Comparison of Core and Cube Compressive Strength of Hardened Concrete

ISSN 2320 – 6020

IJBSTR RESEARCH PAPER VOL 1 [ISSUE 6] JUNE 2013

Comparison of Core and Cube Compressive Strength of Hardened Concrete Rohit Rai1, Ambareesh Kumar2, Ashish Singh3, Kanhaiya Lal Pandey4 and R D Patel* ABSTRACT- This paper presents the result of core and cube compressive strength of hardened concrete. Non-destructive test methods are used to investigate the properties and strength of hardened concrete. In existing concrete structures there was no direct relation between the results of non-destructive tests. This paper describes the relation between core compressive strength and cube compressive strength of hardened concrete in existing structures. Sixteen cores (diameter 75mm) were extracted from the hardened concrete of 15- years’ old building. Fifteen cores (48mm diameter) were also extracted from the same hardened concrete building.

Two sizes of cubes (150mm x 150 x 150mm) and (100mm x 100mm x 100mm) were made from the hardened concrete from stone pieces which were taken from the structural members of building with help of stone cutting machine. The cores and cubes compressive strength was determined in compression testing machine. It was found that smaller size cubes (100mm x 100mm x 100mm) and cores (48mm diameter) show greater strength as compared to larger size cubes (150mm x 150 x 150mm) and cores (diameter 75mm). It was also found that the cubes (150mm x 150 x 150mm) compressive strength is 74% the cubes (100mm x 100mm x 100mm) compressive strength. The 75mm diameter cores compressive strength is 65% the compressive strength of cubes (150mm x 150 x 150mm).

KEY WORDS: Compressive Strength, flexure Strength, Core.

INTRODUCTION Once concrete has hardened it can be subjected to wide range of tests to prove its ability to perform as planned or to discover its characteristics if its history is unknown. For new concrete this usually involves casting specimens from fresh concrete and testing them for various properties as the concrete matures. The concrete cube tests, concrete cylinder tests are the most familiar tests and are used as the standard method of measuring compressive strength for quality control purposes.

Concrete beam specimens are cast to test for flexural strength and cast cylinder cone be used for tensile strength. Specimens for many other tests can be made at the same time to assess other properties. For existing concrete samples will need to be taken from the structure. Non-destructive testing methods are useful in some instances and can help identify areas from which samples should be taken. The normal method of concrete sampling is by coring although same chemical analysis techniques can be carried out on drilling dust samples.

*

Associate Professor, Civil Engineering Department, MMM Engineering College, Gorakhpur-273010 1,2,3,4

Research Scholar, “structural engineering” (Civil Engineering Department) MMM Engineering College, Gorakhpur-273010

In laboratory many techniques can be used to examine and test hardened concrete to assess a wide Varity of properties. Nondestructive and semi-destructive methods play an important role in evaluating the existing structure conditions. Nondestructive test methods are used to indicate properties other than strength. British Standard defined non-destructive testing as a test that does not impair the intended performance of the element or member under investigation. The non-destructive evaluation techniques are used to assess the condition of concrete structures, to predict future performance and allow minor repair system. The best known non-destructive evaluation techniques use ultrasonic waves, core testing, Schmidt hammer test. The estimation of in place concrete strength requires that a known relationship between the results of in-situ testing and the strength of concrete. For existing construction the relationship has to be assessed on site correlating non-destructive test results to strength of core.

There was no any relation between core strength and cube strength of hardened concrete. In this research work a relationship between core strength and cube strength was developed.

This paper also describes the effect of diameter of core on compressive strength of concrete and effect of size of cube on compressive strength of concrete.

Cubes The compression text is carried out on specimens cubical and cylindrical in shape. The cube specimen is of the size 150mm.

If the largest nominal size of the aggregate does not exceed 20cm and 10cm size cubes may also be used as an alternative.

For each mix 15 cube specimens of size 150 x 150 x 150mm

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IJBSTR RESEARCH PAPER VOL 1 [ISSUE 6] JUNE 2013 were cast in C.I. moulds cubes was tested at 7 days, 14 days and 28 days of curing. Each compressive strength result is the average of test results.

The test was conducted as per IS 516 -1959. t h e c u b e o f s t a n d a r d s i z e 150x150x150mm were uses to find the compressive strength of were placed on the bearing surface of UTM, of capacity 2000KN without eccentricity and a uniform of loading of 140 kg per cm2 per minute was applied till the failure of the cube at failure, the failure of the maximum load was noted and the compressive strength was calculated. Cube compressive strength σcc (mpa)=Pf/Ab Pf=failure load (KN) Ab=bearing area of the cube (mm2) Compacting The test cube specimens made are soon as practicable after mixing and in such a way as produce full compaction of the concrete with neither segregation nor excessive laitance. The concrete is filled into the mould in layers approximately 5mm deep.

in the table – 1 & 2. The concrete stones were made into the regular cube shape of size 150mm x 150mm x 150mm and 100mm x100mm x 100mm by stone cutting machine. The cores were extracted by using rotary cutting machine with diamond bits. The extracted cores were trimmed to make smooth edges parallel and then capped with sulphur to make the ends smooth. After capping, cores were tested in compression testing machine in the concrete laboratory. No rebar were found in the cores. The results of cores are shown in table 1 & 2. After making the regular shape of cubes of sizes 150mm x150mm x 150mm and 100mm x100mm x 100mm they were tested in compression testing machine. The results of cubes are shown in table – 3. The average compressive strength of 150mm x 150mm x 150mm cubes was 24MPa [3508psi] and the average compressive strength of 100mm x 100mm x 100mm was found to be 32MPa [4608psi]. The average corrected compressive strength of cores having diameter 75mm was 17MPa [2466psi]. The average compressive strength of cores having diameter of 48mm was 18MPa [2611psi]. All cores were tested in dry condition. The strength of cores was calculated after applying all the factors given in ASTM C42-90.

CUBE TEST 

Compacting by hand

 

The cubical specimens, in no case should be the concrete be subject to less than 35 strokes per layer for 15cm or 25 strokes per layer for 10cm cubes

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OBJECTIVES 

The objectives of this research were: (1) To develop a relationship between core compressive strength and cube compressive strength.

(2) To study the effect of core diameter on the compressive strength of concrete.

It is to be noted that the cube is tested at right angles to the position at cast, therefore, eliminating the need for capping or grinding because the bearing faces are sufficiently plane.

CYLINDER TEST

(3) To study the effect of cube size on compressive strength of concrete.

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EXPERIMENTAL PROGRAM



Fifteen cores of 75mm diameters and 48mm diameters were extracted from the 15-years old concrete buildings at different locations of the structural members. Some cores were taken from columns, beams and some cores were taken from base slab, floor slabs and walls. In the same location concrete stone pieces were taken. The length of cores was variable as shown

Cube test developed by the BS is very popularly used in UK for determining compressive strength of concrete in the lab The test cube has a size of 150 mm (6 in.)

BS (British Standards) 1881: Part 108: 1983 prescribes filling the mold in layers All the treatments of the test cube after casting and before testing are covered by BS 1881: Part 111: 1983 Testing of the prepared concrete cubes is covered by BS 1881: Part 116: 1983

 

Cylinder test developed by the ASTM is very popularly used in USA for determining compressive strength of concrete in the lab The test cylinder has a diameter of 150 mm (6 in.) and length as 300mm (12 in.) to match with l/d ratio of 2:1 The specimens are prepared and tested as per the ASTM specifications, as follows: Testing of cylinders for splitting tensile strength

After 28 days of curing the cylinders were removed from the curing tank, weighed and tested for splitting tensile strength in a 2000KN ultimate testing machine as per IS456 –2000 at a

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IJBSTR RESEARCH PAPER VOL 1 [ISSUE 6] JUNE 2013 rate of loading, (1.2 to 2.4) (π/2) l*d, N/min. The maximum load applied on the specimen was recorded Here l = 300mm and d = 150mm. The experimental splitting tensile stress was calculated according to the above equation.

interpretation of the test results of compressive strength of such small cores. It has also been reported that the correction factor for the value of L/D is dependent on the diameter of the core.

CORE

CORE TEST

The taking of cores most commonly occurs when the results of tests on standard test cylinders to determine the 28-day compressive strength indicate noncompliance with the specification. Such noncompliance may be due to the fact that the concrete that was placed in a given part of the structure, as well as in the test cylinders, is indeed noncompliant because its 28-day strength is lower than specified. It may also happen that while the strength of test cylinders is satisfactory, there are suspicions that the concrete being placed in the actual structure has segregated or has been inadequately consolidated. This can be resolved by the inspection and testing of cores. There are other situations where taking cores may be desirable, or even essential. For example, it may be required to subject an existing structure to heavier loads than hitherto; or a change of use may be proposed, and the loadcarrying capacity of the structure needs to be verified; or it may be necessary to ascertain that the strength of concrete has not been impaired by overloading or by fatigue, fire, explosion, chemical attack, or some other deleterious agent.



Core test is commonly used to determine the compressive strength of concrete in the actual structure.

Cylindrical cores of sufficient length and diameter are cut from the structure using a coring machine.

These cores are then used for determining the compressive strength of concrete in the laboratory.

The methods for determining the compressive strength of cores are prescribed by BS 1881: Part 120: 1983 and by ASTM C 42, both are essentially similar.

  

The diameter of the core should not be less than 3 times the maximum size of aggregate. The length of the core should be between 1 and 2 times the diameter. After determining the average strength (fcore) of the moist cores, the estimated actual cube strength (fcube) is obtained from the following equation: (fcube) = (D × fcore) / (1.5 + 1/λ) Where,

The core cutter machine at given different size core in 48mm, 75 mm, 100mm and 150mm

D = 2.5 for cores drilled horizontally = 2.3 for cores drilled vertically

SIZE OF CORES It is considered preferable to use cores with a diameter of 48mm and 75 mm. It allows the use of smaller diameters when it is impossible to obtain a core with L/D of at least 1, but only for “cases other than load-bearing situations.” According to ASTM C 42, the minimum diameter of the cores is governed by the maximum aggregate size: it “should preferably be at least three times the nominal maximum size of the coarse aggregate and must be at least twice the nominal maximum size” European Standard BS EN 12504-1:2000 simply says that, when the core diameter approaches a value that is less than 3 times the maximum aggregate size, there is “a significant influence on the measured strength”. The reason for the limitations on the core size is that, unlike a molded cylinder, in a core some coarse aggregate particles are cut in the drilling process and are, therefore, not wholly bonded to the cement paste matrix. The adverse effect of incomplete bond is aggravated by the difference in the modulus of elasticity between the aggregate and the cement paste. When a significant proportion of coarse particles is in that state, some of them may become partially loosened during the test and cease to carry their share of the applied load. When this happens, a lower value of compressive strength is recorded.

This situation is recognized in the assessment of precision of tests on cores of various sizes. The confidence limits of the predicted strength of test cubes from the strength of such small cores are very wide, and I, for one, remain skeptical about the

λ = finished length/diameter ratio of the core Table – 1 Results of Cores (75mm diameter) Sr. No.

Dia.( mm)

Heigh t(mm)

Weig ht(gm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

75.00 74.80 73.50 74.30 72.90 72.30 74.50 74.70 73.90 72.90 74.50 72.30 72.90 73.70 74.50

154.6 156.0 157.0 158.2 161.8 150.0 158.4 157.2 152.0 161.0 163.0 152.6 166.0 159.0 159.0

1320 1320 1350 1370 1350 1350 1370 1310 1300 1310 1380 1330 1340 1300 1310

Crushi ng Load( KN) 105 100 75 110 80 80 120 90 80 100 130 100 90 90 80

Corrected Compressive strength of core(MPai) 25.00 24.00 17.92 26.79 19.48 18.45 26.80 19.90 17.04 23.96 29.11 24.36 21.50 19.65 20.47

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IJBSTR RESEARCH PAPER VOL 1 [ISSUE 6] JUNE 2013 Table – 2 Results of Core (48mm diameter)

Sr. No.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Dai.(mm)

47.40 47.30 46.40 47.20 45.90 46.70 47.30 47.50 46.80 45.90 47.50 47.30 45.60 46.00 46.30

Height (mm)

98.50 97.60 94.40 96.30 95.20 99.70 95.30 100.00 94.80 96.10 97.90 98.80 94.40 93.30 98.30

Weight (gm)

330.0 330.0 337.5 342.5 337.5 337.5 342.5 327.5 325.0 327.5 345.0 332.5 335.0 325.0 327.5

Corrected compressiv e strength of core(MPai) 16.64 20.14 21.02 22.40 19.50 17.30 24.36 16.88 18.36 21.43 24.64 23.64 19.94 17.37 19.43

Table – 3 Result of cubes testing Sr. No.

Cube size(mm)

Weight(gm)

Load(KN)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

150×150×150 150×150×150 150×150×150 150×150×150 100×100×100 100×100×100 100×100×100 100×100×100 100×100×100 100×100×100 100×100×100 100×100×100 100×100×100 100×100×100 100×100×100

8190 8150 8480 8550 8300 8160 8120 8510 8530 8210 8140 7840 8160 8440 8300

510 910 540 850 890 560 630 710 760 570 650 450 780 910 780

Corrected compressive strength 22.60 40.45 24.00 37.75 40.10 24.89 28.18 31.98 33.77 25.34 29.10 20.26 35.13 41.26 35.10

6 7 8 9 10 11 12 13 14 15

18.45 26.80 19.90 17.04 23.96 29.11 24.36 21.50 19.68 20.47

23.06 33.50 24.88 21.30 29.35 36.38 30.45 26.80 24.60 25.50

RESULTS AND DISCUSSION The cores (diameter 75mm) show greater compressive strength of concrete as compared to the equivalent cube compressive strength. The cubes standard size (150mm x 150mm x 150mm) show greater compressive strength. The cores (diameter 48mm) show greater compressive strength of concrete as compared to the cores (diameter 75mm). The smaller size cores have smaller cross-sectional area. The compressive strength is load per unit area. Due to smaller size, the strength of smaller diameter cores was greater as compared to the larger size diameter of cores. Similarly the smaller size cubes give greater strength as compared to larger size cubes.

Relation between Core Compressive Strength and Cube Strength The compressive strength of cores (75mm diameter) is 0.632 times the compressive strength of cubes (150mm x 150mm x150mm). The compressive strength of cores (75mm diameter) is 0.508 times the compressive strength of cubes (100mm x 100mm x 100mm). The core (48mm diameter) compressive strength is 0.752 times the compressive strength of cubes (150mm x 150mm x 150mm).The compressive strength of cores (48mm diameter) is 0.572 times the compressive strength of cubes (100mm x 100mm x 100mm).

Core strength of 75mm dia. to compressive cube strength

40.00 Equivalent cube strength is 516:195, 6 Relation between core strength and equivalent cube strength. Cube strength = 5/4 × corrected crushing strength.

Sr. No. 1 2 3 4 5

Core strength 25.00 24.00 17.92 26.79 19.48

20.00 0.00

cube strength

Equivalent cube strength 31.25 30.00 22.40 33.48 24.38

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IJBSTR RESEARCH PAPER VOL 1 [ISSUE 6] JUNE 2013 Compressive strength of 75mm and 48mm core dia.

Strength to compressive cube strength

The relation of 48mm dia. Core to corrected compressive cube strength

Chart Title core strength 75mm dia

core dia 48mm

cube strength 75mm dia

48mm core dia

core strength 48mm dia.

50 40 30

40.45 37.75 40.1 41.26 33.77 31.98 22.6 20.14 22.4 21.02 25.00 2426.79 19.5 28.18 24.36 16.64 35.13 35.1 24.00 24.89 29.1 26.80 24.64 17.3 25.34 21.43 23.64 29.11 17.92 19.48 18.36 16.88 18.45 19.90 23.96 24.36 19.94 20.26 19.43 17.37 17.04 21.50 19.68 20.47

20 10 0

1 3 5 7 9 11 13 15

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

75mm dia core strength to corrected compressive strength of cube

47.4 47.3 47.2 46.4 47.3 47.5 46.7 45.9 46.847.545.6 40.45 46 46.3 40.1 45.947.3 41.26 37.75 33.77 35.13 35.1 31.98 28.18 29.1 corrected 24.89 22.624 25.34 24.36 22.4 24.64 21.02 20.14 23.64 19.5 compressive 21.43 16.64 20.26 17.316.88 18.36 19.94 19.43 17.37 strength of cube corrected compressive strength of 48mm core dia.

Core 75mm dia. Is greater than compressive strength of concrete as compared to the equivalent cube compressive strength

80 70 60 50 40 30 20 10 0

80 70 60 1 3 5 7 9 11 13 15

75mm dia core

75mm dia. core

75mm dia. core

75mm dia core

1 2 3 4 5 6 7 8 9

50 40

compressive strength of 75mm dia. core

30 20

1 1 1 1 1 1 0 1 2 3 4 5

equivalent cube compressive strength

10

75mm dia core 757574747372757574737572737475 corrected compressive strength of core75mm dia

75mm dia core

252418271918272017242924222020

0 1 3 5 7 9 11 13 15

corrected compressive 234024384025283234252920354135 strength of cube

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IJBSTR RESEARCH PAPER VOL 1 [ISSUE 6] JUNE 2013 Core 48mm show greater compressive strength of concrete as compared to the core dia 75mm

5.

6.

48mm dia. core

50

Rasheed Uzzafar, D and Gahtani, A.S “Corrosion of reinforcement in concrete structure in the middle of east” concrete international, American concrete institute, vol., 7. No 9. Sep.1985, PP, 48-55.

Rasheed Uzzafar Dakhil, F.H and Bader, A.M toward solving the concrete determination problem in the gulf region, the Arabian , Journal of science and Engineering , theme issue on concrete durability vol.11nov.

48mm dia core

40 30 compressive strength of 75mm dia. core

20 10 0 1

4

7 10 13

compressive strength of 48mm dia. core

CONCLUSION From the results of the tested specimen the following conclusion were made:1. The compare strength of cubes was greater than the compressive strength of cores.

2. The smaller size cubes (100mm x 100mm x 100mm) give greater compressive strength of cubes as compared to larger size (150mm x 150mm x 150mm).

3. The smaller size cubes strength is 1.271 times larger size cube compressive strength.

4. The strength of cores (48mm) is 5% greater than cores (75mm).

Reference 1.

2.

3.

4.

Is 456 2000 plain and reinforced concrete code of practice. Concrete technology by M.S. Shetty.

British standard institution, Guide to the use of nondestructive methods of test for hardened concrete, BS, 881, 1986, part 201.

Fookes P.G “concrete in the middle test past, present and future, review” Damage Assessment Repair Techniques and strategies for reinforcement concrete Macmillan, G.L ed. Bahrain society of Engineers 1991.

5.Pocock ,D.C “ the selection o cost-effective repair strategies for corrosion damage concrete an determination and respire of reinforced concrete In the Persian Gulf , oct,1997, Bahrain vol. ,PP147-161.

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