Bs 1881 Part 116.pdf
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Bs 1881 Part 116.pdf - A Guide to Testing Concrete Cubes
Concrete is one of the most widely used construction materials in the world. It is composed of cement, water, aggregates and sometimes admixtures. The properties of concrete depend on the quality and proportions of these ingredients, as well as the curing conditions and the age of the concrete. To ensure that concrete meets the desired specifications and performance criteria, it is necessary to test its strength, durability, workability and other characteristics.
One of the most common methods of testing concrete strength is by using concrete cubes. These are small specimens of concrete, usually with a size of 150 mm x 150 mm x 150 mm, that are cast and cured under controlled conditions. The cubes are then subjected to a compressive load until they fail. The maximum load that the cube can withstand divided by its cross-sectional area gives the compressive strength of the concrete.
The British Standard BS 1881: Part 116: 1983 provides a method for determining the compressive strength of concrete cubes. This standard specifies the apparatus, test specimens, procedure, type of failure, calculation and expression of results, and test report for this test. The standard also gives some precision data for measurements of the compressive strength of hardened concrete, expressed as percentages of the mean of the two cube strengths whose difference is to be compared with r or R.
The standard states that the test specimens should be made from representative samples of fresh concrete, taken in accordance with BS 1881: Part 101. The specimens should be cast in steel or cast iron moulds that are rigid, watertight and have smooth internal faces. The moulds should be lightly oiled before use and cleaned after each casting. The specimens should be compacted either by hand or by vibration, depending on the consistence of the concrete. The specimens should be stored in a moist condition at a temperature of 20 5 C for at least 16 hours before demoulding. The specimens should then be cured either in water or in moist air until they are tested.
The standard states that the test should be carried out as soon as possible after the specimens have been removed from the curing medium. The specimens should be wiped clean and any loose material removed from their surfaces. The dimensions of each specimen should be measured to the nearest 0.2 mm and their mass recorded. The specimens should be placed between the platens of a compression testing machine that can apply a load at a constant rate of stress within the range of 0.2 to 0.4 N/mm/min. The load should be applied without shock and increased continuously until failure occurs.
The standard states that the type of failure should be recorded according to the diagrams shown in Figures 1 and 2. Satisfactory failures are those that occur along one or more vertical planes without any sign of twisting, shearing or buckling. Some unsatisfactory failures are those that occur due to misalignment, eccentricity, variation in specimen dimensions, or machine faults. If any specimen shows an unsatisfactory failure, it should be discarded and another specimen tested.
The standard states that the compressive strength of each specimen should be calculated by dividing the maximum load carried by the specimen by the average cross-sectional area based on the measured dimensions. The compressive strength should be expressed to the nearest 0.5 N/mm. The mean compressive strength of the group of specimens should also be calculated and reported. If any individual strength differs from the mean by more than 15 %, it should be considered as suspect and excluded from the calculation.
The standard states that the test report should include the following information:
- identification and date of testing;
- specification of concrete mix (e.g. strength grade);
- consistence of concrete;
- air content of concrete (if air-entrained);
- age and curing conditions of specimens;
- dimensions and mass of each specimen;
- maximum load and compressive strength of each specimen;
- type of failure of each specimen;
- mean compressive strength and number of valid results;
- any deviations from the standard method.
The standard also provides a table of precision data for measurements of the compressive strength of hardened concrete, based on the results of an inter-laboratory test programme. The table gives the values of r and R, which are the differences between two single test results that are expected to be exceeded in only 5 % of cases in normal and acceptable test results, respectively. The table also gives the values of s and sr, which are the standard deviations of individual and average test results, respectively. The table can be used to assess the accuracy and reliability of the test results.
In conclusion, BS 1881: Part 116: 1983 is a standard method for determining the compressive strength of concrete cubes. It is widely used in the construction industry to ensure that concrete meets the required specifications and performance criteria. The method is simple, reliable and accurate, provided that the specimens are prepared, cured and tested in accordance with the standard. The method can also be used to compare the quality of different batches of concrete or to investigate the effects of various factors on the strength of concrete.
References:
- BS 1881: Part 116: 1983. Testing concrete. Method for determination of compressive strength of concrete cubes. British Standards Institution.
- BS 1881: Part 101: 1983. Testing concrete. Method for sampling fresh concrete on site. British Standards Institution.