Hardened Concrete Properties Tests Essay Examples & Outline
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Hardened Concrete Properties Tests
The main objective of this experiment is to determine the 28 day compressive strength and indirect compressive strength of the hardened specimens of concrete.
Being one of the most widely used building material, concrete compressive and tensile strengths are the most important concrete performance measures used in structural design. Compressive strength is a measure of the ability of concrete to resist crushing loads. The results of this test are mainly used establish that the delivered concrete mixture meets the specified strength requirements of a given project specification. Concrete structures are subjected to tensile stress from applied loading and other kinds of effects, which can result in tensile cracking.
The tensile strength is usually about 10-20% of the compressive strength. Splitting test is an indirect standard test method used to determine tensile strength of a concrete specimen. Plywood strips placed between the loading platens on the compression machine and specimen allow for uniform distribution of the applied load, and also reduces the compressive stresses at the point of the applied load.
The compressive strength of concrete is measured by breaking cylindrical specimens of concrete in a compression testing machine. The specimens can also be cubic, with cubes measuring 150mm by 150mm by 150mm or 100mm by 100mm by 100mm, depending on the aggregate size used.
In this lab experiment, two compressive tests one splitting tests were performed on cylinders specimens from batch A and D.
-Compression testing machine
[A] Compression Test
Cylinder diameter (d) = 100mm
Cylinder height (h) = 200mm
Two cylinder specimens were cast and cured to attain 28 day strengths. After wiping the specimen to remove excess water, it was placed vertically on the platform of the compression machine. A load of 160kN/min was then applied uniformly without shock until the specimen failed. The failure load (Pmax) was recorded down. The failed sample was inspected to make detailed observations about the failure mode. The steps above were repeated for the second specimen.
[B] Splitting test
-Cylinder diameter (d) = 150mm
-Cylinder height (h) = 300mm
The 28 days concrete specimens cured under standard conditions of temperature and humidity were set in a splitting apparatus. The load was then applied diametrically and uniformly along the cylinder length at a rate of 106kN/min until it failed along the vertical diameter, then the failure load attained (Pmax.) recorded. The failed sample was inspected to make detailed observations about the failure mode.
RESULTS AND DISCUSSION
Compression Test Results
Batch A Cylinder 1 Cylinder 2
Failure Load Pmax (kN) 95 123.2
(MPa) 12.10 15.69
For cylinder 1:
Compressive strength (Pmax)
For cylinder 2:
Batch D Cylinder 1 Cylinder 2
Failure Load Pmax (kN) 166.3 139.5
(MPa) 21.18 17.76
For cylinder 1:
Compressive strength (Pmax in N/ Area in mm2)
For cylinder 2:
There are certainly a number of possible reasons as to why the concrete strengths may vary within a particular batch. One such reason is air content in the batch. Controlling air content in a concrete mixture is probably hard to control. High air content reduces concrete strength. Concrete strength reduces by about 4-5% with a 1% increase in air content. Personal errors can sometimes be a culprit in a concrete mix, which may lead to varied strength for the same batch. Routine inspections and paying attention during the batching process can reduce errors due to handling.
Splitting Strength Test Results
Batch D Batch A
Failure Load Pmax (kN) 231.8 231.7
(MPa) 3.279 3.278
For batch D:
Tensile splitting strength,
For batch A:
Tensile splitting strength,
Theoretical Tensile splitting strength values
The indirect tensile stress generated by the poison’s effect applied causes the concrete specimens to split along the vertical plane into two halves. An element along the vertical diameter of the specimen is subjected to both vertical compressive stress as well as horizontal stress due to the applied compressive load. As a result of this loading condition, a high compressive strength is produced below the loading points immediately. The larger portion of the specimen that corresponds to its depth is subjected to a tensile strength that is uniform and acts horizontally. Under this test, the cylinder specimens failed by breaking into two halves.
The experimental splitting values are higher compared to theoretical values. These differences are brought about by a number of departures from the theoretical splitting. First, the theory assumes that the concrete material is homogenous and linear elastic, which is not. Second, the load is distributed within a plywood strip rather than being applied along a line. Loading on the strip produces large compressive stresses that lie perpendicular to the diametric plane under the loading strips. The specimen diameter, length and shape, the bearing strips, the specimen moisture condition, and the loading rate all have an effect on the experimental splitting strength.
Compressive stress and tensile stress are the two most important concrete properties that have to be established in the initial design of concrete structures. The tests for this properties are performed mostly after 28 days from the day of casting. However, the tests can also be done at 1, 3, 7, 14 or 56 days. The 28 day age is preferred because at this age, concrete has achieved about 98% of its strength. The average compressive strengths of the specimens are 13.895 MPa and 17.76 MPa in batch A and D respectively. The results of the compressive strength are used to determine if the concrete mixture meets the specified strength in a project specification. The experimental tensile strengths are 3.279 MPa and 3.278 MPa for batch A and D respectively. The tensile strength test is a model of how a concrete beam will be loaded.
Kett, Irving. Engineered Concrete: Mix Design and Test Methods, Second Edition. New York: CRC Press, 2009.