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Cross-sectional strength gradients in high strength concrete columns
CEMENT and CONCRETE RESEARCH. VoL 24, pp. 139-149, 1994. Printed in the USA. 0008-8846/94. $6.00+00. 1993 Pergamon Press Ltd.
CROSS-SECTIONAL STRENGTH GRADIENTS IN HIGH STRENGTH CONCRETE COLUMNS S.L.Mak*, M.M.Attardt, D.W.S.Ho* and P.LeP.Darvallt * CSIRO Division of Building, Construction and Engineering P.O. Box 56, Highett, Victoria 3190, Australia t Department of Civil Engineering, Monash University, Clayton, Victoria 3168, Australia (Refereed) (Received Oct. 21, 1991; in final form Sept. 30, 1993)
ABSTRACT Vertical variations of strength in a structural element is a well-established phenomenon. In large cross-sections, horizontal strength gradients can also occur because of local differences in humidity and temperature conditions. When the heat of hydration of concrete is free to dissipate from a cross-section, the temperature variations within a cross-section imply not only differing rates of strength development but also a risk of thermal cracking due to high temperature gradients. A series of columns of two cross-sectional dimensions were cast in the laboratory for concrete mixes with 28-day compressive strengths ranging from 40 to 115 MPa. Compressive strength results of vertically drilled cores showed the presence of significant cross-sectional strength gradients. Three-dimensional core strength distributions were constructed from which effective cross-sectional strengths of unreinforced in-situ concrete were calculated. Introduction The presence of strength gradients in the vertical direction of a structural element is well known. The in-situ core compressive strength of concrete at the top of a structural element is generally weaker than the concrete at the bottom (1-8). This phenomena has been attributed to a combination of the rise of bleeding water, segregation, hydrostatic pressure and revibration of the preceding concrete layers. In large structural elements, and especially when cross-sectional temperature and humidity conditions vary greatly, horizontal cross-sectional strength gradients can also occur. This may be due to differing rates of strength development at different locations within a cross-section because of different temperature and humidity histories. In addition, high temperature gradients within a cross-section result in thermal cracking which can cause strength loss (9-11). High strength concrete columns can experience a very high temperature rise in the first few days of cement hydration (12,13). This is due mainly to the high cement contents typically used in high strength concrete mixes. Depending on cross-sectional dimensions, ambient conditions, the insulative properties of formwork, formwork stripping times and the provision of thermal curing, large temperature differences can occur within a cross-section. 139
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A laboratory-based investigation of in-situ strength properties of square unreinforced concrete columns was undertaken for concrete mixes with 28-day cylinder strengths ranging from 40 to 115 MPa (6). This paper presents the cross-sectional strength characteristics at 28 days.
Experimental Investigation Concrete mixes and materials Six mixes with 28-day specified standard moist-cured cylinder strengths ranging from 40 to 115 MPa were investigated. Mix proportions are shown in Table 1. Mixes P32C, P50C, P70C and P100C were plain mixes which did not contain supplementary cementitious materials. Mixes P100S and PI20S contained silica fume. Mixes P32C and P50C will be referred to as normal strength mixes, whilst the remaining are high strength mixes. Mix P32C was supplied commercially and mix details are not available. A formaldehyde condensate type superplasticiser was used for all mixes except P32C and P50C. Superplasticiser dosages were added to attain a minimum slump of 80 mm. Type A Normal Portland cement was used for all the mixes. Table 2 shows the chemical and physical characteristics, provided by the suppliers, of the cement and silica fume. The coarse aggregate was a 14 mm old basalt with a specific gravity of 2.95 and 24-hour absorbence of 1.8%. The sand had a specific gravity of 2.65 and 24-hour absorbence of 0.5%. Sepcific gravity was determined on an SSD basis. TABLE 1 Mix Proportions Per m 3
Cement (kg) Silica fume (kg) Total aggregate (kg) Des. water (kg) Superpl. (1) Slump (mm) Water/binder ratio 28-day cylinder strength* (MPa) Initial concr, temp. (°C)
P32C
P50C
P70C
P100C
P100S
P120S
not available
40.5
380 0 2000 170 0 120 0.45 60.5
500 0 1925 150 5 150+ 0.30 69.0
550 0 1935 150 6 80 0.27 93.0
460 40 1920 150 6 130 0.30 95.5
500 45 1920 135 10 150+ 0.25 115.5
25.3
17.7
26.2
27.5
27.3
18.9
* Median strengths from sets of four 100 × 200 mm cylinders cured in lime-saturated baths. Casting program Square unreinforced columns of two cross-sectional dimensions (400 × 400 mm and 800 × 800 mm) were investigated. The 800 × 800 mm cross-section was similar to those used in the Melbourne Central project, whilst the smaller 400 × 400 mm cross-section provided a comparison of member size effects. The columns were 1200 mm high. All columns were cast under shelter inside the concrete laboratory. For the duration of the project, the mean ambient temperature in the laboratory ranged between 14 and 21°C and the mean relative humidity was 60%.
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TABLE 2 Chemical and Physical Characteristics of Type A Normal Portland Cement and Silica Fume, as Provided by the Suppliers Type A Normal Portland cement SiO 2 A 1 2 0 3 + Fe203 (%) TiO 2 (%) (%) 21.0 5.8 2.9
CaO (%) 64.1
C3A (%) 10.5
C4AF (%) 9.0
C3S (%) 51.0
C2S (%) 22.0
LOI (%) 1.6
SSA (m2/kg) 300
Silica fume SiO 2 A1203+ (%) TiO2 (%) 92.3 0.99
CaO (%) 0.27
C (%) 2.7
S (%) 0.11
P205 (%) 0.08
MnO2 (%) 0
LOI (%) 2.55
SSA (m2/g) 17-20
Fe203 (%) 0.87
The columns were cast in layers to allow easy separation and coring in the vertical direction. Figure 1 shows a column in elevation. Each column consisted of primary layers (layers 1, 3 and 5), from which cores were taken, and sacrificial layers (layers 2, 4 and 6), which separated the primary layers. The primary and sacrificial layers were separated by 5 mm masonite sheets which minimised moisture movement in the vertical direction. A segmented steel formwork system was used and no thermal insulation was provided on the forrnwork walls. However, the top and bottom ends of the columns were insulated with 25 mm thick expanded polystyrene foam sheets. The type and thickness of insulation was determined after a trial pour was conducted where vertical temperature distributions within a column were evaluated (6). Formwork was removed after 70 hours, when temperature measurements were also stopped. The external joints between the primary and sacrificial layers were sealed with a silicone sealant and taped over with waterproof electrical ducting tape to minimise moisture loss from the boundaries. 1,3,5 - Primary 2,4,6 - Sacrificial
Polystyrene Insulation
I
100 1200 300
400 or 800
FIG. 1 Column in elevation.
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One set of columns of each size was then covered with plastic sheeting whilst the other was left exposed to the laboratory air (6). The results reported in this paper are for the columns exposed to air, similar to an internal column of a building. Temperature measurements A total of 16 thermocouples were placed on a horizontal plane within each 800 × 800 mm crosssection in an octant, as shown in Fig. 2. In the 400 x 400 mm columns, temperatures were monitored with three thermocouples. Temperature readings were recorded at 20-minute intervals. Coring and testing In-situ strengths were evaluated by means of coring each primary layer in the vertical direction. Coring in the vertical direction was not only consistent with the direction of casting and loading but also provided sufficient core samples for studying cross-sectional strength properties. A supplementary investigation was undertaken to evaluate the effect of possible coring damage on core compressive strengths (6). For four normal and high strength mixes, 100 mm diameter cores were drilled from 150 × 300 mm moist-cured cylinders at 28 days and trimmed to 200 mm in length. Table 3 shows the core strength results, as well as 100 and 150 mm diameter moistcured cylinder su'engths. When compared to 100 × 200 mm cylinder strengths, the core strengths were all marginally higher at 28 days. The same trend was observed when core strengths were compared with 150 × 300 mm cylinder strengths. On the basis of these results, it was concluded that no core strength reduction could be attributed to the coring process. Using the coring patterns shown in Fig. 3, a total of 17 cores were cut from each of the 800 × 800 mm cross-sections, whilst 7 cores were cut from the 400 x 400 mm crosssections. To enable construction of three-
800
400
20O 100
4 i
31
21
i-." 3
"/
230
170
FIG. 2 Thermocouple locations in 800 x 800 mm cross-section.
dimensional strength profiles, strength zones were designated for each cross-section. The strength zones are labelled A to D and E to F for the 800 × 800 mm and 400 × 400 mm cross-sections respectively, as shown in Fig. 3. All the cores were approximately 300 mm in length, with nominal diameters of 95 mm. After trimming from both ends to between 190 and 200 mm, the average height to diameter ratios of cores were between 2 and 2.1. All trimmed cores were capped on both ends with a high strength sulphur capping compound. Capped cores were allowed to cure in the air for at least five hours before being tested to ensure sufficient strength in the sulphur caps. All cores were tested in a 5000 kN AMSLER testing machine, in accordance with AS 1012.9-86 (14). The mean coefficient of variation for sets of four cores from all strength zones and mixes was 5% at 28 days.
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TABLE 3 Compressive Strength Comparison at 28 days of 100 x 200 mm Cores Extracted from 150 x 300 mm Moist-cured Cylinders with 100 x 200 mm and 150 x 300 mm Moist-cured Cylinders Mix
Compressive strength (MPa) 100 × 200 mm cores from 150 x 300 mm cylinders
100 x 200 mm cylinders
150 x 300 mm cylinders
42.5 61.0 91.0 116.0
40.5 60.0 87.0 115.0
40.0 60.0 92.5 114.5
P32C P50C P 100L* P120S
* Mix P100L is a proprietary mix containing slag, studied as part of another project. Standard cylinders were scheduled for testing at 7, 28, 90 and 450 days, whilst cores were to be cut and tested at 28, 90 and 450 days. For mixes P32C and P50C, only 28-day cores were taken.
Results and Discussion Cross-sectional core strength distributions The extensive coring undertaken made it possible to examine cross-sectional strength properties in detail. Table 4 shows the core strength results obtained at 28 days for the various strength zones. The distribution of core strengths taken along the diagonal of an 800 x 800 mm cross-section is shown in Fig. 4. The values plotted for each strength zone are median values of sets of cores from the same strength zone. Cross-sectional strength gradients were found in all columns for all mixes. The highest core compressive strength within a cross-section was found at the center. Between the center and the 800 X S 0 0
O
O
0
•
•
•
800
0
•
• •
¸
A - corner B - intermediate C - center
D - side
•
400 x400 E - corner F - center
•
•
•
•
•
•
wit
'" .200 ~' 21110 '
4"
400
FIG. 3 Coring pattern for 800 x 800 and 400 × 400 mm cross-sections.
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TABLE 4 28-day Core, Effective and Standard Moist-cured Cylinder Strengths (MPa) and Core Strength Ratios P32C 400 x 400 mm Comer (E) Center (F) Effective 800 x 800 mm Comer (A) Side (D) Interm. (B) Center (C) Effective 100 mm cylinder Strength ratios f(E)/f(F) f(A)/f(C) f(B)/f(C) f(D)/f(C) fe(400)/f(F) fe(800)/f(C)
P50C
34.5
53.0 54.5 55.5 57.0 55.0 60.5
36.5 38.5 36.5 40.5
0.90 0.95
0.93 0.97 0.96
0.95
0.96
P70C
P100C
P100S
P120S
51.5 61.0 57.0
80.5
70.5 81.5 76.5
93.5 106.0 100.5
56.0 57.0 59.0 63.0 58.5 95.5
86.5 83.0 83.0 93.0 84.5 115.5
46.0 49.0 52.0 58.0 52.0 69.0
73.0 77.5 81.5 78.0 93.0
0.84 0.79 0.89 0.84 0.93 0.90
0.87 0.89 0.94 0.91 0.91 0.93
0.90 0.95
0.96
0.88 0.93 0.89 0.89 0.95 0.91
f(A-F) = median core strengths at each strength zone. fe(400) = effective strength of 400 × 400 mm column. fe(800) = effective strength of 800 × 800 mm column. 100 90-
~
P120~
~
PlO0(
/
80" 7o-
u
5040-
- ~ p a 2 c
o
"-' 302o-
lo-
115
230
3,45
460 575 690 805 Distancealongdiagonal(nun)
920
1035
1150
FIG. 4 Median core strength distribution along diagonal of 800 x 800 mm cross-sections at 28 days.
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outer regions of a column, a gradual and substantial strength reduction occurred. The lowest strength within a cross-section was generally found in the corner strength zone. Comparing cores taken from the comer to those taken from the center, the comer to center strength ratios, f(A)/f(C) and f(E)/f(F), were calculated and are shown in Table 4. In the 800 x 800 mm crosssections, cores from the center zone were generally 7-10% stronger than the cores at the comer zones. For mix P70C, the cores from the corner were 20% weaker than those from the center. The strength gradients could have been caused by a combination of factors, which include differences in compaction, higher rates of evaporation in the outer zones, different rates of hydration due to different temperature histories and thermal microcracking. Uniformity of compaction was assessed by examining the core density distributions of each cross-section. Surface dry densities of cores in the outer strength zones, measured immediately after trimming, were not consistently lower than cores from the center. No systematic link between density variations and strength gradients could be found (6). Under laboratory air-drying conditions, the extent of moisture loss and its effect on the effective strength of a cross-section becomes less important in large specimens (15,16). The concrete at the center of a large column is not likely to be affected by moisture loss due to evaporation. In the outer zones, the effect of evaporation is unlikely to extend beyond the cover region of a column. Limited humidity measurements taken in an 800 x 800 mm column for mix P120S at 28 days, showed that the relative humidity at the comer strength zone (A) was 84% compared to 80% at the center (C) of the column (6). These results were also observed in the 400 x 400 mm columns. Therefore, there was no evidence of a reduction in humidity at the comer strength zone (A). The slightly higher humidities in the outer regions of a column suggest that the local variations in humidity were governed by different rates of moisture consumption due to different hydration rates. The heat evolved during cement hydration was free to dissipate to the surrounding since the formwork walls were not thermally insulated. Therefore, at each strength zone, the local temperature-time relationships were different. As a result, temperature regimes varied greatly within a cross-section. Table 5 shows the maximum rise in temperature above the temperature of concrete at the time of casting for each strength zone. As expected, the highest temperatures occurred at the center of a column. The side zones (D) had higher temperatures compared to the TABLE 5 Maximum Temperature Rise, T n (°C), Above Concrete Temperature at Time of Casting for Various Strength Zones
800 x 800 mm Comer (A) Side (D) Interm. (B) Center (C) 400 x 400 mm Comer (E) Center (F) Tn(C) - Tn(A)
P32C
P50C
P70C
P100C
P100S
P120S
10.4 13.8 17.1 22.0
13.5 17.8 21.3 26.9
20.7 29.2 34.4 42.4
18.1 27.1 33.7 37.0
16.3 23.0 29.1 39.2
16.0 21.8 25.5 33.0
20.7 26.1
18.1
13.2 17.8
12.9 16.5
21.7
18.9
22.9
17.0
11.6
13.4
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corners (A) because the exposed surface area for heat dissipation was lower at the side when compared to the corner. The maximum temperature rise at the center of the 800 × 800 mm cross-sections were generally double that at the center of the 400 × 400 mm cross-sections. Different local temperature regimes suggest that the rates of early strength development will vary within a cross-section. It also implies that both the medium- and long-term strengths within a cross-section may be variable. It is well known that high curing temperatures can lead to lowered medium and long-term strengths (9,15,17,18). The strength development of silica fume concretes is also very sensitive to high temperatures, because the initiation and extent of pozzolanic reactions are temperature sensitive. In the 800 × 800 mm cross-sections, the early rate of hydration in the center of a column would be faster than that of the outer strength zones because of the higher temperatures. If the development of compressive strength can be simply related to maturity or a unique temperature/time relationship, the differences between center (C) and outer zone (A and D) 28-day strengths in the 800 × 800 mm cross-sections would be consistent with the differences in accumulated maturity. But, the core strength results from the center zone (F) of the 400 × 400 mm columns contradict this assumption. The accumulated maturity at 28 days for the concrete at the center of the 400 × 400 mm columns (F) would have been lower than that of the center of the 800 × 800 mm columns (C) by virtue of the lower temperatures achieved. However, the strength of zone F cores were the highest of all the strength zones considered. This suggests that the lower temperatures in the small columns led to higher 28-day strengths. Therefore, the cross-sectional strength gradients could not be accounted for by differences in concrete maturity but may be influenced by another factor. Large temperature variations within a cross-section will induce stresses which can cause thermal microcracking. These stresses result from volume changes which occur both in the heating and cooling phases during the first few days of cement hydration, and is brought about by the incompatible thermal expansion characteristics of the different mix constituents (9). The extent to which cracking occurs is also dependent on the degree of restraint to free expansion imposed by the cross-sectional geometry. If thermal microcracking occurs, the strength of the matrix and aggregate-paste interface could be impaired, resulting in the loss of compressive strength. Depending on ambient temperature conditions, the highest temperature gradients in the 800 × 800 mm cross-sections were found near the surface and generally within the outer 100 mm of a column. In the corner strength zones of the 800 × 800 mm cross-sections, maximum thermal gradients ranged from 47°C/m for mix P50C up to 99°C/m for mix P100S (6). The lowest strength gradients were found for mixes P32C and P50C which also had the lowest thermal gradients. Figure 5 shows a contour plot of maximum thermal gradients within an 800 × 800 mm cross-section for mix P70C. The regions of highest thermal gradients corresponded with strength zones A and D which had the lowest core strengths within a cross-section. Based on the temperature data in Table 5, mixes P70C and P100S showed the largest difference in maximum temperatures between the center and the corner of an 800 × 800 mm cross-section. This also corresponded to the lowest f(A)/f(C) ratios for all the mixes investigated, i.e. 0.79 and 0.89 for P70C and P100S respectively. However, it must be noted that for a particular cross-sectional geometry and thermal gradient, the extent of thermal microcracking would also depend on the thermal expansion properties of the concrete. This is, in turn, a function of mix proportions and mix constituents. Effective cross-sectional strength The presence of strength gradients effectively reduced the overall load bearing capacity of a cross-section. To incorporate the effect of strength gradients for uniaxial compression, an average load-bearing capacity of in-situ concrete or effective strength was calculated for each crosssection. To achieve this, three-dimensional core strength distributions for all the cross-sections were constructed (6). A typical core strength distribution in an 800 × 800 mm cross-section is
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FIG. 5 Distribution o f m a x i m u m thermal gradients in an 800 x 800 m m cross-section - mix P70C.
FIG. 6 Three-dimensional core strength distribution of 800 x 800 m m cross-section mix P100S at 28 days. -
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shown in Fig. 6. These profiles were constructed by gridding and interpolation of the median strengths of each strength zone, carried out using a proprietary software called SURFER. The median core strength of each strength zone is based on four core strength results, except for zones C and F where five and three cores were taken respectively. The effective strengths were calculated by integrating the volume enclosed under the three-dimensional core strength distributions (6). Effective strength results are shown in Table 4. To quantify the effect of the strength gradients, effective to center, or fe(400)/f(F) and fe(800)/f(C), ratios were calculated. These strength ratios provided a measure of the overall reduction of insitu load-bearing capacity in relation to the highest strength within a cross-section. The 28-day strength ratios are shown in Table 4. In the 800 x 800 mm cross-sections, for mixes P70C and P120S, the strength gradients led to a 10% reduction of effective strength when compared to the strength at the center of the column. For the other mixes, the reduction in load-bearing capacity due to strength gradients were approximately 5%.
Implications of cross-sectional strength gradients The non-uniform distribution of core compressive strengths across a cross-section suggests that engineering properties will also vary across a cross-section. For instance, the stress-strain behaviour of in-situ concrete will vary with depth from the surface of a column when large strength gradients are present. Apart from the obvious reduction of load-bearing capacity of the concrete in axial compression, the presence of strength gradients has important implications for structural design, especially for columns subjected to bending. Cross-sectional strength gradients also imply that the depth to which cores are drilled from the surface of a column will be important when cores are required for assessment of concrete strength compliance. Limited results of cross-sectional strength variations of horizontally drilled cores have been published (13). A significant reduction of concrete strength towards the outer region of a structural element imply that the concrete in the outer region is of lower quality when compared to the remainder of the cross-section. This has important implications for durability and fire resistance. The 28-day results show that in-situ strengths were appreciably lower than standard cylinder strengths. The discrepancies between in-situ and cylinder strengths were further aggravated because of the cross-sectional strength reduction by horizontal strength gradients. These discrepancies have important implications for the strength reduction factors used in column design which are intended to partially account for in-situ and cylinder strength differences. A more detailed discussion of the differences between in-situ and cylinder strengths has been provided elsewhere (6). Results from 90- and 450-day tests will be published as they become available.
Conclusions The distribution of core compressive strengths of in-situ concretes for a range of high strength concretes cast in 400 × 400 mm and 800 × 800 mm cross-sections were not uniform. Compressive strengths of vertically drilled cores were highest in the center of a cross-section. The reduction of core strength from the center to the corner of an 800 x 800 mm cross-section was generally 10% but was as high as 20% in a plain 70 MPa mix. The available evidence suggests that these strength gradients were a result of a complex combination of temperature and humidity effects. These included thermal microcracking, and differing rates and extents of strength development. To incorporate the effects of the non-uniform cross-sectional strength distributions, effective cross-sectional strengths were calculated. The presence of strength gradients in a column crosssection resulted in a reduced load-bearing capacity of in-situ concrete of between 5 and 10% for the mixes considered. The non-uniformity of cross-sectional strengths means that engineering properties can vary across the cross-section. This has implications for the design of columns in bending, durability and fire
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resistance. When cores are drilled from the surface of a column to evaluate the strength of in-situ concrete, the depth to which cores are drilled will also be very important.
Acknowledgments The technical and material support from PBM Concrete, CRL Pty Ltd, TEMCO Pty Ltd, CSIRO Division of Building, Construction and Engineering, and the Department of Civil Engineering, Monash University are gratefully acknowledged. The authors are particularly grateful to Mr M. Graham from Monash University and Mr R. Harrison and Mr E. Christie from CSIRO. This project formed part of the first author's PhD dissertation undertaken at Monash University with the support of a Monash Graduate Scholarship and Monash Research Scholarships.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
Bhargava, J., 'Strength of concrete members cast in deep forms', National Swedish Institute of Building Research, Document No. 5 (1965). Bhargava, J., 'Strength of concrete in high strength revibrated walls', National Swedish Institute of Building Research, Document No. 6 (1969). Bloem, D.L., 'Concrete strength measurement - cores vs. cylinders', Proc. American Society for Testing and Materials, 65, 668-696 (1965). Hang, A.K. and Jakobsen, B., 'In-situ and design strength for concrete in off-shore platforms', Proc. 2nd Int. Symp. on Utilization of High Strength Concrete, Univ. California, Berkeley, May (1990). Hoshino, M., 'Relationship between bleeding, coarse aggregate and specimen height of concrete', ACI Materials Journal, 86(2), 185-190 (1989). Mak, S.L., Attard, M.M., Ho, D.W.S. and Darvall, RLeR, 'In-situ strength of high strength concrete', Civil Engineering Research Report No. 4/1990, Monash University, 132 pp (1990). Petersens, N., 'Strength of concrete in finished structures', Transactions Royal Institute of Technology, No. 232, Stockholm, Sweden (1964). Sangha, C.M. and Dhir, R.K., 'Core and cube relationships in concrete', in Advances in Ready-mixed Concrete Technology, Ed. R.K. Dhir, Pergamon Press, UK (1976). Alexanderson, J., 'Strength losses in heat cured concrete', Proc. Swedish Cement and Concrete Institute, No. 43, Stockholm (1972). Carlson, R.W., Houghton, D.I. and Polivka, M., 'Causes and control of cracking in unreinforced mass concrete', Proc., ACI Journal, No. 7, July, 821-837 (1979). Springenschmid, R. and Breitenbucher, R., 'Technological aspects for high strength concrete in thick structural members', Proc. Utilization of High Strength Concrete Conf., Stavanger, Norway, Tapir Publishers, June, 487-496 (1987). Aitcin, P.C. and Riad, N., 'Curing temperature and very high strength concrete', Concrete International: Design & Construction, 10(10), 69-72 (1988). Yuan, R.L., Ragah, M., Hill, R.E. and Cook, J.E., 'Evaluation of core strength in high strength concrete', Concrete International, 13(5), 30-34 (1991). Australian Standard 1012, Part 9-1986, 'Method for the determination of the compressive strength of concrete specimens', Standards Association of Australia, Sydney (1986). Mak, S.L., Ho, D.W.S. and Darvall, RLeP., 'The effects of moist curing on the compressive strengths of some very high strength concretes', Proc. Concrete for the Nineties Conf., Leura, New South Wales, September, 12 pp (1990). Urpani, D., Mak, S.L. and Attard, M.M., 'Curing of very high strength concretes', Civil Engineering Report No. 1, Monash University (1991). Kjellsen, K.O., Detweiler, R.J. and Gjorv, O.E., 'Development of microstructures in plain cement pastes hydrated at different temperatures', Cement and Concrete Research, 21(1), 179-189 (1991). Klieger, E, 'Effect of mixing and curing temperature on concrete strength' ACI Journal, 54, 1063-1081 (1958).
CROSS-SECTIONAL STRENGTH GRADIENTS IN HIGH STRENGTH CONCRETE COLUMNS S.L.Mak*, M.M.Attardt, D.W.S.Ho* and P.LeP.Darvallt * CSIRO Division of Building, Construction and Engineering P.O. Box 56, Highett, Victoria 3190, Australia t Department of Civil Engineering, Monash University, Clayton, Victoria 3168, Australia (Refereed) (Received Oct. 21, 1991; in final form Sept. 30, 1993)
ABSTRACT Vertical variations of strength in a structural element is a well-established phenomenon. In large cross-sections, horizontal strength gradients can also occur because of local differences in humidity and temperature conditions. When the heat of hydration of concrete is free to dissipate from a cross-section, the temperature variations within a cross-section imply not only differing rates of strength development but also a risk of thermal cracking due to high temperature gradients. A series of columns of two cross-sectional dimensions were cast in the laboratory for concrete mixes with 28-day compressive strengths ranging from 40 to 115 MPa. Compressive strength results of vertically drilled cores showed the presence of significant cross-sectional strength gradients. Three-dimensional core strength distributions were constructed from which effective cross-sectional strengths of unreinforced in-situ concrete were calculated. Introduction The presence of strength gradients in the vertical direction of a structural element is well known. The in-situ core compressive strength of concrete at the top of a structural element is generally weaker than the concrete at the bottom (1-8). This phenomena has been attributed to a combination of the rise of bleeding water, segregation, hydrostatic pressure and revibration of the preceding concrete layers. In large structural elements, and especially when cross-sectional temperature and humidity conditions vary greatly, horizontal cross-sectional strength gradients can also occur. This may be due to differing rates of strength development at different locations within a cross-section because of different temperature and humidity histories. In addition, high temperature gradients within a cross-section result in thermal cracking which can cause strength loss (9-11). High strength concrete columns can experience a very high temperature rise in the first few days of cement hydration (12,13). This is due mainly to the high cement contents typically used in high strength concrete mixes. Depending on cross-sectional dimensions, ambient conditions, the insulative properties of formwork, formwork stripping times and the provision of thermal curing, large temperature differences can occur within a cross-section. 139
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A laboratory-based investigation of in-situ strength properties of square unreinforced concrete columns was undertaken for concrete mixes with 28-day cylinder strengths ranging from 40 to 115 MPa (6). This paper presents the cross-sectional strength characteristics at 28 days.
Experimental Investigation Concrete mixes and materials Six mixes with 28-day specified standard moist-cured cylinder strengths ranging from 40 to 115 MPa were investigated. Mix proportions are shown in Table 1. Mixes P32C, P50C, P70C and P100C were plain mixes which did not contain supplementary cementitious materials. Mixes P100S and PI20S contained silica fume. Mixes P32C and P50C will be referred to as normal strength mixes, whilst the remaining are high strength mixes. Mix P32C was supplied commercially and mix details are not available. A formaldehyde condensate type superplasticiser was used for all mixes except P32C and P50C. Superplasticiser dosages were added to attain a minimum slump of 80 mm. Type A Normal Portland cement was used for all the mixes. Table 2 shows the chemical and physical characteristics, provided by the suppliers, of the cement and silica fume. The coarse aggregate was a 14 mm old basalt with a specific gravity of 2.95 and 24-hour absorbence of 1.8%. The sand had a specific gravity of 2.65 and 24-hour absorbence of 0.5%. Sepcific gravity was determined on an SSD basis. TABLE 1 Mix Proportions Per m 3
Cement (kg) Silica fume (kg) Total aggregate (kg) Des. water (kg) Superpl. (1) Slump (mm) Water/binder ratio 28-day cylinder strength* (MPa) Initial concr, temp. (°C)
P32C
P50C
P70C
P100C
P100S
P120S
not available
40.5
380 0 2000 170 0 120 0.45 60.5
500 0 1925 150 5 150+ 0.30 69.0
550 0 1935 150 6 80 0.27 93.0
460 40 1920 150 6 130 0.30 95.5
500 45 1920 135 10 150+ 0.25 115.5
25.3
17.7
26.2
27.5
27.3
18.9
* Median strengths from sets of four 100 × 200 mm cylinders cured in lime-saturated baths. Casting program Square unreinforced columns of two cross-sectional dimensions (400 × 400 mm and 800 × 800 mm) were investigated. The 800 × 800 mm cross-section was similar to those used in the Melbourne Central project, whilst the smaller 400 × 400 mm cross-section provided a comparison of member size effects. The columns were 1200 mm high. All columns were cast under shelter inside the concrete laboratory. For the duration of the project, the mean ambient temperature in the laboratory ranged between 14 and 21°C and the mean relative humidity was 60%.
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TABLE 2 Chemical and Physical Characteristics of Type A Normal Portland Cement and Silica Fume, as Provided by the Suppliers Type A Normal Portland cement SiO 2 A 1 2 0 3 + Fe203 (%) TiO 2 (%) (%) 21.0 5.8 2.9
CaO (%) 64.1
C3A (%) 10.5
C4AF (%) 9.0
C3S (%) 51.0
C2S (%) 22.0
LOI (%) 1.6
SSA (m2/kg) 300
Silica fume SiO 2 A1203+ (%) TiO2 (%) 92.3 0.99
CaO (%) 0.27
C (%) 2.7
S (%) 0.11
P205 (%) 0.08
MnO2 (%) 0
LOI (%) 2.55
SSA (m2/g) 17-20
Fe203 (%) 0.87
The columns were cast in layers to allow easy separation and coring in the vertical direction. Figure 1 shows a column in elevation. Each column consisted of primary layers (layers 1, 3 and 5), from which cores were taken, and sacrificial layers (layers 2, 4 and 6), which separated the primary layers. The primary and sacrificial layers were separated by 5 mm masonite sheets which minimised moisture movement in the vertical direction. A segmented steel formwork system was used and no thermal insulation was provided on the forrnwork walls. However, the top and bottom ends of the columns were insulated with 25 mm thick expanded polystyrene foam sheets. The type and thickness of insulation was determined after a trial pour was conducted where vertical temperature distributions within a column were evaluated (6). Formwork was removed after 70 hours, when temperature measurements were also stopped. The external joints between the primary and sacrificial layers were sealed with a silicone sealant and taped over with waterproof electrical ducting tape to minimise moisture loss from the boundaries. 1,3,5 - Primary 2,4,6 - Sacrificial
Polystyrene Insulation
I
100 1200 300
400 or 800
FIG. 1 Column in elevation.
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One set of columns of each size was then covered with plastic sheeting whilst the other was left exposed to the laboratory air (6). The results reported in this paper are for the columns exposed to air, similar to an internal column of a building. Temperature measurements A total of 16 thermocouples were placed on a horizontal plane within each 800 × 800 mm crosssection in an octant, as shown in Fig. 2. In the 400 x 400 mm columns, temperatures were monitored with three thermocouples. Temperature readings were recorded at 20-minute intervals. Coring and testing In-situ strengths were evaluated by means of coring each primary layer in the vertical direction. Coring in the vertical direction was not only consistent with the direction of casting and loading but also provided sufficient core samples for studying cross-sectional strength properties. A supplementary investigation was undertaken to evaluate the effect of possible coring damage on core compressive strengths (6). For four normal and high strength mixes, 100 mm diameter cores were drilled from 150 × 300 mm moist-cured cylinders at 28 days and trimmed to 200 mm in length. Table 3 shows the core strength results, as well as 100 and 150 mm diameter moistcured cylinder su'engths. When compared to 100 × 200 mm cylinder strengths, the core strengths were all marginally higher at 28 days. The same trend was observed when core strengths were compared with 150 × 300 mm cylinder strengths. On the basis of these results, it was concluded that no core strength reduction could be attributed to the coring process. Using the coring patterns shown in Fig. 3, a total of 17 cores were cut from each of the 800 × 800 mm cross-sections, whilst 7 cores were cut from the 400 x 400 mm crosssections. To enable construction of three-
800
400
20O 100
4 i
31
21
i-." 3
"/
230
170
FIG. 2 Thermocouple locations in 800 x 800 mm cross-section.
dimensional strength profiles, strength zones were designated for each cross-section. The strength zones are labelled A to D and E to F for the 800 × 800 mm and 400 × 400 mm cross-sections respectively, as shown in Fig. 3. All the cores were approximately 300 mm in length, with nominal diameters of 95 mm. After trimming from both ends to between 190 and 200 mm, the average height to diameter ratios of cores were between 2 and 2.1. All trimmed cores were capped on both ends with a high strength sulphur capping compound. Capped cores were allowed to cure in the air for at least five hours before being tested to ensure sufficient strength in the sulphur caps. All cores were tested in a 5000 kN AMSLER testing machine, in accordance with AS 1012.9-86 (14). The mean coefficient of variation for sets of four cores from all strength zones and mixes was 5% at 28 days.
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TABLE 3 Compressive Strength Comparison at 28 days of 100 x 200 mm Cores Extracted from 150 x 300 mm Moist-cured Cylinders with 100 x 200 mm and 150 x 300 mm Moist-cured Cylinders Mix
Compressive strength (MPa) 100 × 200 mm cores from 150 x 300 mm cylinders
100 x 200 mm cylinders
150 x 300 mm cylinders
42.5 61.0 91.0 116.0
40.5 60.0 87.0 115.0
40.0 60.0 92.5 114.5
P32C P50C P 100L* P120S
* Mix P100L is a proprietary mix containing slag, studied as part of another project. Standard cylinders were scheduled for testing at 7, 28, 90 and 450 days, whilst cores were to be cut and tested at 28, 90 and 450 days. For mixes P32C and P50C, only 28-day cores were taken.
Results and Discussion Cross-sectional core strength distributions The extensive coring undertaken made it possible to examine cross-sectional strength properties in detail. Table 4 shows the core strength results obtained at 28 days for the various strength zones. The distribution of core strengths taken along the diagonal of an 800 x 800 mm cross-section is shown in Fig. 4. The values plotted for each strength zone are median values of sets of cores from the same strength zone. Cross-sectional strength gradients were found in all columns for all mixes. The highest core compressive strength within a cross-section was found at the center. Between the center and the 800 X S 0 0
O
O
0
•
•
•
800
0
•
• •
¸
A - corner B - intermediate C - center
D - side
•
400 x400 E - corner F - center
•
•
•
•
•
•
wit
'" .200 ~' 21110 '
4"
400
FIG. 3 Coring pattern for 800 x 800 and 400 × 400 mm cross-sections.
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TABLE 4 28-day Core, Effective and Standard Moist-cured Cylinder Strengths (MPa) and Core Strength Ratios P32C 400 x 400 mm Comer (E) Center (F) Effective 800 x 800 mm Comer (A) Side (D) Interm. (B) Center (C) Effective 100 mm cylinder Strength ratios f(E)/f(F) f(A)/f(C) f(B)/f(C) f(D)/f(C) fe(400)/f(F) fe(800)/f(C)
P50C
34.5
53.0 54.5 55.5 57.0 55.0 60.5
36.5 38.5 36.5 40.5
0.90 0.95
0.93 0.97 0.96
0.95
0.96
P70C
P100C
P100S
P120S
51.5 61.0 57.0
80.5
70.5 81.5 76.5
93.5 106.0 100.5
56.0 57.0 59.0 63.0 58.5 95.5
86.5 83.0 83.0 93.0 84.5 115.5
46.0 49.0 52.0 58.0 52.0 69.0
73.0 77.5 81.5 78.0 93.0
0.84 0.79 0.89 0.84 0.93 0.90
0.87 0.89 0.94 0.91 0.91 0.93
0.90 0.95
0.96
0.88 0.93 0.89 0.89 0.95 0.91
f(A-F) = median core strengths at each strength zone. fe(400) = effective strength of 400 × 400 mm column. fe(800) = effective strength of 800 × 800 mm column. 100 90-
~
P120~
~
PlO0(
/
80" 7o-
u
5040-
- ~ p a 2 c
o
"-' 302o-
lo-
115
230
3,45
460 575 690 805 Distancealongdiagonal(nun)
920
1035
1150
FIG. 4 Median core strength distribution along diagonal of 800 x 800 mm cross-sections at 28 days.
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outer regions of a column, a gradual and substantial strength reduction occurred. The lowest strength within a cross-section was generally found in the corner strength zone. Comparing cores taken from the comer to those taken from the center, the comer to center strength ratios, f(A)/f(C) and f(E)/f(F), were calculated and are shown in Table 4. In the 800 x 800 mm crosssections, cores from the center zone were generally 7-10% stronger than the cores at the comer zones. For mix P70C, the cores from the corner were 20% weaker than those from the center. The strength gradients could have been caused by a combination of factors, which include differences in compaction, higher rates of evaporation in the outer zones, different rates of hydration due to different temperature histories and thermal microcracking. Uniformity of compaction was assessed by examining the core density distributions of each cross-section. Surface dry densities of cores in the outer strength zones, measured immediately after trimming, were not consistently lower than cores from the center. No systematic link between density variations and strength gradients could be found (6). Under laboratory air-drying conditions, the extent of moisture loss and its effect on the effective strength of a cross-section becomes less important in large specimens (15,16). The concrete at the center of a large column is not likely to be affected by moisture loss due to evaporation. In the outer zones, the effect of evaporation is unlikely to extend beyond the cover region of a column. Limited humidity measurements taken in an 800 x 800 mm column for mix P120S at 28 days, showed that the relative humidity at the comer strength zone (A) was 84% compared to 80% at the center (C) of the column (6). These results were also observed in the 400 x 400 mm columns. Therefore, there was no evidence of a reduction in humidity at the comer strength zone (A). The slightly higher humidities in the outer regions of a column suggest that the local variations in humidity were governed by different rates of moisture consumption due to different hydration rates. The heat evolved during cement hydration was free to dissipate to the surrounding since the formwork walls were not thermally insulated. Therefore, at each strength zone, the local temperature-time relationships were different. As a result, temperature regimes varied greatly within a cross-section. Table 5 shows the maximum rise in temperature above the temperature of concrete at the time of casting for each strength zone. As expected, the highest temperatures occurred at the center of a column. The side zones (D) had higher temperatures compared to the TABLE 5 Maximum Temperature Rise, T n (°C), Above Concrete Temperature at Time of Casting for Various Strength Zones
800 x 800 mm Comer (A) Side (D) Interm. (B) Center (C) 400 x 400 mm Comer (E) Center (F) Tn(C) - Tn(A)
P32C
P50C
P70C
P100C
P100S
P120S
10.4 13.8 17.1 22.0
13.5 17.8 21.3 26.9
20.7 29.2 34.4 42.4
18.1 27.1 33.7 37.0
16.3 23.0 29.1 39.2
16.0 21.8 25.5 33.0
20.7 26.1
18.1
13.2 17.8
12.9 16.5
21.7
18.9
22.9
17.0
11.6
13.4
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corners (A) because the exposed surface area for heat dissipation was lower at the side when compared to the corner. The maximum temperature rise at the center of the 800 × 800 mm cross-sections were generally double that at the center of the 400 × 400 mm cross-sections. Different local temperature regimes suggest that the rates of early strength development will vary within a cross-section. It also implies that both the medium- and long-term strengths within a cross-section may be variable. It is well known that high curing temperatures can lead to lowered medium and long-term strengths (9,15,17,18). The strength development of silica fume concretes is also very sensitive to high temperatures, because the initiation and extent of pozzolanic reactions are temperature sensitive. In the 800 × 800 mm cross-sections, the early rate of hydration in the center of a column would be faster than that of the outer strength zones because of the higher temperatures. If the development of compressive strength can be simply related to maturity or a unique temperature/time relationship, the differences between center (C) and outer zone (A and D) 28-day strengths in the 800 × 800 mm cross-sections would be consistent with the differences in accumulated maturity. But, the core strength results from the center zone (F) of the 400 × 400 mm columns contradict this assumption. The accumulated maturity at 28 days for the concrete at the center of the 400 × 400 mm columns (F) would have been lower than that of the center of the 800 × 800 mm columns (C) by virtue of the lower temperatures achieved. However, the strength of zone F cores were the highest of all the strength zones considered. This suggests that the lower temperatures in the small columns led to higher 28-day strengths. Therefore, the cross-sectional strength gradients could not be accounted for by differences in concrete maturity but may be influenced by another factor. Large temperature variations within a cross-section will induce stresses which can cause thermal microcracking. These stresses result from volume changes which occur both in the heating and cooling phases during the first few days of cement hydration, and is brought about by the incompatible thermal expansion characteristics of the different mix constituents (9). The extent to which cracking occurs is also dependent on the degree of restraint to free expansion imposed by the cross-sectional geometry. If thermal microcracking occurs, the strength of the matrix and aggregate-paste interface could be impaired, resulting in the loss of compressive strength. Depending on ambient temperature conditions, the highest temperature gradients in the 800 × 800 mm cross-sections were found near the surface and generally within the outer 100 mm of a column. In the corner strength zones of the 800 × 800 mm cross-sections, maximum thermal gradients ranged from 47°C/m for mix P50C up to 99°C/m for mix P100S (6). The lowest strength gradients were found for mixes P32C and P50C which also had the lowest thermal gradients. Figure 5 shows a contour plot of maximum thermal gradients within an 800 × 800 mm cross-section for mix P70C. The regions of highest thermal gradients corresponded with strength zones A and D which had the lowest core strengths within a cross-section. Based on the temperature data in Table 5, mixes P70C and P100S showed the largest difference in maximum temperatures between the center and the corner of an 800 × 800 mm cross-section. This also corresponded to the lowest f(A)/f(C) ratios for all the mixes investigated, i.e. 0.79 and 0.89 for P70C and P100S respectively. However, it must be noted that for a particular cross-sectional geometry and thermal gradient, the extent of thermal microcracking would also depend on the thermal expansion properties of the concrete. This is, in turn, a function of mix proportions and mix constituents. Effective cross-sectional strength The presence of strength gradients effectively reduced the overall load bearing capacity of a cross-section. To incorporate the effect of strength gradients for uniaxial compression, an average load-bearing capacity of in-situ concrete or effective strength was calculated for each crosssection. To achieve this, three-dimensional core strength distributions for all the cross-sections were constructed (6). A typical core strength distribution in an 800 × 800 mm cross-section is
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FIG. 5 Distribution o f m a x i m u m thermal gradients in an 800 x 800 m m cross-section - mix P70C.
FIG. 6 Three-dimensional core strength distribution of 800 x 800 m m cross-section mix P100S at 28 days. -
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shown in Fig. 6. These profiles were constructed by gridding and interpolation of the median strengths of each strength zone, carried out using a proprietary software called SURFER. The median core strength of each strength zone is based on four core strength results, except for zones C and F where five and three cores were taken respectively. The effective strengths were calculated by integrating the volume enclosed under the three-dimensional core strength distributions (6). Effective strength results are shown in Table 4. To quantify the effect of the strength gradients, effective to center, or fe(400)/f(F) and fe(800)/f(C), ratios were calculated. These strength ratios provided a measure of the overall reduction of insitu load-bearing capacity in relation to the highest strength within a cross-section. The 28-day strength ratios are shown in Table 4. In the 800 x 800 mm cross-sections, for mixes P70C and P120S, the strength gradients led to a 10% reduction of effective strength when compared to the strength at the center of the column. For the other mixes, the reduction in load-bearing capacity due to strength gradients were approximately 5%.
Implications of cross-sectional strength gradients The non-uniform distribution of core compressive strengths across a cross-section suggests that engineering properties will also vary across a cross-section. For instance, the stress-strain behaviour of in-situ concrete will vary with depth from the surface of a column when large strength gradients are present. Apart from the obvious reduction of load-bearing capacity of the concrete in axial compression, the presence of strength gradients has important implications for structural design, especially for columns subjected to bending. Cross-sectional strength gradients also imply that the depth to which cores are drilled from the surface of a column will be important when cores are required for assessment of concrete strength compliance. Limited results of cross-sectional strength variations of horizontally drilled cores have been published (13). A significant reduction of concrete strength towards the outer region of a structural element imply that the concrete in the outer region is of lower quality when compared to the remainder of the cross-section. This has important implications for durability and fire resistance. The 28-day results show that in-situ strengths were appreciably lower than standard cylinder strengths. The discrepancies between in-situ and cylinder strengths were further aggravated because of the cross-sectional strength reduction by horizontal strength gradients. These discrepancies have important implications for the strength reduction factors used in column design which are intended to partially account for in-situ and cylinder strength differences. A more detailed discussion of the differences between in-situ and cylinder strengths has been provided elsewhere (6). Results from 90- and 450-day tests will be published as they become available.
Conclusions The distribution of core compressive strengths of in-situ concretes for a range of high strength concretes cast in 400 × 400 mm and 800 × 800 mm cross-sections were not uniform. Compressive strengths of vertically drilled cores were highest in the center of a cross-section. The reduction of core strength from the center to the corner of an 800 x 800 mm cross-section was generally 10% but was as high as 20% in a plain 70 MPa mix. The available evidence suggests that these strength gradients were a result of a complex combination of temperature and humidity effects. These included thermal microcracking, and differing rates and extents of strength development. To incorporate the effects of the non-uniform cross-sectional strength distributions, effective cross-sectional strengths were calculated. The presence of strength gradients in a column crosssection resulted in a reduced load-bearing capacity of in-situ concrete of between 5 and 10% for the mixes considered. The non-uniformity of cross-sectional strengths means that engineering properties can vary across the cross-section. This has implications for the design of columns in bending, durability and fire
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resistance. When cores are drilled from the surface of a column to evaluate the strength of in-situ concrete, the depth to which cores are drilled will also be very important.
Acknowledgments The technical and material support from PBM Concrete, CRL Pty Ltd, TEMCO Pty Ltd, CSIRO Division of Building, Construction and Engineering, and the Department of Civil Engineering, Monash University are gratefully acknowledged. The authors are particularly grateful to Mr M. Graham from Monash University and Mr R. Harrison and Mr E. Christie from CSIRO. This project formed part of the first author's PhD dissertation undertaken at Monash University with the support of a Monash Graduate Scholarship and Monash Research Scholarships.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
Bhargava, J., 'Strength of concrete members cast in deep forms', National Swedish Institute of Building Research, Document No. 5 (1965). Bhargava, J., 'Strength of concrete in high strength revibrated walls', National Swedish Institute of Building Research, Document No. 6 (1969). Bloem, D.L., 'Concrete strength measurement - cores vs. cylinders', Proc. American Society for Testing and Materials, 65, 668-696 (1965). Hang, A.K. and Jakobsen, B., 'In-situ and design strength for concrete in off-shore platforms', Proc. 2nd Int. Symp. on Utilization of High Strength Concrete, Univ. California, Berkeley, May (1990). Hoshino, M., 'Relationship between bleeding, coarse aggregate and specimen height of concrete', ACI Materials Journal, 86(2), 185-190 (1989). Mak, S.L., Attard, M.M., Ho, D.W.S. and Darvall, RLeR, 'In-situ strength of high strength concrete', Civil Engineering Research Report No. 4/1990, Monash University, 132 pp (1990). Petersens, N., 'Strength of concrete in finished structures', Transactions Royal Institute of Technology, No. 232, Stockholm, Sweden (1964). Sangha, C.M. and Dhir, R.K., 'Core and cube relationships in concrete', in Advances in Ready-mixed Concrete Technology, Ed. R.K. Dhir, Pergamon Press, UK (1976). Alexanderson, J., 'Strength losses in heat cured concrete', Proc. Swedish Cement and Concrete Institute, No. 43, Stockholm (1972). Carlson, R.W., Houghton, D.I. and Polivka, M., 'Causes and control of cracking in unreinforced mass concrete', Proc., ACI Journal, No. 7, July, 821-837 (1979). Springenschmid, R. and Breitenbucher, R., 'Technological aspects for high strength concrete in thick structural members', Proc. Utilization of High Strength Concrete Conf., Stavanger, Norway, Tapir Publishers, June, 487-496 (1987). Aitcin, P.C. and Riad, N., 'Curing temperature and very high strength concrete', Concrete International: Design & Construction, 10(10), 69-72 (1988). Yuan, R.L., Ragah, M., Hill, R.E. and Cook, J.E., 'Evaluation of core strength in high strength concrete', Concrete International, 13(5), 30-34 (1991). Australian Standard 1012, Part 9-1986, 'Method for the determination of the compressive strength of concrete specimens', Standards Association of Australia, Sydney (1986). Mak, S.L., Ho, D.W.S. and Darvall, RLeP., 'The effects of moist curing on the compressive strengths of some very high strength concretes', Proc. Concrete for the Nineties Conf., Leura, New South Wales, September, 12 pp (1990). Urpani, D., Mak, S.L. and Attard, M.M., 'Curing of very high strength concretes', Civil Engineering Report No. 1, Monash University (1991). Kjellsen, K.O., Detweiler, R.J. and Gjorv, O.E., 'Development of microstructures in plain cement pastes hydrated at different temperatures', Cement and Concrete Research, 21(1), 179-189 (1991). Klieger, E, 'Effect of mixing and curing temperature on concrete strength' ACI Journal, 54, 1063-1081 (1958).