s o n a b l y strengthened and much toughened cellulose-paper-cement c o m p o s i t e can be obtained. It should be also noted that increasing the volume f r a c t i o n of papers increases both o b and R as
5
Vol. 12, No. 5
663 FRACTURE TOUGHNESS, STRENGTH,
INTERLAMINAR BONDING, MORTAR
shown in the table. A careful examination of the fractured specimens reveals that the controls failed in a typical brittle manner with a flat fractured surface but that the perforated paper modified cement mortars failed with an irregular fracture surface with obvious areas of delamination which gave rise to the large fracture energy absorptions. Of course there are many other problems with cellulose-cement composites, notably moisture absorption, but these will not be discussed in this note. Insofar as imparting toughness to the cement mortar is concerned, the method described in this communication seems useful. REFERENCES i.
R. Andonian, Y.W. Mai and B. Cotterell, "Strength and fracture properties of cellulose fibre reinforced cement composites", Int. J. Cement Composites, ~, 151 (1979).
2,
T.K. Sia. Y.W. M~i and B. Cotterell, "Strength and fracture properties of epoxy-cement composites", Proc. 2nd Australian Conference on Engineering Materials, Ed. D.J. Cook, pp 515-529, 1981.
NOTE IMPARTING FRACTURE RESISTANCE TO CEMENT MORTAR BY INTERMITTENT INTERLAMINAR BONDING
¥.W. Mai, M. Hakeem and B. Cotterell Department of Mechanical Engineering, University of Sydney, Sydney. N.S.W. 2006 Australia.
(Communicated by D.M. Roy) (Received March 16, 1982)
It is well-known that cement mortars do not have sufficient tensile strength and fracture toughness to render them useful for any engineering applications. Fibres are therefore added to provide improvements in both of these propertles. For building products used in Australia, asbestos fibres have been traditionally used but for many reasons they are gradually giving way to cellulose fibres, particularly for non-structural applications. Although reasonable bend strengths (~ 20 MPa) may be obtained in these cellulose fibre-cement composites their fracture toughnesses are very low (~ 500 J/m 2) [I]. In a recent paper [2] we have used epoxy resins in an endeavour to improve these mechanical properties by pre-mixing the cement-silica slurry with the cellulose fibres and then adding the epoxy resins to the mixture. However, the improvements in strength and toughness are only marginal [2]. In the present note we report a method whereby we can impart considerable toughness to the cement-si±ica mortar so that if this is to be used Jointly with the cellulose fibres a material with a reasonable strength and high toughness can be manufactured. The basic principle of the method is to have many layers of perforated papers or some other suitable material stacked up within the cement-matrix so that good bonds are maintained through the perforations and poor bonds exist in regions where the paper separates the matrix. When loads are applied in bending, due to the differential interlaminar stresses across the sheet thickness, delamlnation in the weak regions occurs giving rise to a sink of fracture energy abaorptlon. However, sufficient regions of good bond through the perforations are required to give a reasonable bend strength. For the control samples equal amounts of cement and silica with a water cement ratio of 0.8 were pre-m/xed in a planetary mixer and poured into a steel mould meaauring 190 mm square. This was then hand-trowelled to a 661
662
Vol. Y.W.
Mai,
12, No.
et al.
fiat surface and followed by hand v i b r a t i o n s to get rid of the trapped air bubbles. For the cement m o r t a r s w i t h intermittent interlaminar bonds, copier papers which were punched w i t h circular holes of 6 mm diameter were hand laid in the mould over its entire surface area as the cement slurry was g r a d u a l l y added. The p e r f o r a t e d papers were kept approximately equidistant apart. For this p r e l i m i n a r y w o r k the proportion of perforations per paper was ~11% and the v o l u m e fraction of the papers varied between 3% to 7%. The cast cement pads were then compressed at about 6 MPa and water cured for 24 hours at r o o m temperature. These castings were further a u t o c l a v e d for 8 hours at 175°C and o v e n - d r i e d for 24 hours at ll6°C. Rectangular strips a p p r o x i m a t e l y 30 x 120 mm 2 were cut from these castings for the modulus of rupture and f r a c t u r e toughness tests. The thickness of the specimens varied from 12 m m to 20 mm. The fracture toughness specimens were m a c h i n e d with notches to about half the beam width. All tests were conducted with the Instron u n i v e r s a l testing machine. The bend strength (o b ) was c a l c u l a t e d from the m a x i m u m load using the elastic beam theory and the specific fracture r e s i s t a n c e (R) was obtained by dividing the work area under the l o a d - d i s p l a c e m e n t d i a g r a m of the fracture test with the ligament area [2]. A summary of these e x p e r i m e n t a l results is given in Table i. Note that two different types of cements were used and these are designated as coarse and fine cements.
TABLE
Condition
i.
MECHANICAL
P R O P E R T I E S OF CONTROL AND MODIFIED C E M E N T MORTARS
Volume f r a c t i o n of papers
(vf) Control (coarse
Modulus of rupture (0 b)
Specific fracture resistance
(re'a)
R(j/m 2 )
7.7 _+ 0.6
20
8.5 ± 0.5
63.5
-+ i
cement)
Control (fine cement)
-+ 0.5
Paper modified (coarse cement)
0.045
3.7+-0.1
405 ± 15
Paper modified (fine cement)
0.037
4.5 _+ 0.6
265 ± 15
0.073
5.7 i O.6
584 ± 60
It is clear from these results that the modified cement mortars have much larger fracture r e s i s t a n c e s compared with the controls. However, the large increase (200% to 800%) of fracture work is at the expense of a 30% to 50% reduction in bend strength. This strength loss may not be a d e s i r a b l e result but it can be c o m p e n s a t e d for by the addition of som~ c e l l u l o s e fibres so that a r e a s o n a b l y strengthened and much toughened cellulose-paper-cement c o m p o s i t e can be obtained. It should be also noted that increasing the volume f r a c t i o n of papers increases both o b and R as
5
Vol. 12, No. 5
663 FRACTURE TOUGHNESS, STRENGTH,
INTERLAMINAR BONDING, MORTAR
shown in the table. A careful examination of the fractured specimens reveals that the controls failed in a typical brittle manner with a flat fractured surface but that the perforated paper modified cement mortars failed with an irregular fracture surface with obvious areas of delamination which gave rise to the large fracture energy absorptions. Of course there are many other problems with cellulose-cement composites, notably moisture absorption, but these will not be discussed in this note. Insofar as imparting toughness to the cement mortar is concerned, the method described in this communication seems useful. REFERENCES i.
R. Andonian, Y.W. Mai and B. Cotterell, "Strength and fracture properties of cellulose fibre reinforced cement composites", Int. J. Cement Composites, ~, 151 (1979).
2,
T.K. Sia. Y.W. M~i and B. Cotterell, "Strength and fracture properties of epoxy-cement composites", Proc. 2nd Australian Conference on Engineering Materials, Ed. D.J. Cook, pp 515-529, 1981.