Study on Estimated In-situ Cube Strength from Cores and Cube Strength

Transcription

1 Study on Estimated In-situ Cube Strength from Cores and Cube Strength Undergraduate Research Opportunities Program (UROP) Report submitted by SEAH JIN WERN JAIME W-11 Supervisor: A. Prof. Swaddiwudhipong Somsak Faculty of Engineering Department of Civil Engineering

2 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 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 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

3 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

4 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 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 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 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, x 100 mm cube specimens and x 110 x 110 mm beams are obtained. From each of the 3 beams, 3 cores are drilled perpendicular to the casting direction - 3 -

5 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 Cubes Cored Cylinder Cube/Cylinder (MPa) (MPa)

6 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 - 5 -

7 References 1. Indelicato, F. A statistical method for the assessment of concrete strength through micropores. Materials and Structures, 26, No. 159, pp Concrete Society, Concrete core testing for strength. Technical Report No. 11, pp. 44, London Murdock, J.W., Kelser, C.E. Effect of Length to Diameter Ratio of Specimen on the Apparent Compressive Strength of Concrete. ASTM Bull. pp April Neville, A.M., Properties of Concrete. Addison Wesley Longman Limited th Edition. 5. Petersons, N. Should standard cube test specimens be replaced by test specimens taken from structures? Materials and structures, 1, No. 5, pp Paris