Study on Estimated In-situ Cube Strength from Cores
and Cube Strength
Undergraduate Research Opportunities Program (UROP)
Report submitted by
SEAH JIN WERN JAIME
99-6004W-11
Supervisor: A. Prof. Swaddiwudhipong Somsak
Faculty of Engineering
Department of Civil Engineering
Study on Estimated In-situ Cube Strength from Cores and Cube Strength
UROP Report
1.
ABSTRACT
The strength of cored cylinders to be converted into estimated in-situ cube strength is dependent
on their Length/Diameter (L/D) ratio as well as their coring direction. The conversion equations
can be found in SS 78:A20:1987 (BS1881: Part120). This study aims to compare the
compressive strength of cores drilled from plain concrete beams with accompanying cubes. The
direction of coring is perpendicular to the casting direction of the beams and the widths of the
beams are 110 mm. The cores tested are 100mm in diameter and grinded to a length of 100mm.
2.
LITERATURE REVIEW
2.1
Testing of hardened concrete
There exists various method of testing the strength of hardened concrete. They can be broadly
classified into 2 types:
1. Mechanical tests
¾ Specimens are tested to the point of destruction. The purpose of this is to determine the
maximum loading a concrete structure can take before collapse. A common mechanical
test widely used is the compressive strength test.
2. Non-destructive tests
¾ These tests can be carried out on specimens as well as on the actual structure. As the
specimen / structure is intact after testing, such tests are good for monitoring the change
of properties in the structure over time. One such test is the Ultrasonic Pulse Velocity
(UPV) Test.
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Study on Estimated In-situ Cube Strength from Cores and Cube Strength
UROP Report
It is important to know the influence of each test method on the measured property of the
concrete as different methods and techniques are used in different countries.
2.2
Compressive Strength Tests
Compressive strength tests are most commonly used and are simple to perform. As concrete is
often designed to take compressive loads, the compressive strength of concrete is therefore an
important property.
In Great Britain, Germany and other European countries, cube specimens of 100 mm and 150
mm are used. During testing, cubes are placed perpendicular to their as-cast positions. In the
United States, France, Canada, Australia and New Zealand, cylinders are the typical specimens.
2 common cylinder sizes are 100 mm in diameter X 200 mm in length and 150 mm in diameter
X 300 mm in length. During testing, the top surface of the cylinder is in contact with the
platens. Hence they need to be grinded or capped to ensure that they are sufficiently flat.
2.3
Ultrasonic Pulse Velocity (UPV) Tests
The UPV test consists of the measurement of the time taken by an ultrasonic pulse to travel the
distance of the length of the specimen. The ultrasonic wave velocity is then related to the
density of the concrete. Generally, a higher value of UPV implies higher compressive strength
when moisture conditions are kept constant. This is because the pulse usually travels faster
through a water-filled void than an air-filled one.
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Study on Estimated In-situ Cube Strength from Cores and Cube Strength
UROP Report
2.4
Comparison of strength of standard cubes and standard cylinders
Standard Cubes (L/D = 1) and standard cylinders (L/D = 2) derived from the same batch of
concrete display differences in compressive strength. This is because of the difference in
influence of the platen’s restraining effects.
According to BS 8110: Part 120:1983, the strength of a cylinder is approximately 0.8 that of a
cube. Nonetheless, research has shown that the ratio of strength of a cylinder vs cube increases
as the strength of concrete increases. At strengths of more than 100 MPa, the ratio approaches 1.
2.5
Effect of Length / Diameter (L/D) ratio on the strength of cylinders
Standard cylinders cast in moulds have a L/D ratio of 2, while cored cylinders depend on the
size of the coring tool and the thickness or width of the slab or beam it is derived from.
Cylindrical cores of L/D ratio lesser than 2 tend to give higher measured strength as compared
to those with larger L/D ratio. Table A1 in the appendix shows the correction factors for
strength of cylinders with different L/D ratio as provided by ASTM C 42-90 and BS 1881:Part
120:1983. Higher strength concrete however are less affected by the L/D ratio as found by
Murdock and Kesler (1957) (Figure A1). Neville (1998) infers that there is comparatively little
difference between the strengths of cube and cylinder with h/d ratio of 1.
3.
METHODLOGY
To carry out the research, 5 batches of concrete of different targeted strength are cast. The range
of strengths from 20 MPa to 100 MPa can be found in Table A2 in the appendix. For each
batch of concrete, 9 100 x 100 mm cube specimens and 3 1000 x 110 x 110 mm beams are
obtained. From each of the 3 beams, 3 cores are drilled perpendicular to the casting direction
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Study on Estimated In-situ Cube Strength from Cores and Cube Strength
UROP Report
and another 3 parallel to the casting direction. The former 3 cores will be used for this project
while the later 3 cores are being used for research purposes beyond the scope of this project
(Figure A2). The cores are each grinded to lengths of 100 mm. The cube and cylindrical
specimens are cured and tested at 7, 14 and 28 days. For each test, 3 samples are used and the
mean strength of the 3 specimens is found.
4.
RESULTS AND DISCUSSION
Table 4.1 shows the compressive strength results for the 2 types of specimens and ratio of their
strengths. Details of the results can be found in Table B1 in Appendix B.
Table 4.1: Comparison of compressive strengths of 100 x 100 cubes and cored cylinders
Compressive Strength
Strength Ratio
100 x 100
mm
100 x 100 mm
Cube/Cylinder
Cubes
Cored Cylinder
(MPa)
(MPa)
19.6
18.6
1.05
23.3
21.1
1.10
26.6
24.1
1.10
30.5
27.1
1.12
40.1
36.8
1.09
48.1
42.8
1.13
49.2
41.6
1.18
59.7
51.0
1.17
70.4
59.2
1.19
52.3
43.2
1.21
66.6
55.4
1.20
74.6
60.7
1.23
69.9
57.6
1.21
90.8
75.4
1.20
91.4
75.3
1.21
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Study on Estimated In-situ Cube Strength from Cores and Cube Strength
UROP Report
Table 4.1 shows that the strength of cubes is generally higher than that of cores drilled parallel
to the potential layering of concrete. It can be seen that the cube to cylinder strength ratio for all
15 specimens are greater than 1. Also, the ratio also tends to increase with the increase in
strength of specimens.
One reason why the cored specimens are likely to generate lower results is that during the
coring process, the drilling operations weaken the bonds between the aggregate and the
surrounding hardened cement paste. Also, in high strength concrete, the bonds between cement
paste and aggregates are higher and more cohesive. This results in more resistance during the
coring operation and greater shearing between the coring bit and the concrete surface. This
would cause greater damage to higher strength concrete as coípared to low strength concrete.
Hence, the cube to cylinder strength ratio would drop.
Since both specimens have L/D = 1, it is concluded that coring indeed lowers the strength of the
specimens
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Study on Estimated In-situ Cube Strength from Cores and Cube Strength
UROP Report
References
1.
Indelicato, F. A statistical method for the assessment of concrete strength through
micropores. Materials and Structures, 26, No. 159, pp. 261-7. 1975.
2.
Concrete Society, Concrete core testing for strength. Technical Report No. 11, pp. 44,
London. 1976.
3.
Murdock, J.W., Kelser, C.E. Effect of Length to Diameter Ratio of Specimen on the
Apparent Compressive Strength of Concrete. ASTM Bull. pp.68-73. April 1957
4.
Neville, A.M., Properties of Concrete. Addison Wesley Longman Limited. 1998. 4th
Edition.
5.
Petersons, N. Should standard cube test specimens be replaced by test specimens taken
from structures? Materials and structures, 1, No. 5, pp. 425-35. Paris. 1968.
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