EARLY STRENGTH TEST FOR QUALITY CONTROL OF CONCRETE

Transcription

1 EARLY STRENGTH TEST FOR QUALITY CONTROL OF CONCRETE Thomas W. Brockenbrough, College of Engneerng, Unversty of Delaware; and Robert R. Larason, Wllam G. Major Assocates, Inc. The tme-honored approach to ensure qualty of concrete s totake samples at the job ste, to mold test cylnders, and then to cure the cylnders under standard condtons for 28 days before testng. The procedure s convenent, the test s smple, and the equpment s dependable. The dsadvantage of the 28-day curng s the delay tme that nether leads to the early correcton of a materal problem nor facltates the prompt removal of defectve concrete. Several researchers have demonstrated the feasblty of 24-hour tests to ensure qualty control of concrete. Most early tests have been accelerated strength tests employng hot ar, steam, or water for curng. The hgher temperatures requre specal equpment, make the specmens dffcult to handle, and ntroduce specal problems when ar-entranng agents and other addtves are used. The merts of a smple 24-hour, 1 F (37.8 C) hot-water curng and testng procedure are descrbed. Results acheved wth the procedure are presented and analyzed. It s concluded that a hot-water curng temperature of 1 F (37.8 C) s adequate. The temperature s low enough to be completely safe for personnel nvolved and to permt careful handlng of test specmens. The equpment requred s smple and nexpensve. THE prncpal aspects of early strength testng of portland cement concrete are curng tme, curng temperature, and expermental procedure. A curng tme of 24 hours has consderable mert because t s convenent and leads to prompt results. The curng temperature s much more arbtrary. Intal tests were run to observe the effects of curng temperature on the level and relablty of 24-hour compressve strength. TESTS PERFORMED The study of curng temperatures was made by comparng results obtaned from curng smlar batches of concrete n dfferent water temperatures. The strength correlatons were acheved by curng and testng varous grades of concrete by both the 28- day standard procedure and the accelerated hot-water procedure. The concrete mx desgn used s gven n Table 1. Tests were conducted by usng hot-water curng temperatures of 9, 11, 13, and 15 F (32.2, 43.3, 54.4, and 65.6 C). Three 6-n.-dameter (15.2-cm) cylnders were molded from varous batches of concrete and cured for 24 hours before testng. The 24-hour strength versus the curng temperature s shown n Fgure 1. From the same batches of concrete, three addtonal cylnders were cast, cured for 28 days n a most room, and then tested. ANALYSIS OF RESULTS The average compressve.strengths of the cylnders cured at 9, 11, 13 and 15 F (32.2, 43.3, 54.4, and 65.6 C) were 1,879, 2,822, 2,99, and 3,17 ps (13, 19.5, 2.6, and 21.4 MPa) repectvely. The results are shown n Fgure 2. 61

2 Table 1. Concrete mx desgn. Water- Mx Cement Cement Desg- Water Factor Rato Sand naton (lb/yd') (sacks/yd') (by weght) (lb/ yd 3 ) A ,32 B ,2 c ,31 D ,4 E ,49 Note: 1 lb/yd'=.593 kg/m'. Coarse Aggrcnle (lb/yd) 1, 71 1,82 1,8 1,77 1, 74 Fgur 1. Twenty-four-hour strength versus curng temperature.. ".,. tf., " -;:. G <> g., J, 2, 1, I I / v 1--- / "' Curllj! To111pere.ture ( F,) 16 Fgure 2. Percentage of strength ganed n 24 hours versus curng t!!mperature. 'cl so <>. II, 4 :!' "' " CD "' 6 "' N 'l " "' 6 JO / I v I lbo

3 The tests demonstrate a substantal strength gan n 24 hours at a temperature as low as 1 F (37.8 C). Snce these results were suffcently nterestng, we performed addtonal hot-water curng tests at 1 F (37.8 C). The 1 F (37.8 C) temperature s low enough to be regarded as a normal rather than an elevated temperature for accelerated curng. The merts of avodng excessve temperatures are numerous and readly apparent. Snce there s no threat to the safety of techncans, they are able to handle the specmens wth care. When abnormal temperatures are not used, questons about thermal shock, the expanson of entraned ar, and artfcal changes n the hardenng process do not arse. The equpment should add nether excessve heat nor humdty to the testng laboratory. 63 PROPOSED EARLY STRENGTH TEST Procedure The descrpton of the procedure followed n conductng the early strength tests s recommended for further tests leadng to the eventual adopton of standard specfcatons. When mxng was completed, the propertes of the plastc concrete were determned n accordance wth ASTM specfcatons. Sx standard 6 by 12-n. (15.2 by 3-cm) cylnders were prepared n cast-ron molds by followng the procedure of ASTM C 192. The molds were capped wth %-n.-thck (1.27-cm) steel plates. They were turned on ther sdes and tapped several tmes at each end. Ffteen mn after the moldng began, three cylnders were placed n the hot-water bath and three were placed n a most room, where a standard temperature of 73.4 ± 3 F (23 C) was mantaned. The cylnders placed n the hot-water bath were cured for 231/z hours. They were then removed, strpped, and tested at 24 hours accordng to ASTM C 39. The cylnders placed n the most room were cured for 28 days and then removed, strpped, and tested n accordance wth ASTM C 39. The procedure produced cylnders wth very good ends; no cappng was needed. Equpment Perhaps the only equpment of nterest s the hot-water bath, whch s shown n Fgure 3. The bath was smply constructed n our shop. The base to support the tank was constructed of 2 by 2 by %-n. (5.1 by 5.1 by.6-cm) steel angles. The tank was made of.13-n. (.32-cm) steel plates welded together along ther edges. The top was ftted wth a wooden ld. The 16-ft 3 (.45-m 3 ) tank s large enough to hold 14 cylnders: Crculaton of the water s provded by a small electrc mpeller. Heat s suppled by a 6-n. (15.2-cm) rng gas burner. A solonod valve s actvated by a metallc expanson thermometer. The tank and ts ld were nsulated because the testng program ncluded curng temperatures as hgh as 15 F (65.6 C). The tank was large enough for the tests conducted and provded temperature control for the water. Tanks of other szes and shapes should perform equally well, Addtonal equpment for producng and testng concrete conformed to ASTM C 31-66, C 39-66, C , C , and C The cast-ron cylnder molds were ftted wth %-n. (1.3-cm) plates that could easly be swung nto poston and clamped. The end plates led to cylnders wth good ends and elmnated the need for cappng. EARLY STRENGTH TEST EXPERIMENTS Testng Program Tests were performed to observe the merts of the 24-hour, 1 F (37.8 C) hot-water

4 64 Fgure 3. Hot-water bath..&. - I, lfettn.1:' Bl<pmlrtvn--type' 'n1e11uoj11etol" E - Inouated Ld ' B - So1:enor4 Val'H F - KJ:eotl"c Swtch l1 - Kleo'Crf.c llotor ana Impeller G - Plot Lght D - S 1x In oh Rng Burner Fgure 4. Twenty-four-hour strength versus 28-day strength.. J. = 1 a,oe T"YJ>e 1 Pa.tlUll oement, no adlll.lxturea R 1N1111on Eqt1atlon 1, Y 49S Jt - o.45&x2 '--'---'L--' l, 2, 3, 4, 24 Hour Cornprr. tve Str-ength (p.e,.)

5 65 Fgure 5. Compressve strength versus water-cement rato. 2B Day Test t J,1-->.r-1--.,,-:::1 "'. -;:. ( 2,ooo f--+.o,,:.,_,,,,,f tj 2J Hour Tent o. o.s O. (J.1 o.b Water-Cenent Rato (By Weght) Fgure 6. Effect of admxtures on strength correlaton. 6,.;.;, A ;.,. t r tj ro- A,,ooo 4, J, 2, 1, " ( ) "' "' & A ' TJ'P l o rnent, no admxtures Alr entraned concrete Retarder ueed a 1, ocro 2,oaa 3, 4, 24 Hour Compl" 1H Strength (p,.1,)

6 66 Fgure 7. Effect of retarder on correlaton between strength and water-cement rato. : 4,.: z,ooo 3, 1, 24 Hour T t Wator-C1ent Rato ( By Weght) Table 2. Statstcal analyss of results for each mx. Average Compressve Standard Mx Strength Devaton Coeffcent Desgn Test (ps) (ps) of Varaton B 24-hour 2, B 28-day 5, hour 1, c 28-day 4, D 24-hour 1, D 28-day 3, E 24-hour 1, E 28-day 3, Lmts of Uncertanty" (ps) ±73.9 ±112.5 ±52. ±116.6 ±43.2 ±79.9 ±21.1 ±118.3 Note: 1 ps = 6.9 kpa. P =.9.

7 curng. They were planned to determne (a) the correlaton between the 24-hour test and the standard 28-day test (Fgure 4) and (b) the correlaton between 24-hour and 28-day compressve strength and the water-cement rato (Fgure 5). Nnety standard cylnders were molded from the mxes wthout admxtures (Table 1). Thrty-sx cylnders were prepared wth a commercal retarder-densfer addtve, and 12 were prepared wth a commercal ar-entranng agent. Fgure 6 shows the effect of a retarder and an ar-entranng agent on strength correlaton, and Fgure 7 shows the effect of a retarder on the correlaton between strength and the watercement rato. The ar content ranged between 6 and 7.5 percent. The procedure descrbed prevously was followed. 67 Analyss of Test Results The compressve strength of each cylnder, the average strength, the standard devatons, and the coeffcents of varaton are gven elsewhere (13). Table 2 gves the average compressve strengths, standard devatons, coeffcents of varaton, and lmts of uncertanty obtaned for each mx used. As the average 28-day strength ncreased from 3,6 to 5,148 ps (2.7 to 35.5 MPa), the strength ganed n the 24 hours ncreased from 35 to 54 percent of the 28-day strength. It s noteworthy that the coeffcents of varaton (rato of standard devaton to compressve strength) of the 24-hour strengths and 28-day strengths were much alke. The average values were 4.13 and 4.19 percent respectvely. CONCLUSIONS AND RECOMMENDATIONS Ths expermental study leads to the followng conclusons: 1. Twenty-four-hour compressve strengths can be used to predct 28-day strengths wth good precson; 2. A hot-water curng temperature of 1 F (37.8 C) s adequate for a 24-hour early strength test; and 3. Test specmens of good qualty can be produced wth cast-ron molds equpped wth steel end plates. Based on the experments descrbed and addtonal research performed at the Unversty of Delaware, we recommend the followng: 1. A broadly based testng program leadng to the development of an early strength test for concrete, 2. The use of hot-water curng, 3. A curng temperature low enough to ensure the careful handlng of test specmens and the safety of the techncans nvolved, and 4. The eventual elmnaton of the 28-day test as a bass for desgn and qualty control of concrete. REFERENCES 1. M. S. Gerend. Steam Curng Cylnders Gve 28-Day Concrete Strength n 48 Hours. Engneerng News Record, Vol. 98, No. 7, Feb. 1927, p Person and Brckett. Studes of Hgh Pressure Steam Curng. ACI Journal, 1932, p G. Patch. An 8-Hour Accelerated Strength Test for Feld Concrete Control. ACI Journal, Proc., Vol. 29, March-Aprl 1933, p Concrete Manual, Methods for Summarzng Test Data. U.S. Bureau of Reclamaton, U.S. Department of the Interor, Sectons 62 and 65, Jan. 1941, pp. 17 and 181.

8 68 5. J. Shdeler and W. Chamberlan. Early Strength of Concrete as Affected by Steam Curng. ACI Journal, 1949, p J. W. H. Kng. Concrete Qualty Control-A Technque of Accelerated Testng Developed at Queen Mary College for the Port of London Authorty. Chartered Cvl Engneer, Bulletn, Insttuton of Cvl Engneers, London, Nov. 1955, pp. 44 to T. N. W. Akroyd. The Accelerated Curng of Concrete Test Cubes. Proc., Insttute of Cvl Engneers, London, May 1961, p V. M. Malhotra. Analyss of Accelerated 24-Hour Concrete Strengths From Feld Tests. Internatonal Unon of Testng and Research Laboratores for Materals and Structures, Bulletn 31, June 1966, p G. H. Schaller. Accelerated Strength Testng of Concrete. Unv. of Delaware, MCE thess, P. Smth and H. Tede. Earler Determnaton of Concrete Strength Potental. Ontaro Department of Hghways, Report RR124, Jan J. W. H. Kng. An Accelerated Test for Concrete. Proc., Insttute of Cvl Engneers, London, Vol. 4, May 1968, p Concrete and Mneral Aggregates. In Book of ASTM Standards, Amercan Socety for Testng and Materals, Part 1, Phladelpha, Oct R. A. Larason. Concrete Compressve Strength: 28 Days or 24 Hours. Unv. of Delaware, MCE thess, D. J. Wlls. Strength Testng of Structural Lght Weght Concrete. Unv. of Delaware, MCE thess, 1971.