Creep and drying shrinkage of calcium silicate pastes I. Specimen preparation and mechanical properties

CEMENT and CONCRETERESEARCH. Vol. 8, pp. 591-600, 1978. Pergamon Press, Inc. Printed in the United States.

CREEP AND DRYING SHRINKAGE OF CALCIUM SILICATE PASTES I. SPECIMEN PREP~RATION AND MECHANICAL PROPERTIES

S. Mindess,* J. F. Young and F. V. Lawrence Department of Civil Engineering University of Illinois at Urbana-Champaign Urbana, Illinois 61801

(Communicated by S. Diamond) (Received June 30, 1978)

ABSTRACT In this first paper of a series dealing with the creep and shrinkage of calcium silicate pastes, the materials, specimen preparation methods, mechanical test procedures and results are discussed. Pastes were prepared with a w/s ratio of 0.4 using C3S, B-C2S or a C~S/CgS blend. Thin-wall, hollow-cylinder specimens were cast and §ubjected-to ~arious conditions of load and drying. The structural and chemical modifications resulting from these treatments will be covered in subsequence papers. In dieser erste Beitrag einer Serie, die das Kriechen und Schrumpfen von Kalzium Silikatstein umhandelt, werden die Materialen Probenpriparierung, mechanische Testverfahren und Resultats dikutiert. Kalzium-Silikatstein mit einem Wasser-Feststof Verh~itnis yon 0.4 wurden vorbereitet aus C^S,~ B-C~S oder eine Mischung aus C3S/C2S. Dunn-Wand, HSle-Zylinder-proSen wurgen ausgeformt und verschiedene-Zust~nde von Belastung und trockenleit unterworfen. Die Struckturellen und chemischen Anderungen als Resultat der Berhandlungen werdem in nachfolgenden Beitragen diskutiert.

*Department of Civil Engineering, University of British Columbia, Vancouver, Canada

591

592

Vol. 8, No. 5 S. Mindess, J.F. Young, F.V. Lawrence Introduction

The time-dependent dimensional changes of hydrated cement paste, drying shrinkage and creep, are properties of great concern to engineers. However, despite considerable study and the development of useful predictive equations, there is still no consensus as to the fundamental mechanisms responsible for these volume changes. This is, perhaps, due to the fact that most theories that have been proposed are based on microstructural and physico-chemical characteristics of the paste which are themselves still subject to differing interpretations. It is generally agreed that creep and shrinkage of cement pastes result from atomic scale changes. Some of the hypotheses that have been advanced are: decreased spacing between C-S-H particles (hindered adsorption) (i); changes in interlayer spacing (2); micro-shearing of C-S-H sheets (3,4); changes in chemical bonding (5); surface diffusion of solid material (6); and changes in disjoining pressure (4). These and other explanations of creep and shrinkage have been reviewed by Neville (7) and Brown and Hope (8). In a discussion of creep and shrinkage, it is convenient to use the terms defined by Neville (7) and illustrated in Fig. i. Drying shrinkage in the absence of load is designated e ~, and basic creep in the absence of rh change is e, . When loading and drying occur simultaneously, the total • DC tlme-dependent (creep) strain, etc' includes a drying creep component, Edc, in excess of basic creep and shrinkage,

i.e.,

etc = esh + Sbc + edc The elastic strain, c , is assumed to be independent of treatment and is sube tracted out at the beginnmng of the experiment.

Shrinkoqe

(ih .... Time

P

Fig. i BASK; Creep

I to

(e = tf Time

P

Load 8, D r y i ~

to

tf Time

Definitions of Strain Occurring in Shrinkage, Basic Creep and Loading and Drying Specimens.

Vol. 8, No. 5

593 CREEP,SHRINKAGE, DRYING, CALCIUM SILICATE PASTES Scope

Most previous studies of creep and shrinkage have consisted of measurements of time-dependent deformations as a function of water:solids ratio (w/s), degree of hydration (~), loads to ultimate compressive strength ratio ( o / f ) , relative humidity (rh) and temperature. Only a few studies C . . (9-15) have attempted to determlne the mlcrostructural and chemical changes induced by creep and shrinkage. The intent of this present study has been to elucidate the microstructural changes in cement pastes which result from creep and shrinkage and to relate these changes to observed mechanical behavior - reversible and irriversible creep and shrinkage strains. The experimental plan is shown in Fig. 2. Specimens were subjected to various combinations of load and drying: * * * *

Shrinkage (SH) - drying to 53 percent rh Basic Creep (BC) - loading under i0 percent of fc and at i00 percent rh Load and Drying (LD) - loading under i0 percent rh while drying to 53 percent rh Control (CT) - maintained at i00 percent rhwithout application of load

Measurements were made of the time dependent deformations resulting from each of the above experimental conditions and are reported in this paper. Particular attention was paid to the irrecoverable strains after the removal of load and/or after resaturation. The microstructural changes induced in the C-S-H by these mechanical and hygral pretreatments were then studied through determination of surface area, pore size distribution and degree of silicate polymerization. The results to be presented are for pastes prepared from pure calcium silicate: C3S, C_Sand a C3S/C2Sblend. These pure systems are far simpler than portland cement and ~ l l o w the microstructural and chemical changes to be more easily followed and more readily interpreted. An interpretation of the results will be given in subsequent papers (20,21).

Fig. 2 Schematic Plan.

I00

_i

of Experimental

53

Jo

30

20 If p' O

wt.

wt., H$, Si-pol

I

I

T@m*

I

-

,

594

Vol. S. Mindess, J.F.

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Young, F.V. Lawrence

Experimental ~[aterials Characterization The monoclinic C3S used in this study was obtained from the2Portland Cement Association and was ground to a Blaine surface area of 3800 cm /g. The ~-C2S, synthesized from reagent grade Ca0 and Si02, was stabilized with 0.5 percent B203 and ground to a Blaine surface area of 3700 cm /g. The compositions of these materials are given in Table i. X-ray diffraction analysis of both Table 1 Oxide Analyses of the Calcium Silicates

Free

CaO*

Si02

A1203+ Fe203 Mg0

C3S

71.33

25.70

0.00

0.14

~-C2S

64.06

33.28

0.12

0.i0

Na20 K20

S03

Ca0 LOI** Insol. #

0.64

0.01

0.00

0.05

0.72

0.76

0.22

0.48

0.00

0.00

0.04

0.00

1.00

0.44

*Excluding free lime; + includes Ti0~ and P^0.. ,!~ Z .b **LOI = Loss on Ignition; ~r Insol. = Insoluble Residue.

materials showed no other phases. and was used as a third material.

A blend of 50 percent C3S and C2S was prepared

Pastes were prepared by vacuum mixing with de-ionized water using a w/s ratio of 0.4 Cubes (12.7 mm) were cast for the determination of compressive strength with time, as shown in Fig. 3. Fragments of these specimens were dried and used to determine degree of hydration (~) using quantitative x-ray diffraction (QXRD) (see Fig. 4) and, in some cases, nonevaporable water methods. For the C3S/C2S blend, ~ was taken as the weight fraction of the reacted silicate which is equal to the average of the of the C3S and C2S constituents. A cross-plot of compressive strength and ~ is

14,00(: I 2 ,0 0 (

I 0,000 i

Fig. 3 Compressive Strength as a Function of Time.

8000 6000

4OOO ZOO0 00

I0

20

30

40

~0 60 Time (cloy}

70

80

90

I00

Vol. 8, No. 5

595 CREEP,SHRINKAGE, DRYING, CALCIUM SILICATE PASTES I(Xl C3$

8(

Fig. 4 <~

Degree of Hydration as a Function of Time.

6(:

4C =

~

2C l

IO

[

20

I

t

30

I 50

40

I 60

I 70

[

80

!

90

iO0

Time (doy)

shown in Fig. 5. All three materials show a similar variation of strength with a particularly at high a. At a given, low ~, C3S pastes are less strong than either the C2S or the C3S/C2S pastes. t4,000 0

~2poc - -

Fig. 5 Compressive Strength Versus Degree of Hydration (a). The Compressive Strengths of the Pastes Tested are Indicated by the Arrows.

0

C3S CzS

~/ e/

~C3S

/./

I 0,0(~ --

8000--

C 3S/C2$

~C3S/C2S

400C ~'21:XX

C3$

~CzS 20

Specimen Preparation

40

60

Degree of Hydration

80

I00

(doy$)

Creep and shrinkage specimens were cast in the form of thin walled, hollow cylinders having the following dimensions: 15 mm O.D., 13.2 nun I.D., (0.9 mm wall thickness) and as-cast length of I00 mm. This type of specimen permits the rapid attainment of hygral equilibrium and has been previously used by Mullen and Dolch (16), Jessop (17) and Bazant (18,19). The specimen mold is shown in Fig. 6 and was patterned after those of Bazant (19). The following de-molding procedure was used. Since the cylinders were very fragile at early ages, they were left in the mold until a paste compressive strength of I000 psi was attained. Then, liquid methanol, cooled with dry ice, was poured through the core of the mandrel for 5-8 minutes, causing it to contract and separate from the paste cylinder. The bolt which kept the mold closed was loosened and the mold and cylinder were then placed in water for 20-25 minutes after which the cylinder was slowly pushed out by hand. Only a 50 percent success rate was experienced; many cylinders cracked or contained holes. After de-molding, the cylinders were placed in lime water at 23°C until the desired ~ was reached; they were then stored in water at about 4°C until they were tested. Further hydration was greatly reduced but not completely eliminated by this low temperature storage. Before testing, the specimens were trimmed with a diamond saw to a length of about 77 mm, and the ends were ground plane and parallel.

596

Vol. 8, No. 5 S. Mindess, J.F. Young, F.V. Lawrence

~

PtexiqlaslCap

Brass Moldn Mandrel

PasteCylinder StOllllessSteelTube Stainless SteelRod BrassBody Atmos~lreinlet O~tlet .

---

0- RinqSeal LVDTCore LVDT

$pri~ - - B r a s s Tube

Threaded

Fig. 6

BrassBase

J

Cross-Section of Mold for Thin-Walled Paste Cylinders.

Rod

Fig. 7 Cylinders were prepared having the following ~: C3S: ~ = 42% (low)

C2S: ~ = 29% (low)

Creep and Shrinkage Cells C3S/C2S:

= 85% (high)

~ = 48% (low) = 69% (high)

Mechanical Pretreatment - Creep and Shrinkase Testing Creep tests were performed on the thin-walled, hollow cylinders using the specially designed loading apparatus shown in Fig. 7. Shrinkage tests were performed in a s i m i l a r apparatus but without the application of

Fig. 8 Pump

h

~ ~

~J

CreepCell

CO;,

Scrubber

Cell

RH

Bath

Circuit for Control of Relative Humidity.

Vol. 8, No. 5

597 CREEP, SHRINKAGE, DRYING, CALCIUM SILICATE PASTES

load. Creep and shrinkage test cells being subjected to a given rh were interconnected as shown in Fig. 8. A completely closed system was used: the test atmosphere was maintained at the desired rh by passing the air through a large resevoir containing a saturated salt solution, and C02 was scavenged from the system with a Na0H solution. Test humidities of i00 percent rh and 53 percent rh (satd. Mg(N03)2"6 H20) were used, and the test temperature was approximately 25°C. The creep specimens were loaded in a universal testing machine to a stress/strength ratio 0.i. This load was maintained by a spring (see Fig. 7) having a spring constant (0.71 Kg/mm) which permitted small length changes of the specimen without any appreciable reductions in load. Both the creep and shrinkage strains were measured with LVDTs with an accuracy of 0.0025 mm. The times of loadin~ and/or drying were fixed at i, i0 and 30 days; in some cases measurements were made at 2 days. No attempt was made to attain steady state or to determine terminal readings during the loading and/or dryingpretreatment periods. However, at the end of thispretreatment period, the specimen were unloaded andresaturated (100% R.H.) until no further recoverable strain could be observed (2-7 days). Results and Discussion Typical strain vs. time curves of LD, BC and SH specimens are shown in Figs. 9-13. Individual data points are not shown, but each curve represents the average of two different tests. In order to facilitate comparisons of the time-dependent strains, the elastic strain (e e) which occurred upon loading and unloading have been subtracted. It should be noted that when the specimens were first placed in the creep or shrinkage cells, some free water was also inadvertently introduced into the cells. Therefore, drying shrinkage did not begin till this free water had disappeared, usually a matter of several hours. The times shown in Figs. 9-13 have been corrected for this "pre-drying" period. The value of ~ noted for the specimens are those at the time when the specimens were placed in the creep and shrinkage cells. It was assumed that further hydration would not take place during the tests. This assumption is probably justified for the SH and LD specimens but is not strictly valid for the BC specimens and CT specimens which were maintained at a relative humidity of 100% during the entire test. Corrections were made for changes

2O

r

~

C35 -42% LooO ~ Ory~fXl - - - - - - Shr ,nkoqe as,¢ Creep

,

,

,

r

,

,

T

.

.

.

.

.

.

.

,

C3S - 8~ % • Load & Oryifx; - - - - - - Shr mllClg~l ..... 8as.: Cte4m

-

S

Z

I \ \

I

2

3

4

5

6

7 T,me

8

9

~E]

II

I~

J'3

~4

15

(days) T;me

Fig. 9 Time-Dependent Strain as a Function of Time. C3S - 42%

(days)

Fig. i0 Time-Dependent Strain as a Function of Time. C3S - 85%

598

Vol. S.

Mindess,

J.F.

25

Young,

C35/CZ$ -4.8% -----......

F.V.

25[

-

Load ~ Otv,ng Snt ,n~age 9as~c Creep

2O

8,

No.

Lawrence

:3S/CzS -----......

2O

- 69%

Sn,,nwQqe 9QS,C Zteep

'0

iSF

,i \ \

/I ,'.. ...... "'i 0

I

2

3

' 5

6

:'''° 7 T me

9

9

0

"u

,2

1'3

14

ot --":~-.

15

:CyS)

. . . . . . . . . . . . .

..................................... T,me

Fig. ii Time-Dependent Strain as a Function of Time. C3S/C2S - 48%

25

(~cy$)

Fig. 12 Time-Dependent Strain as a Function of Time. C3S/C2S - 69%

CzS-Z9 % Load ~ Dry."9 - - - - - Sflt tnWaqe

2o

'c2 ~

15

Fig. 13 Time-Dependent Strain as a Function of Time. C3S - ~-9/, Ic

\ \\

i -~-3

4

~

6

7 s Time

9

,o

,

,2 ,3 ,4

{dc~sl

in ~ in the case of C^S pastes of low ~; in the other pastes, were considered to ~e within experimental error.

the changes in

Table II summarizes the creep and shrinkage data in terms of total strain at the end of the loading/drying period and also the amount of irreversible strain after unloading and resaturation of the specimens. Conclusions A detailed interpretation of the data will be found in Part III (21) of this study, where it will be discussed in conjunction with changes in the properties of the C-S-H. However, some general observations can be made at this time: i) The creep and drying shrinkage of pure calcium silicate pastes is less than that of portland cement under the same conditions. However, the variation with time is very similar to that of portland cement, as was observed by Helmuth & Turk (5) for drying shrinkage. 2) The profiles of the creep and drying shrinkage curves are very similar. The rate of deformation is most rapid during the first 24 hours (when water is being lost), but deformation continues even after the specimen has attained apparent hygral equilibrium.

5

Vol. 8, No. 5

599 CREEP, SHRINKAGE, DRYING, CALCIUM SILICATE PASTES

Table 2 Creep Test Results

Total and (Irreversible) Paste Composition

Degree of Hydration

i-DAY

Creep Strain (xlO -4)

IO-DAY

Basic Creep

Shrinkage

Load + Drying

Basic Creep

Shrinkage

30-DAY Load + Drying

C38

42% (low)

1.2 (0.i)

12.7 (3.8)

i~.0 (9.9)

2.0 (0.6)

18.7 (8.1)

23.5 (16.8)

C38

85% (high)

3.8* (0.6*)

ii.8" (5.0*)

21.7" (12.9")

5.8 (2.7)

15.7 (6.8)

31.6 (15.0)

C28

29% (low)

1.8 (0.i)

14.5 (6.2)

18.4 (12.0)

3.1 (2.0)

19.2 (13.0)

20.9 (15.6)

C3S/C28

48% (low)

2.0 (i.0)

9.2 (2.4)

13.3 (3.6)

3.8 (0.6)

14.6 (6.5)

18.3 (11.6)

C38/C28

69% (high)

1.7 (0.8)

12.7 (2.7)

17.0 (4.i)

2.8 (1.3)

17.8 (8.0)

24.6 (10.5)

Basic Creep

Shrinkage

Load ~ Drying

NA NA

19.0 (9.0)

34.0 (22.0)

)2-day results

creep. low ~.

3) As has been widely noted, well hydrated pastes exhibit drying However, no significant drying creep was observed with pastes of

4) The amount of irreversible strain increases with the time of loading and drying. After the first 2 days, the additional strain is predominantly irreversible and drying creep is also mostly irreversible. Moreover, the percentage of irreversible strain was greater for specimens with low ~ that were subjected to both loading and drying even though no drying creep was observed. Acknowledgements This study was sponsored by the National Science Foundation under the grant NSF ENG 75-00422. The authors wish to acknowledge the help and assistance of their co-workers -- Dr. A. Bentur, Dr. N. B. Milestone, Professor R. L. Berger and Professor J. A. Nelson. Portions of the experimental work were carried out by Mr. J. Kung and Mr. J. O'Conner. Mr. P. S. Grindrod and the Canada Cement Lafarge Ltd., Belleville, Ontario kindly supplied the chemical analyses. References i.

T. C. Powers, in A. E. Brooks and K. Newman (eds.), "The Structure of Concrete," Proc. Intern. Conf. London, 1965, p. 314. Cement and Concrete Association, London (1968).

2.

R. F. Feldman,

3.

W. Ruetz, Ref. i, pp. 365-387.

4.

F. H. Wittman in "Hydraulic Cement Pastes: Their Structure and Properties," Proc. Conf. Sheffield, 1976, p. 96. (Cement and Concrete Association, Slough, 1976.

Cem. Concr. Res., i, 521 (1972).

600

Vol. 8, No. 5 S. Mindess, J.F. Young, F.V. Lawrence

5.

R. A. Helmuth and D. Turk, Journal of the PCA Res., Dev. Labs., 9, 8 (1967).

6.

Z. P. Bazant and Z. Moschovidis, J. Amer. Ceram. Soc. 56, 235 (1973).

7.

A. M. Neville, "Creep of Concrete: Plain, Reinforced and Prestressed." (North Holland Publ. Co., Amsterdam, 1970).

8.

N. H. Brown and B. B. Hope, "Theories of Creep of Concrete," Civil Engineering Report No. 72, Queen's University at Kingston, Ontario, July (1972).

9.

C. M. Hunt, L. A. Tomes and R. L. Blaine, J. Res. Natl. Bur. Stds. 64A, 163 (1960).

i0.

D. N. Winslow and S. Diamond, J. Amer. Ceram. Soc. 57, 143 (1974).

ii.

R. H. Mills, in M. Te'eni (ed.), "Structure, Solid Mechanics and Design," Part I, Proc. Civil Eng. Mater. Conf., Southampton, 1969, p. 751. (Wiley Interscience, London, 1971).

12.

R. F

Feldman, Cem. Concr. Res., i, 285 (1971).

13.

R. F

Feldman, Cem. Concr. Res., ~, 777 (1973).

14.

R. F

Feldman, Cem. Concr. Res., i, i (1974).

15.

B. B

Hope and N. H. Brown, Cem. Concr. Res., ~, 577 (1975).

16.

W. G

Mullen and W. L. Dolch, Proc. ASTM, 64, 1146 (1964).

17.

E. L Jessop, "Creep and Creep Recovery in Concrete Materials," Ph.D. Thesis, Civil Engineering Department, University of Calgary, 1964.

18.

Z. P. Bazant, I. H. Hermann, H. Koller and L. J. Najjar, Mater. Constr. (Paris) 6, 277 (1973).

19.

Z. P. Bazant, A. A. Asghari and J. Schmidt, Mater. Constr. 279 (1976).

20.

A. Bentur, N. B. Milesone and J. F. Young, "Creep and Drying Shrinkage of Calcium Silicate Pastes II. Induced Microstructural and Chemical Changes," to be published in this journal.

21.

A. Bentur, R. L. Berger, N. B. Milestone, S. Mindess, F. V. Lawrence and J. F. Young, "Creep and Drying Shrinkage of Calcium Silicate Pastes III. Hypothesis for Irreversible Strains," to be published in this journal.

(Paris), 9,

A