Compressive strength testing of high performance concrete cylinders using confined caps

Construction and Building Materials, Vol. 10, No. 8, pp. 589-595, 1996 © 1997 Elsevier Science Ltd Printed in Gieat Britain. All rights reserved 0950-0618/96 $15.00+0.00 ELSEVIER

PII:S0950-0618(96)00020-7

Compressive strength testing of high performance concrete cylinders using confined caps S. Ali Mirza and Claude D. Johnson Department of Civil Engineering, Lakehead University, Thunder Bay, ON P7B 5E1, Canada Received 15 April 1995; revised 30 June 1996; accepted 31 July 1996 The confined capping system, reported in this paper, provides a simple approach for compressive strength testing of high performance concrete cylinders. This method employs standard concrete laboratory testing equipment and an inexpensive customized capping apparatus for preparing the cylinder ends. The method ensures cap confinement without tight controls on the cylinder end roughness prior to capping, and on the cap thickness itself. This paper explains and documents the use of the method for testing concrete cylinders having strengths up to and exceeding 100 MPa. © 1997 Elsevier Science Ltd. All rights reserved.

Keywords: capping; capping confinement; compressive strength

Introduction

high performance concrete mixes, and tested to establish the suitability of the method presented in this paper. Mix proportions, including cement, silica fume, water, aggregates and naphthalene-based superplasticizer are given in Table 1. Two mixes, DM9-1 and DM19-1, had the same water to cementing material ratio, 0.23, but different maximum aggregate sizes, 9 mm and 19 mm, respectively, as well as different aggregate blends. The mix DM9-2 was similar to the mix DM9-1, except that the water to cementing material ratio was held at 0.20. The coarse aggregate used in all mixes was crushed dolomite from Manitoulin Island and the fine aggregate had a fineness modulus of 2.6. All cylinders were cast in cardboard cylinder molds, and rodded using the standard North American procedure of rodding three layers, 25 times each. The tops of the cylinders were then finished by trowelling. The cylinders were cured together at a constant temperature of 24°C under a moist burlap enclosed within a polyethylene sheet. Compressive strength tests were conducted at 7, 14, 28, and 61 days, under a loading rate of approximately 0.24 MPa/s. Two different cylinder end conditions were considered; namely, "regular" ground ends, and capped ends with confining rings (confined caps developed at the Lakehead University). Capping compound--having the manufacturer's specified ultimate compressive strength of 35 MPa after 5 min and 55 MPa after 48 hIWaS used for making the confined caps. Laboratory tests on this capping compound, using standard 50 mm×50 mmx50 mm cubes, produced ultimate compressive strengths of 44.2, 50.3, 49.5 and 51.1 MPa after 1, 24, 48, and 72 h, respectively. All specimens were removed from the moist curing environment, 24 h prior to testing, so that the capping and grinding operations could be completed before testing. The cylinders

Specifications, detailing a standard procedure for the compressive strength testing of high performance concrete cylinders, are yet to emerge. At present, cylinders of different sizes are being used and a number of different procedures are being followed to prepare the cylinder ends for compressive testing. These factors can have a significant effect on the measured compressive strength of high performance concrete cylinders. To obtain consistent test results and an accurate measure of the compressive strength, rather than just a measure of the shortcomings of the testing equipment and procedure, these factors need to be carefully controlled. With most of the testing procedures being followed today, the equipment required to conduct compressive tests on high performance concrete cylinders may not be available in a concrete laboratory that has been set up for the testing of normal strength concrete specimens. The purchase of a special grinding equipment for use in the preparation of the end conditions of specimens, made up of concrete having a compressive strength of the order of 100 MPa or higher, may involve a significant capital outlay for small laboratories. This paper presents a simple test method developed by the authors at the Lakehead University for compressive strength testing of high performance concrete cylinders. The method employs standard concrete laboratory testing equipment and an inexpensive customized capping apparatas for preparing the specimen end conditions.

Concrete mixes and specimen preparation A total of 78 test cylinders, 1 0 0 m m x 2 0 0 m m and 150 mmx300 nun in size, were cast from three different

589

Compressive strength testing of HPC cylinders: S. Ali Mirza and C. D. Johnson

590 Table 1

Concrete mix proportions

Material (per m 3)

Mix DM9-1

Mix DM9-2

Mix DM19-1

Cement (kg) Silica fume (kg) Water (1)

495 55 126.5 (120.5) a 19 1169 576 0.23 C 250

495 55 110.0 (104.0) a 20 1169 576 0.20 C 130

495 55 126.5 (118.5) a 20 1134b 611 0.23 c 210

Superplasticizer (1) Coarse aggregate (kg) Fine aggregate (kg) Water to cementing material ratio Measured slump (nun)

~Quantities adjusted for moisture content of aggregates bMix DM19-1 had a blend of 715 kg of 19 m m coarse aggregate and 419 kg of 9 m m coarse aggregate CNot counting the water in the superplasticizer

were capped approximately 1 h prior to being tested. All specimens were tested using a 2500 kN universal testing machine, with the upper bearing block sphere 189 mm in diameter, and the upper bearing block face 254 mm in diameter.

Specimen end conditions Capped end conditions are a standard for normal strength concrete testing, and sulfur capping compounds having compressive strengths of the order of 50 MPa, are readily available for use in normal strength concrete testing. If standard capping procedures (with usual cap thicknesses) are employed, these capping compounds are not of sufficient strength for use in the testing of the high performance concrete cylinders, that have compressive strengths of the order of 100 MPa or higher. Because of this, the cylinder ends of the high performance concrete cylinders are usually prepared by grinding them to form two, relatively smooth, plane, parallel surfaces. A specially designed grinding machine is usually employed to accomplish this objective. Some laboratories have adapted a large metal lathe for this purpose. The purchase of either of these equipments may be expensive for small testing laboratories. To eliminate the need for this special equipment, a simple specimen end preparation procedure, developed at the Lakehead University, is presented here, in which the capped cylinder ends are simply confined using circular steel rings; one of them is shown seated on the base of the capping apparatus in Figure l(a). The role of the steel ring at each end of the specimen is to provide lateral confinement to the specimen caps. This confinement restricts the lateral deformation of the cap, thus increasing its vertical compressive strength. In the present study, the confining rings were cut from a steel pipe of standard wall thickness of 6.4 ram. Pipes having outer diameter of 125.6 mm and 175.6 mm were used for obtaining confining rings for the concrete cylinders having diameter of 100 mm and 150 ram, respectively. All confining rings were 10 mm in depth. The dimensions of the cap and the confining ring are shown in Figure 2. To facilitate the end capping of the cylinders, special steel capping guides were fabricated, as shown in Figure l(a). Note that in this photograph, one of the confining rings is shown in a position, ready for capping.

The steel plate at the bottom was machined using a recess for a quick and accurate positioning of the confining ring, before the hot capping compound was poured into the ring and the cylinder guided into position (Figure l(b)), to make the confined cap (Figure l(c)). The machined surface of the base plate complied with the standard flatness tolerances, used in North America. For an easy removal of the capped cylinder from the capping guide, a thin layer of oil was applied to the recess on the base plate , before the confining ring was positioned into the recess and the melted sulfur compound poured into the ring. The inner surface of the confining ring was kept oil-free. After a cap had been placed, the confining ring was left on the capped end until the completion of the cylinder test. Once both the ends were prepared in this manner, the cylinder was tested for its compressive strength in the usual manner. Capping operations explained in the above-mentioned paragraphs are as simple as those used for normal strength concrete cylinders. The confining rings can be reused numerous times, if the rings, after a test, are removed carefully without damaging their shape and flatness. A test method, that uses a neoprene pad in a steel restraining pot at each end of the cylinder reported by Carrasquillo and Carrasquillo l, seems to have given satisfactory results for concrete strengths up to 76 MPa. However, for higher strengths, the neoprene pads must be changed for each cylinder tested, making the method uneconomical for high performance concrete cylinders2. Another method of end capping, that uses the so-called "sand box", has been recently reported by Boulay and de Larrard2. This method requires an elaborate preparation of the cylinder ends, making it cumbersome for the laboratory and impractical for field testing. The proposed method using the confining rings is simple enough both for the laboratory and field applications.

Testing machines and test specimens With compressive strengths of high performance concrete cylinders of the order of 100MPa now being fairly common, a testing machine having a capacity of 2500 kN is required to test a 150 ram×300 mm cylinder. A testing machine of this capacity may not be adequate even, if the actual design overcapacity of the machine does not make the testing machine stiff enough. Testing machines used for compressive strength testing of the high performance concrete cylinders must be relatively stiff, so that the elasticity of the machine itself does not produce a sudden release of energy at the time of the specimen failure that can affect the measured test result. Lessard et aL 3 have recently recommended that the testing machine capacity should be approximately 1.5 times the average expected cylinder strength, primarily not only for the testing machine characteristics being stiff enough but also a sudden release of energy can affect the calibration of the testing machine. Although this does seem to be a reasonable guideline to follow, the actual design and overcapacity of the testing machine should be considered in making this judgement. It is noted that by the criterion recommended by Lessard et

Compressive strength testing of HPC cylinders: S. Ali Mirza and C. D. Johnson

591

(a)

(b)

(c) Figure 1 Confined capping apparatus developed at the Lakehead University for compressive strength testing of HPC cylinders: (a) capping guide with rentovable confining ring in place; (b) cylinder placed on melted capping compound inside confining ring and held vertically against capping guide until capping compound hardened; and (c) prepared confined cap with ring in place at one of the cylinder ends after removal from apparatus

Compressive strength testing of

592 y

HPC

cylinders: S. Ali Mirza and C. D. Johnson

CONFINING STEEL RING CONFINED CAP

magnitude are commonly available in most concrete testing laboratories. Of course, the maximum aggregate size used in the specimen must be taken into consideration while selecting the appropriate cylinder size. For maximum aggregate sizes up to 19 ram, 100 m m x 2 0 0 m m cylinder test results are v e r y c o n s i s t e n t with t h o s e for 150 nurlx300 mm cylinders, throughout the age range 7 61 days, as shown in Table 2. The average compressive strengths given in this table are also plotted in Figures 3 and 4 for the DM9-1 and DM19-1 concrete mixes, respectively. The average relative strength of specimens of the size 100 mm x 200 mm compared to specimens of the size 1 5 0 m m x 3 0 0 m m is about 103% for each mix, considering all the ages. The average relative strength is also 103% for the combined set of cases for both the mixes, and both the 28- and 61-day strengths being taken as a group. The confined caps, discussed in greater detail in the following section, were used to prepare the ends of the specimens reported in Table 2 and Figures 3 and 4.

~i ~< 6.4MM ' ._V._

f//z///z~

- - - - A -.>-

r,/ IM A

z~ ..J )t..)

~--1L.O 7-

CYLINDER DIAMETER

Comparison of test results ~<6,4MM

Figure2 Verticalcross-sectionof a cylinder with prepared confined cap at both ends

The compressive strength test results for the three high performance concrete mixes, obtained on the basis of two different cylinder sizes with two different cylinder end conditions, are presented in Tables 3 and 4. The average values and coefficients of variation for both the 28- and 61-

al. 3, testing machine capacities of 2650 kN and 4000 kN would be required for the testing of 1 5 0 m m x 3 0 0 m m cylinders at compressive strengths of 1 0 0 M P a and 150 MPa, respectively. Most concrete testing laboratories, that are set up for testing normal strength concrete specimens, do not have testing machines of these capacities. As a consequence, 1 0 0 m m x 2 0 0 m m cylinders are gaining more acceptance in the literature and may emerge as the standard specimen size to be used in the compressive strength testing of the high performance concrete cylinders. Using the previously stated criterion, concrete cylinders of the size 100 m i n x 2 0 0 mm, having compressive strengths of 100 MPa and 150 MPa, could be tested using 1200 kN and 1 8 0 0 k N capacity testing machines, respectively. Machines having load capacities in these orders of

1

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

| oo .>

7 Days

14Days

28Days

61 Days

Age Figure3 Averagecompressive strength of cylinders with confined caps tested at various ages for concrete mix DM9-1

Table 2 Statistics of compressive strength of cylinders with confined caps tested at various ages 7-day s~ength Cylinder size (mmxmm)

Average value (MPa)

14-day s~ength

Coefficient A v e r a g e of variation value (%) (MPa)

28-day s~ength

Coefficient A v e r a g e of variation value (%) (MPa)

Coefficient of variation (%)

61-day strength Average value (MPa)

Coefficient of variation (%)

Mix DM9-1 100x 200 150x300

75.4 75.7

1.6 1.9

84.4 85.7

4.9 0.9

94.8 89.6

2.6 2.6

97.9 3.8"

1.6 1.8a

90.3" 90.9

1.2a 2.8

94.9 91.8

1.9 3.6

Mix DM19-1 100x200 150x 300

75.0 68.6a

1.0 1.6'~

82.5 81.1

2.9 2.1

aEach of these values represents two tests; the rest are based on three tests

Compressive strength testing of HPC cylinders: S. Ali Mirza and C. D. Johnson 120 I

: '

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

7 Days

14 Days

1so x 3oo]

28 Days

61 days

Age Figure 4 Averagecompressivestrength of cylinderswith confinedcaps tested at various ages for concrete mix DM19-1

Table 3 Averagecompressivestrength(MPa) for differentend conditions 28-day strength Cylinder size (nunxmm)

Ground ends

Ends with confined caps

61-day suength Ground ends

Ends with confined caps

102.6 95.6

97.9 93.8a

-

-

Mix DM9-1 100x200 150x300

98.2 91.6

94.8 89.6 Mix DM9-2b

100×200 150x300

107.7 106.8

105.5 102.6 Mix DM19-1

100x200 150×300

96.3 89.5a

90.3" 90.9

94.9a 94.7a

94.9 91.8

aEach of these values represents the average of two tests; the rest are averages based on three tests bCylindersmade up of DM9-2 mix were tested only at 28 days Table 4 Compressivestrength coefficientof variation (%) for different end conditions

Cylinder size (mmx mm)

28-day strength

61-day s~ength

Ground ends

Ground ends

Ends with confined caps

1.8 1.9

1.6 1.9a

Ends with confined caps

593

4 are the averages from three test specimens, with a few exceptions, as indicated in these tables. An examination of coefficients of variation of the c o m p r e s s i v e s t r e n g t h s (Table 4) r e v e a l s that the 1 5 0 m m × 3 0 0 m m cylinders, made up of the DM19-1 mix, produced a higher than expected coefficient of variation (5.9%) in one of the cases. The 100 m m x 2 0 0 m m cylinders, made up of the DM19-1 mix, and cylinders of both the sizes in DM9-1 and DM9-2 mixes, gave lower coefficients of variation (less than 5%), as indicated in Table 4. Hence, according to the ACI Committee 214 criteria4 for within-test coefficients of variation of laboratory batches, the concrete is classified fair to excellent. For the purpose of making comparisons, the compressive strength of a 150 m m x 3 0 0 m m cylinder with ground ends is taken to represent the "standard" compressive strength. Relative compressive strengths for the other cylinder size ( 1 0 0 m m x 2 0 0 m m ) and cylinders having the confined cap end condition are compared with the "standard" in Figure 5. Each relative strength value in Figure 5 is the average of approximately 12 tests. This includes the test data for all the three concrete mixes and both the test ages given in Table 3. As shown in Figure 5, the average relative test strengths for cylinders utilizing confined caps are very close to those for the "standard" (size 150 m m × 300 mm) test specimens with g r o u n d ends. For the c y l i n d e r s of the size 150 m m × 3 0 0 m m with confined caps, the relative strength is 98.1% of the "standard" strength. The results for the cylinders of the size 100 m m x 2 0 0 m m are also in close agreement with those for the "standard" strength considering the effect of cylinder size, with relative strengths of 101.2% and 104.6% for confined caps and ground ends, respectively. The coefficients of variation of the test data in Figure 5 ranged approximately from 3.5% for cylinders of the size 150 m m x 3 0 0 m m with confined caps to 2.7% for 100 m m x 2 0 0 m m cylinders with both confined caps and ground ends. However, the coefficients of variation for individual groups of data in Table 4 vary in the range 0.9%-5.9% for cylinders with ground ends, and in the range 1.2%-3.6% for cylinders with confined caps, indicating slightly higher dispersions in the test data for

Mix DM9-1 100×200 150×300

1.5 4.5

2.6 2.6

120 Ground

I

Mix DM9-2b 100x200 150x300

2.1 0.9

2.6 3.6

Confined Cap

-

-

5.0~ 2.6a

1.9 3.6

IT"

Mix DM19-1 lOOx200 150×300

1.9 5.9~

1.2' 2.8

aEach of these valuesrepresents the coefficientsof variationbased on two tests. "]'herest of the valuesgivethe coefficientsof variationbased on three tests bCylindersmade up of DM9-2 mix were tested only at 28 days day ,,,trengths are reported in these tables. End conditions include ground ends, and capped ends with confining rings (contined caps developed at the Lakehead University). The test results presented, for a given condition, in Tables 3 and

100 X 200

150 X 300

Cylinder Size (mm)

Figure 5 Averagecompressive strength of cylindersrelative to strength of standard 150 mmx 300 nun cylinderswith ground ends

Compressive strength testing of HPC cylinders: S. Ali Mirza and C. 19. Johnson

594

v

E z°

,

lOO .

.

.

.

.

.

i

Ground

lOO

,,.,

...........

~

. . . . . . . . .

lConfined l Cap

.... lOOX~

l~X~

Cylinder Size (ram)

Figure 6 Average compressive strength of cylinders relative to strength of cylinders of same size with ground ends

cylinders with ground ends. Although a definite conclusion cannot be drawn for the present time due to limited number of cylinder tests reported here, it may be slightly more difficult to obtain consistency in test results for specimens with ground end conditions. Imperfections, resulting in a slightly non-plane ground surface can affect the failure mode of a specimen, and thus, the measured cylinder strength. In this context, it may be noted, for the study reported here, the cylinders were ground at the Ontario Ministry of Transportation Laboratory in Thunder Bay, Ontario, Canada, met the planeness and perpendicularity requirements of the standard North American practice, and are expected to represent the typical ground end conditions for the industry. The relative compressive strengths of cylinders with confined capped end are compared with those of the corresponding cylinders of the same size but with ground end conditions, as shown in Figure 6. Each relative strength in Figure 6 represents the average of approximately 12 tests. This includes the test data from all three concrete mixes and both test ages, given in Table 3. The results are very similar to those discussed previously. Cylinders with confined caps have relative compressive strengths in close agreement with specimens of the same size having ground ends. For cylinders, 1 5 0 m m × 3 0 0 m m and

Figure 8

Typical failure modes of 150 mmx300 mm cylinders

100 mm×200 mm in size, the relative strengths of those with confined caps are 98.1% and 96.7%, respectively, as indicated in Figure 6. The respective coefficients of variation computed were approximately 3.5% and 2.5%. In Figures 7 and 8, typical photographs of the specimens after failure for cylinders of sizes 100 mmx200 mm and 150 mmx300 mm, respectively, are shown. It should be noted that the cylinders with confined caps have conical type failure modes, very similar to those obtained for cylinders with ground ends.

Conclusions The confined capping system, developed by the authors at the Lakehead University provides a simple, inexpensive approach for the compressive strength testing of high performance concrete cylinders. This approach ensures cap confinement without tight controls on the cylinder end roughness prior to capping, and on the cap thickness itself. The method has been successfully used for concretes having compressive strengths in excess of 100MPa. However, the upper limit on cylinder strengths, that can be tested by this method using capping compounds presently available in the market, will be established after further experimental investigations that are currently underway at the Lakehead University. Compressive strength test results obtained for cylinders, both 150 mm×300 mm and 100 mm×200 mm in size, tested using confined caps are in good agreement with the "standard" measured compressive strength of cylinders, 150 mmx300 mm in size, with ground end conditions.

Acknowledgements

Figure 7

Typical failure modes of 100 mmx200 mm cylinders

The authors wish to thank Bruce Beames, Paul Ell, Kelly Kerr, Eric Powell, Edith Ramanathan, Sylvain Rivet and John Rocke for assisting in the specimen testing and preparation of figures. The financial assistance, provided to the first author by the Natural Sciences and Engineering Research Council of Canada and by the Lakehead University Senate Research Committee, is gratefully acknowledged. The grinding support of the Ontario Ministry of Transportation Laboratory, Thunder Bay, Ontario is also appreciated.

Compressive strength testing of HPC cylinders: S. Ali Mirza and C. D. Johnson References 1 C~u'rasquillo, EM. and Carrasquillo, R.L., Effect of using unbonded capping systems on the compressive strength of concrete cylinders. ACI Materials Journal, 1988, 85(3), 141-147. 2 Boulay, C. and de Larrard, E, A new capping system for testing HPC cylinders: the sand-box. Concrete International, 1993, 15(4), 63-66.

595

3 Lessard, M., Chaallal, O. and Aitcin, EC., Testing high-strength concrete compressive strength. ACI Materials Journal, 1993, 90(4), 303-308. 4 ACI Committee 214, Recommended practice for evaluation of strength test results of concrete (ACI 214-77). In AC1 Manual of Concrete Practice (Part 1), Detroit, Michigan, 1989, 214-1 to 214-14.