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Basic creep, drying creep and shrinkage of a mature cement paste after a heat cycle
CEMENTand CONCRETERESEARCH. Vol. 7, pp. 597-604, 1977. Pergamon Press, Inc Printed in the United States.
BASIC CREEP, DRYING CREEP AND SHRINKAGE OF A MATURE CEMENT PASTE AFTER A HEAT CYCLE L. J. Parrott Cement and Concrete Association, U. K. Wexham Sprin&s, Slough SL3 6PL
(Communicated by P. J. Sereda) (Received July 15, 1977)
ABSTRACT Cement paste that was heated in a saturated state at an age of 28 days exhibited reductions in basic creep and shrinkage and an increase in the extent of polymerisation of the hydrated silicates. This behaviour was similar to that of cement pastes heated at early ages. Drying creep was also reduced by the heat treatment, the reductions in drying creep being closely related to the reductions in shrinkage. Measured changes in properties that were related to the structure of the cement paste assisted interpretation of the present deformation and strength observations and of transitional thermal creep data reported in the i it erature.
Im g e s a t t i g t e n z u s t a n d im a l t e r yon 28 t a g e n e r w a r m t e r z e m e n t b r e i z e i g t e e i n e abnahme d e r g r u n d k r i e c h - und s c h r u m p f e i g e n s c h a f t e n sowie eine verstark-te polymerisierung der silikathydrate. Dieses verhalten e n t s p r a c h dem y o n f r u h z e i t i g erwanntem zementbrei. Das t r o c k n u n g s k r i e c h e n wurde durch d i e warmebehandlung e b e n f a l l s v e r r i n g e r t , wobei diese verringerung recht genau der schrumpfungsabnahme folgte. Die der zementbreistruktur entsprechenden gemessenen eigenschaftsanderungen halfen b e i d e r d e u t u n g d e r v e r f o r m u n g s - und f e s t i g k e i t s b e o b a c h t u n g e n sowie der in der literatur zitierten daten uber das ubergangswarmekriechverhalten.
597
598
Vol. 7, No. 5 L. J. Parrott Introduction
Experiments on cement paste have been carried out, as part of a broader study of the deformation of structural concrete, to elucidate in a relatively fundamental way t h e i n f l u e n c e s of curing temperature, duration of heating and age at the start of heating upon creep and shrinkage. Results illustrating the effects of curing temperature and duration of heating upon the recoverable and irrecoverable components of shrinkage and basic creep (i.e. creep without simultaneous loss of moisture} have already been reported (1,2). The present report is mainly concerned with the influence upon basic creep, drying creep and shrinkage of cement paste of delaying heating until an age of 28 days. Delayed heating is of interest for two reasons: a) In practice some structures (e.g. nuclear pressure vessels and oil storage vessels} are heated after construction a n d i t may b e i m p o r t a n t to know the effects of this upon deformation of the structural concrete, and b) fundamentally it is helpful to know if the products of cement hydration can be converted after formation to yield a material with reduced deformation potential. Apart from measuring deformation, data relating to porosity, combined water content and silicate polymerisation of the cement pastes were obtained. Experimental
Details
The experimental procedures that were used in the present study have been reported previously (1,2); thus only the essential points will be stated here. A paste of Ordinary Portland cement (Cement C in reference 1) was cast with a water/cement ratio of O.&? by weight to give a slab measuring 12 x 110 x 290 mm. The slab was cut into 12 x 12 x 110 mm prisms which were then cured in water at 20°C until they were 28 days old. At this stage the prisms were divided into three groups which were water cured for two additional days at 20, 75 or 95°C. The prisms were then dried at 85~ relative humidity and 20°C in a vacuum desiccator (to avoid carbonation). At an age of seven weeks, when the prisms were close to equilibrium with the desiccator atmosphere four prisms from each group were loaded to a stress of 3.7 N/mm 2 in miniature creep rigs. Eight weeks after loading, when the creep rate at 85~ relative humidity and 20°C had diminished considerably, the prisms were subjected to further drying in the desiccator at 55~ relative humidity and 20°C. T h e creep specimens were unloaded after a total of fifteen weeks under load. After creep recovery the creep and control prisms were used to measure swelling and weight gain during resaturation, compressive strength in the dry and resaturated state, silicate polymerisation (using the method of W i e k e r (3) ) and water loss during thermal analysis at a controlled water vapour pressure (I). Results The main results are summarised in Table 1: it can be seen that an elevated temperature heating cycle, applied at an age of 28 days reduced basic creep, drying creep and shrinkage. The water losses during the various stages of drying suggested that there was a change in the pore volume distribution with heat cycling but the corresponding changes in combined water content and compressive strength were relatively small. Polysilicate formation (as indicated by the silicate not complexed with an acidified molybdate solution after a reaction time of 300 seconds, ~oo(1) ) was substantially increased as a result of the additional, elevated temperature curing. The development of creep with time under load is illustrated in Figure 1.
Vol. 7, No. 5
599
CREEP, SHRINKAGE, HEATING, CEMENTPASTE, MATURITY TABLE 1 Summary of main experimental
results
Curing temperature from age of 28 to 30 days (°C)
2o
75
95
Weight loss (@/cc)
0.132
0.165
0.212
Shrinkage
2080
1320
1050
Weight loss (g/cc)
0.100
0.070
0.055
S h r i n k a g e (x 10 ~ )
1850
1080
760
o.158
o.138
0.1o8
0.390
0.373
0.375
Basic creep after 56 days under load (x 10 -6 )
425
255
230
Drying creep after 21 days of drying (x 10-6)
380
295
165
Proportion of silicate not complexed after 300 sec., S~ oo
0.069
0.130
0.182
Combined water lO0-&60°C
0.151
0.167
0.156
Changes at 85% relative humidity, 20°C
Changes at 55% relative humidity, 20°C
Weight loss from 55% relative humidity, to 2% relative humidity, IOO°C (g/cc)
(x 10-6 )
20°C
Sum of weight losses from initial saturated state to 2% relative humidity, IOO°C {g/cc)
Compressive strength of 12 mm cubes
(9/9 of cement) 55% relative humidity
54
54
49
resaturated ~ hour
t~3
~o
35
resaturated
~4
~i
34
48 h o u r s
Discussion a)
Basic creep and shrinkage
The results in Table I clearly show that heating of a mature (28 day old) cement paste reduces basic creep and shrinkage while it increases the formation of polysilicate, as indicated by B o o This situation is the same as that for heating at an early age (1,2) and suggests that the measured changes in basic creep, shrinkage and silicate polymerisation with heating result from a conversion of the products of cement hydration. The changes do not appear to be associated with some special chemical or physical structure that develops only during hydration of the cement at elevated temperature. Figure 2 shows that changes in creep are closely associated with changes in polysilicate formation, as indicated by B o o The data from the present study are in reasonable agreement with those from reference 1 considering that there were some differences in the moisture contents and degrees of hydration of the specimens from the two studies.
600
Vol. 7, No. 5 L. J. Parrott
8O0
Jo zoo 7, drying
creep
0 600
500 Cured from 28 Io30 days at a temperature Of 400
300 t5°C 200
100 In eQudtbr,um at 85~, R H ......... 0 1
30
~,'o
tO
0
4'0
,,,44,,.
s'o
drying at 55~ R H __ I
~'o
zo
~o
9'0
,oo
Days under load
Creep
FIG. 1 versus days
strain
under
load
~- 4 0 0 i
o Symbol o
oo 5
g
O
3oo
O
A
Curing , source temperature I ...... i__ _
•
2o°c
•
•
75 95
O []
2O 70
',Reference
A
95
L ......
Present study
I
1
=
&Q t, 2 0 0
[] & A
100
0 0
Basic
creep
i 0-1 0'2 0 3 , 5 3 0 0 , p r o p o r tIon of silcate not c o m p l e x e d a f t e r 3 0 0 sec
versus
FIG. polysilicate
2
formation
as
indicated
by ~oo
Vol. 7, No. 5
601 CREEP, SHRINKAGE, HEATING, CEMENT PASTE, MATURITY
b)
Transient thermal creep
Recently it has been reported that when Portland cement mortar under load is heated for the first time, substantial additional creep is developed within a few days [4). The time scale for this transient thermal creep development corresponds closely to the rate of polysilicate formation after a temperature change, as may be seen in Figure 3. Any phase change in a material under load would be expected to lead to a local and thus overall increase in compliance, so it seems possible that the molecular rearrangement associated with polysilicate formation may be responsible for transient thermal creep. This possibility will be the subject of future studies. In the present experimental study the phase change was completed well before the samples were loaded so no transient thermal creep was present and a reduction in basic creep was observed. Comparable observations have been made regarding the effects of a phase change due to carbonation of hydrated Portland cement prior to and during creep tests [5,6}: carbonation prior to loading decreased creep while simultaneous carbonation and loading increased creep. ,,c- -
i
4 o - zolt£___ ~
' o 150 × "&
too E Data trom IIIston and Sanders
50
o
4
0
~
8
t6
t~
(4)
I
t4
Days alter temperature
rise
O3
~g 0
m
~
20-~70C
g~ g~
D a t a of a u t h o r
2
I
I
4
6
I 8
10 Days alter
(1)
I
12 14 temperature r,se
FIG. 3 Creep and silicate formation as a function of time. c)
Drying creep
The additional creep due to drying under load is plotted against control specimen shrinkage in Figure 4. There does not appear to be any major influence of heat treatment upon the relationship between shrinkage and drying creep: this suggests that the same methods for predicting drying creep may be applicable to normal and heat cured concretes. Drying creep data for concrete, from the literature (7,8,9,10) have successfully been fitted to the following equation:Drying creep = K x stress x shrinkage Figure 5 shows that the drying creep measurements from the present study on cement paste are consistent with data from a wide range of
602
Vol. 7, No. 5 L. J. P a r r o t t
'O x v
Symbol _
400
0 [] A
O. &_
O
Curing temperature 20°C 75 95
0 0
300 t,_
a
200
100
0
-f&
0
z~O I
I
500
1000
Drying
0.15_
-
1
I
1500 2000 Shrinkage ( x 1 0 -6 )
FIG. 4 creep versus
shrinkage
/
Symbol Reference ,
10
0.10
O~ •
Present study
_ / -!-:
I 0-4
I 0.6
0-05
/
0
0
I/ 0.2
I 0.8
I 1.0
1.2
Water/cement ratio by weight Constant
rIG. 5 K in e q u a t i o n " D r y i n g c r e e p = K x s t r e s s x s h r i n k a g e " versus water/cemen% ratio of mix
Vol. 7, No. 5
CREEP, SHRINKAGE, HEATING, CEMENT PASTE, MATURITY
603
concretes. It seems likely that the drying creep of heat cured concrete will depend mainly upon stress, concurrent shrinkage and the water/cement ratio of the mix. d)
General
The four weeks of water curing prior to heating ensured that hydration of the cement was sufficiently well advanced so that the heat cycles had only a small effect upon the combined water content. The heat cycles did however affect the distribution of water losses during the various drying stages, as indicated in Table 1: there was an apparent increase in the volume of the larger water filled spaces or pores at the expense of the smaller water filled spaces. This might be regarded as consistent with the polymerisation or molecular bonding of existing hydrated silicates within the cement paste. A simple examination of the distribution of the combined water losses as measured during thermo9ravimetric analysis did not indicate that there were any large changes in chemical composition of the cement hydrates with curing temperature which could be associated with changes in creep, shrinkage and polysilicate formation. The compressive strengths of cubes conditioned at 95°C were slightly lower than those of cubes conditioned at 20 and 75°C: this might be due to the increased volume of larger pores. The compressive strength was reduced by short-term (~ hour) and longer-term (2 day) resaturation, regardless of heat treatment. The weight of water gained after ½ hour of resaturation was within a few percent of the final water uptake but the swelling at this stage had reached only about half of its final value. The compressive strengths after ~ hour and 2 days of resaturation were virtually the same for each heat treatment, so any internal stresses associated with the later stages of swelling were clearly of no importance in controlling the crushing b e h a v i o u r Conclusions Ordinary Portland cement paste that was heated in a saturated state at an age of 28 days exhibited reductions in basic creep and shrinkage and an increase in the extent of polymerisation of the hydrated silicates. This behaviour was similar to that of cement pastes heated at early ages. Drying creep was also reduced by the heat treatment, the reductions being closely related to the reductions in shrinkage. Acknowled@ements The author acknowledges with gratitude the assistance of Mr J A Martin and Mr M G Taylor with the deformation and silicate polymerisation measurements References I.
L.J. Parrott. Effect of a heat cycle during moist curing upon the deformation of hardened cement paste. Proc. Conf. Hydraulic Cement Pastes, University of Sheffield, 8/9 April 1976 pp 189-20&. Published by Cement and Concrete Association.
2.
L.J. Parrott. Recoverable and irrecoverable deformation of heat-cured cement paste. Magazine of Concrete Research, Volume 29, Number 98, March 1977 pp 26-30.
.
W. Wieker. New methods of investigation of the hydration processes of Portland cements. Presented at 6th International Congress on the Chemistry of Cements, M o s c o w 1 9 7 ~ . (Not yet published).
604
Vol. 7, No. 5 L. J. Parrott
&.
.
.
.
.
.
10.
J.M. Illston and P.D. Sanders. The effect of temperature change upon the creep of mortar under torsional loading. Magazine of Concrete Research, Volume 25, Number 8&, September 1973 pp 136-1~4. J. Alexandre. Influence de la carbonatation sur le fluage en compression en beton. (Influence on carbonation on the creep of concrete in compression). Revue de Materiaux de Construction No 68~ November 1973 pp 22-29. L.J. Parrott. Increase in creep of hardened cement paste due to carbonation under load. Magazine of Concrete Research, Volume 27, Number 92, September 1975 pp 179-181. I.Ali a n d C.E. Kesler. Mechanisms of creep in concrete. Proc. of Symposium on Creep of Concrete. American Concrete Institute, Special Publication SP-9 1964, pp 35-~7. G.E. Troxell, J.M. Raphael and R.E. Davis. Long-time creep and shrinkage tests of plain and reinforced concrete. Proc. of A.S.T.M. Volume 58, 1958, pp 1 1 0 1 - 1 1 2 0 . J.R. Keeton. Study of creep in concrete. U.S. Naval Civil Engineering Laboratory. Technical Report R333-I, Port Hueneme California, Jan. 1965. L.J.
Parrott.
Some o b s e r v a t i o n s
on the
components
of
creep
in
concrete.
Magazine of Concrete Research, Volume 22, Number 72, September 1970 pp 143-1/.8.
BASIC CREEP, DRYING CREEP AND SHRINKAGE OF A MATURE CEMENT PASTE AFTER A HEAT CYCLE L. J. Parrott Cement and Concrete Association, U. K. Wexham Sprin&s, Slough SL3 6PL
(Communicated by P. J. Sereda) (Received July 15, 1977)
ABSTRACT Cement paste that was heated in a saturated state at an age of 28 days exhibited reductions in basic creep and shrinkage and an increase in the extent of polymerisation of the hydrated silicates. This behaviour was similar to that of cement pastes heated at early ages. Drying creep was also reduced by the heat treatment, the reductions in drying creep being closely related to the reductions in shrinkage. Measured changes in properties that were related to the structure of the cement paste assisted interpretation of the present deformation and strength observations and of transitional thermal creep data reported in the i it erature.
Im g e s a t t i g t e n z u s t a n d im a l t e r yon 28 t a g e n e r w a r m t e r z e m e n t b r e i z e i g t e e i n e abnahme d e r g r u n d k r i e c h - und s c h r u m p f e i g e n s c h a f t e n sowie eine verstark-te polymerisierung der silikathydrate. Dieses verhalten e n t s p r a c h dem y o n f r u h z e i t i g erwanntem zementbrei. Das t r o c k n u n g s k r i e c h e n wurde durch d i e warmebehandlung e b e n f a l l s v e r r i n g e r t , wobei diese verringerung recht genau der schrumpfungsabnahme folgte. Die der zementbreistruktur entsprechenden gemessenen eigenschaftsanderungen halfen b e i d e r d e u t u n g d e r v e r f o r m u n g s - und f e s t i g k e i t s b e o b a c h t u n g e n sowie der in der literatur zitierten daten uber das ubergangswarmekriechverhalten.
597
598
Vol. 7, No. 5 L. J. Parrott Introduction
Experiments on cement paste have been carried out, as part of a broader study of the deformation of structural concrete, to elucidate in a relatively fundamental way t h e i n f l u e n c e s of curing temperature, duration of heating and age at the start of heating upon creep and shrinkage. Results illustrating the effects of curing temperature and duration of heating upon the recoverable and irrecoverable components of shrinkage and basic creep (i.e. creep without simultaneous loss of moisture} have already been reported (1,2). The present report is mainly concerned with the influence upon basic creep, drying creep and shrinkage of cement paste of delaying heating until an age of 28 days. Delayed heating is of interest for two reasons: a) In practice some structures (e.g. nuclear pressure vessels and oil storage vessels} are heated after construction a n d i t may b e i m p o r t a n t to know the effects of this upon deformation of the structural concrete, and b) fundamentally it is helpful to know if the products of cement hydration can be converted after formation to yield a material with reduced deformation potential. Apart from measuring deformation, data relating to porosity, combined water content and silicate polymerisation of the cement pastes were obtained. Experimental
Details
The experimental procedures that were used in the present study have been reported previously (1,2); thus only the essential points will be stated here. A paste of Ordinary Portland cement (Cement C in reference 1) was cast with a water/cement ratio of O.&? by weight to give a slab measuring 12 x 110 x 290 mm. The slab was cut into 12 x 12 x 110 mm prisms which were then cured in water at 20°C until they were 28 days old. At this stage the prisms were divided into three groups which were water cured for two additional days at 20, 75 or 95°C. The prisms were then dried at 85~ relative humidity and 20°C in a vacuum desiccator (to avoid carbonation). At an age of seven weeks, when the prisms were close to equilibrium with the desiccator atmosphere four prisms from each group were loaded to a stress of 3.7 N/mm 2 in miniature creep rigs. Eight weeks after loading, when the creep rate at 85~ relative humidity and 20°C had diminished considerably, the prisms were subjected to further drying in the desiccator at 55~ relative humidity and 20°C. T h e creep specimens were unloaded after a total of fifteen weeks under load. After creep recovery the creep and control prisms were used to measure swelling and weight gain during resaturation, compressive strength in the dry and resaturated state, silicate polymerisation (using the method of W i e k e r (3) ) and water loss during thermal analysis at a controlled water vapour pressure (I). Results The main results are summarised in Table 1: it can be seen that an elevated temperature heating cycle, applied at an age of 28 days reduced basic creep, drying creep and shrinkage. The water losses during the various stages of drying suggested that there was a change in the pore volume distribution with heat cycling but the corresponding changes in combined water content and compressive strength were relatively small. Polysilicate formation (as indicated by the silicate not complexed with an acidified molybdate solution after a reaction time of 300 seconds, ~oo(1) ) was substantially increased as a result of the additional, elevated temperature curing. The development of creep with time under load is illustrated in Figure 1.
Vol. 7, No. 5
599
CREEP, SHRINKAGE, HEATING, CEMENTPASTE, MATURITY TABLE 1 Summary of main experimental
results
Curing temperature from age of 28 to 30 days (°C)
2o
75
95
Weight loss (@/cc)
0.132
0.165
0.212
Shrinkage
2080
1320
1050
Weight loss (g/cc)
0.100
0.070
0.055
S h r i n k a g e (x 10 ~ )
1850
1080
760
o.158
o.138
0.1o8
0.390
0.373
0.375
Basic creep after 56 days under load (x 10 -6 )
425
255
230
Drying creep after 21 days of drying (x 10-6)
380
295
165
Proportion of silicate not complexed after 300 sec., S~ oo
0.069
0.130
0.182
Combined water lO0-&60°C
0.151
0.167
0.156
Changes at 85% relative humidity, 20°C
Changes at 55% relative humidity, 20°C
Weight loss from 55% relative humidity, to 2% relative humidity, IOO°C (g/cc)
(x 10-6 )
20°C
Sum of weight losses from initial saturated state to 2% relative humidity, IOO°C {g/cc)
Compressive strength of 12 mm cubes
(9/9 of cement) 55% relative humidity
54
54
49
resaturated ~ hour
t~3
~o
35
resaturated
~4
~i
34
48 h o u r s
Discussion a)
Basic creep and shrinkage
The results in Table I clearly show that heating of a mature (28 day old) cement paste reduces basic creep and shrinkage while it increases the formation of polysilicate, as indicated by B o o This situation is the same as that for heating at an early age (1,2) and suggests that the measured changes in basic creep, shrinkage and silicate polymerisation with heating result from a conversion of the products of cement hydration. The changes do not appear to be associated with some special chemical or physical structure that develops only during hydration of the cement at elevated temperature. Figure 2 shows that changes in creep are closely associated with changes in polysilicate formation, as indicated by B o o The data from the present study are in reasonable agreement with those from reference 1 considering that there were some differences in the moisture contents and degrees of hydration of the specimens from the two studies.
600
Vol. 7, No. 5 L. J. Parrott
8O0
Jo zoo 7, drying
creep
0 600
500 Cured from 28 Io30 days at a temperature Of 400
300 t5°C 200
100 In eQudtbr,um at 85~, R H ......... 0 1
30
~,'o
tO
0
4'0
,,,44,,.
s'o
drying at 55~ R H __ I
~'o
zo
~o
9'0
,oo
Days under load
Creep
FIG. 1 versus days
strain
under
load
~- 4 0 0 i
o Symbol o
oo 5
g
O
3oo
O
A
Curing , source temperature I ...... i__ _
•
2o°c
•
•
75 95
O []
2O 70
',Reference
A
95
L ......
Present study
I
1
=
&Q t, 2 0 0
[] & A
100
0 0
Basic
creep
i 0-1 0'2 0 3 , 5 3 0 0 , p r o p o r tIon of silcate not c o m p l e x e d a f t e r 3 0 0 sec
versus
FIG. polysilicate
2
formation
as
indicated
by ~oo
Vol. 7, No. 5
601 CREEP, SHRINKAGE, HEATING, CEMENT PASTE, MATURITY
b)
Transient thermal creep
Recently it has been reported that when Portland cement mortar under load is heated for the first time, substantial additional creep is developed within a few days [4). The time scale for this transient thermal creep development corresponds closely to the rate of polysilicate formation after a temperature change, as may be seen in Figure 3. Any phase change in a material under load would be expected to lead to a local and thus overall increase in compliance, so it seems possible that the molecular rearrangement associated with polysilicate formation may be responsible for transient thermal creep. This possibility will be the subject of future studies. In the present experimental study the phase change was completed well before the samples were loaded so no transient thermal creep was present and a reduction in basic creep was observed. Comparable observations have been made regarding the effects of a phase change due to carbonation of hydrated Portland cement prior to and during creep tests [5,6}: carbonation prior to loading decreased creep while simultaneous carbonation and loading increased creep. ,,c- -
i
4 o - zolt£___ ~
' o 150 × "&
too E Data trom IIIston and Sanders
50
o
4
0
~
8
t6
t~
(4)
I
t4
Days alter temperature
rise
O3
~g 0
m
~
20-~70C
g~ g~
D a t a of a u t h o r
2
I
I
4
6
I 8
10 Days alter
(1)
I
12 14 temperature r,se
FIG. 3 Creep and silicate formation as a function of time. c)
Drying creep
The additional creep due to drying under load is plotted against control specimen shrinkage in Figure 4. There does not appear to be any major influence of heat treatment upon the relationship between shrinkage and drying creep: this suggests that the same methods for predicting drying creep may be applicable to normal and heat cured concretes. Drying creep data for concrete, from the literature (7,8,9,10) have successfully been fitted to the following equation:Drying creep = K x stress x shrinkage Figure 5 shows that the drying creep measurements from the present study on cement paste are consistent with data from a wide range of
602
Vol. 7, No. 5 L. J. P a r r o t t
'O x v
Symbol _
400
0 [] A
O. &_
O
Curing temperature 20°C 75 95
0 0
300 t,_
a
200
100
0
-f&
0
z~O I
I
500
1000
Drying
0.15_
-
1
I
1500 2000 Shrinkage ( x 1 0 -6 )
FIG. 4 creep versus
shrinkage
/
Symbol Reference ,
10
0.10
O~ •
Present study
_ / -!-:
I 0-4
I 0.6
0-05
/
0
0
I/ 0.2
I 0.8
I 1.0
1.2
Water/cement ratio by weight Constant
rIG. 5 K in e q u a t i o n " D r y i n g c r e e p = K x s t r e s s x s h r i n k a g e " versus water/cemen% ratio of mix
Vol. 7, No. 5
CREEP, SHRINKAGE, HEATING, CEMENT PASTE, MATURITY
603
concretes. It seems likely that the drying creep of heat cured concrete will depend mainly upon stress, concurrent shrinkage and the water/cement ratio of the mix. d)
General
The four weeks of water curing prior to heating ensured that hydration of the cement was sufficiently well advanced so that the heat cycles had only a small effect upon the combined water content. The heat cycles did however affect the distribution of water losses during the various drying stages, as indicated in Table 1: there was an apparent increase in the volume of the larger water filled spaces or pores at the expense of the smaller water filled spaces. This might be regarded as consistent with the polymerisation or molecular bonding of existing hydrated silicates within the cement paste. A simple examination of the distribution of the combined water losses as measured during thermo9ravimetric analysis did not indicate that there were any large changes in chemical composition of the cement hydrates with curing temperature which could be associated with changes in creep, shrinkage and polysilicate formation. The compressive strengths of cubes conditioned at 95°C were slightly lower than those of cubes conditioned at 20 and 75°C: this might be due to the increased volume of larger pores. The compressive strength was reduced by short-term (~ hour) and longer-term (2 day) resaturation, regardless of heat treatment. The weight of water gained after ½ hour of resaturation was within a few percent of the final water uptake but the swelling at this stage had reached only about half of its final value. The compressive strengths after ~ hour and 2 days of resaturation were virtually the same for each heat treatment, so any internal stresses associated with the later stages of swelling were clearly of no importance in controlling the crushing b e h a v i o u r Conclusions Ordinary Portland cement paste that was heated in a saturated state at an age of 28 days exhibited reductions in basic creep and shrinkage and an increase in the extent of polymerisation of the hydrated silicates. This behaviour was similar to that of cement pastes heated at early ages. Drying creep was also reduced by the heat treatment, the reductions being closely related to the reductions in shrinkage. Acknowled@ements The author acknowledges with gratitude the assistance of Mr J A Martin and Mr M G Taylor with the deformation and silicate polymerisation measurements References I.
L.J. Parrott. Effect of a heat cycle during moist curing upon the deformation of hardened cement paste. Proc. Conf. Hydraulic Cement Pastes, University of Sheffield, 8/9 April 1976 pp 189-20&. Published by Cement and Concrete Association.
2.
L.J. Parrott. Recoverable and irrecoverable deformation of heat-cured cement paste. Magazine of Concrete Research, Volume 29, Number 98, March 1977 pp 26-30.
.
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