The International Jo'Jmal of Cement Composites and Lightweight Concrete. Volume 11, Number 1 Feh_ rtmry 1989

Contro l led t ropical curing method for acce lera ted concrete st rengths

N. C. Das Gupta* and C. T. Tam ~

Synopsis Accelerated curing and testing of concrete can be used for both quality control and quality assurance This paper presents controlled curing and accelerated curing methods for early assessment and prediction of strength of concrete and discusses their r,3ia live merits. The controlled curing method utilises the inherent high ambient temperature of the tropics wi thout the artificial heating needed for other accelerated curing methods. The test specimens are controlled cured for only 24 hours prior to testing.

A series of compressive strength tests were conducted on concrete of varying proport ions Two types of water reducing admixtures were used individually in some mixes. A multiple linear regression technique has been adopted where appropriate to correlate the 28-day standard cured strength with the controlled cured strer~gth, 35 ° cured strength, 55 ° cured strength and standard 2 day and 3 day strengths. The correlation rosults ar~ discussed in detail. The important ohs==r,,ations ari~ing in this study are: the simplicity of the controlled curing method; the reliability of multiple linear regression relations; and the small effect of admixtures on the correlations between strengths.

Keywords Concrete, accelerated curing, compressive strength, tensile strength, accelerated tests, strength development, regression analysis, crushing stre,:gth, curing, strength r)f materials.

INTRODUCTION Accelerated construction schedule and increased rate of concrete placement in a single structure are the two major factors contributing to the need for faster evalu- ation of the quality of f inished concrete. The speed of construction has increased to the extent that the usual 28 day wait before determining the concrete strength and quality is considered to be too long. as it would be economically impractical to rectify concrete if found defective at this late date I l l . A reliable accelerated strength test would therefore add immensely to the control of concrete quality by enabling the concrete engineer to make desired changes in the concrete mixes sooner, and to take early remedial action on any unsatis- factory concrete placed.

Basis of accelerated curi~,.g Development of concrete strength as a function of maturity, is the basis of accelerated curing. A higher curing temperature is used to reduce the t ime taken for the mix to reach a cho.~en maturity and hence its corresponding strength, in general, there are three

* Senior Lecturer and t Associate Professor, Department of Ctvil Engineering. National University of Singapore. Re:eived 11 May 1988 Accepted 29 August 1988 ~) Longman Group UK Ltd 1989 0262-5075/89/11104029/$02.00

important stages of any acnelerated curing regime and each has its duration of t ime and related temperature (Figure 1) as follows:

(a) initial curing period (ICP)- the time between mould- ing of specimens and the start of the accelerated curing period. The ass(~ciated temperature is often that of the environment during casting. A sufficient period of t ime is needed for this period in order tn ensure that the specimens can withstand the change i;~ temperature in the next accelerated ct.~rirrg stage.

(b) Accelerated curing period (ACP)- the time of curing at a high temperature (including the rate of tem- perature rise used in some regimes). The du,atiof~ uf this period and the highest temperature used determine the energy cost of the regime.

(c) Final curing period (FCP)- the t ime between the end of the accelerated curing period and the final testing of the specimens. The associated temperature is generally that of the environment. The duration should be sufficient to return the specimens to the ambient temperature at the t ime of testing.

A more detailed assessment of the maturity con- cept wil l not be presented and this has been undertaken by the National Bureau of Standards [2, 3. 41.

Present study A wide variety of procedures and methods have been

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Con trolled tropical curing method for accelerated con cre [ e s treng ths Gupta and Tam

Table 5 Prediction of F281~. from FCCM and F3D

~F2aL)) A {F28D) P (F28D)P Mix (N/ram 2) (n/mm 2) Ratio (F28D)A

M15 8.90 10.40 0.85 M20 12.30 15.15 0.80 M30 27.10 29.25 0.95 M40 33.35 38.25 0.85 M50 41.75 47.45 0.90 M60 43.£0 48.85 0.90

M25 27.35 28.50 0.95 M25A !6.85 22.10 0.75 M35 39.30 37.15 1.05 M45 50.35 44.70 1.15 M55 44.25 47.10 0.95 M60A 53.65 58.35 0.90

recommend the use of this method of controlled curing in the tropical region utilising the near constant high ambient temperature that persists throughout the year.

2. The correlation coefficients and the standard errors of estimates for the relationship between the 28 day strength and the controlled curing strength ~rc found to be comparable with those obtained for the relationship between the 28 day strength and other e~rly strengths, namely 35 ° curing, 55 ° curing, 2 day and 3 day standard curing. The values of such coefficie.~ts are considered to be satisfactory fo~ the range of mixes tested.

3. In teHns of zhe ease of handling specimens, the low cost recurred, the simplicity of the method and the safety of personnel involved, the controlled curing method outvvelghs any other accelerated curing method.

A C ' ~ ( N O W L E D G E M E N T S The P,.,mors are grateful to Mr. Ong Weng Hui and Mr. Lee .-\~ Seng, who did the experimental work in partial fulfilment of their degree of Bachelor cf Engineering at the National University of Singapore. The work was supported in part by the RDAS Grant C/81!01 on Low Cost Construction Materials from the Ministry of Trade and industry" and administered by trlr~ S~gapore Science Council

R E F E R E N C E S 1. Abdun-Nut, E. A.. 'Accelerated, early and

immediate evaluation of concrete quality', Accelerated Strength Testing. American Concrete Institute PuLlication SP-56, 1978. pp. 1-13.

2. Carino, N. J., I, ew, H. S. and Volz, C. K., 'Early age temperature ~,ffects on concrete strength predic- tion by the m ~turit¥ .method', Amerie.an Concrete Institute Journal, Proceedings Vol. 80. No. 2. March-April 19,93, pp. 93-101.

GO

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~ I ///I/ " 95 / / ~ , ~ / " % ,.ONFIDENCE B6NO 20 c=, ~ " / ~ F280= 5.27° 14,17F55C

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~=~ / r = 0.97 IO * /

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0 5 ~0 15 20 25 30 35 LO &~

55°C CURING STRENGTH (MPA). F551; Figure 6 28 day strength vs. 55 = strength fct M, N and

P-series mixes. (Each point is an average of six tests)

3. Carino, N. J. and Lew. H. S.. 'Temperature effects on strength maturity relations of mortar'. American Concrete Institute Journal, Proceedings Vol. 80. No. 3, May-June 1983, pp. 177-82.

4. Carino, N. J., 'Temperature effects on the strength maturity relation of mortar'. Report No. NBSIR 81 - 2244. National Bureau of Standards, Washington, March 1981, 98 pp.

5. Malhotra, V. M., 'The past, present and future of ae.ce!erated strength testing of concrete', Indian Concrete Journal, Vol, 43. No,. 9, September 1969, pp. 324 49.

6. Ramakrishnan, V. and Dietz, J., 'ASTM Accelerated Strength Tests for Quality Control of Concrete', American Concrete Institute Publication SP56, 1978. pp. 95-116.

7. Wills, M. H,, Jr., "Early assessment of concrete quality by accelerated compressive strength development with heat (Results of ASTM's co- operative test program)', Journal of Testing and Evaluation, Vol. 3, No. 4. July 1975.

8. British Standards Institution, BS 1881: Part 112 (1983), 'Methods of accelerated curing of test cubes', London.

9. American Society for Testing and Materials. ASTM

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Controlled tropical curing method for accelerated concrete strengths Gupta and Tam

10.

11.

Designation C684-81 (1982), 'Standard ,,~ethod of making accelerated curing and testing of concrete compression test specimens'. Philadelphia, U.S.A British Standards Institution, BS 1881: Part 108 (19831, 'Methods of making test cubes from fresh concrete', London. Tam, C. T. and Das Gupta. N. C., 'Accelerated concrete strengths by controlled curing method'. Proceedings of the 10th Conference on Our World in Concrete and Structures, C.I. Premier, Singa- pore, 1985, pp. 374-85.

12. Chin, F. K., 'Relation between strength and matur- ity of concrete', Amedcan Concrete Institute Jour- nal, Proceedings Vol. 68, No. 3, March 1971, pp. 196-203.

t3. Tam, C. T., 'A quick method to predict 28-day strength of concrete', Bulletin of Institution of Engineers, Pet3!i~g J-%'~. M~!-%,_~!_% .a.pr!! 1974, pp. 11-16.

14. Walker. S. and Bloem, D. L,, 'Variation in portland cement', Proceedings of the American Socie~,y ;~or Testing and Materials, Vot. 58, 1958, pp. 1009-32.

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