The testing of cement is done in accordance with the standard procedures in BS EN 196, which is published in different parts for the various physical and chemical tests. The British Standard - which now implements the European Standard - for cement testing covers a wide range of properties including chemical analysis, fineness, strength, setting time, soundness and, in special cases, heat of hydration.
Aggregates
Aggregates
For good quality control, it is important to ensure that the aggregate is clean and does not contain any organic impurities which might retard or prevent the setting of the cement, and that the proportions of the different sizes or particles within a graded material remain uniform.
Cleanness
Accurate tests for determining the proportion of fines (clay, silt and dust) in sand and coarse aggregates are given in Standards, but these tests are only suitable for the laboratory. On site, cleanness can be assessed visually, although for natural sands the 'field settling' test will give an approximate guide to the amount of fines; the test is not appropriate for coarse aggregates or for crushed-rock sand.
The test entails placing about 50 ml of a 1% solution of common salt in water (roughly two teaspoonfuls per liter) in a 250 ml measuring cylinder. Sand is then added gradually until the level of the top of the sand is at the 100 ml mark, and more solution is added to bring the liquid level to the 150 ml mark. The cylinder is shaken vigorously, and the contents allowed to settle for three hours. The thickness of the layer of fines that settles above the sand (Figure 37) is then measured and expressed as a percentage of the height of the sand below the layer. The amount of fines in the sand is compared against previous test results to give a warning of changes in cleanness.
If a measuring cylinder is not available, a jam jar or bottle filled to a depth of 50 mm with sand and to a total depth of 75 mm with the salt solution will give comparable results if the contents are allowed to settle for three hours.
The field settling test gives only an approximate guide. Sands apparently containing large amounts of fines cannot be regarded as having failed to comply with the specification, and further laboratory tests to assess their suitability must be carried out. It is, however, a useful and quick way of detecting changes in the cleanness of sand.
There is no suitable site test for the cleanness of coarse or all-in aggregates or of crushed-rock sand, and reliance is usually placed upon the grading analysis (see below) to show whether there is an excess of fine dust in the material. Problems arise, however, because of the tendency for fines to adhere to the rough surface of crushed coarse aggregate.
Similarly, there is no suitable site test for the cleanness of a gravel coarse aggregate. It is important to ensure that aggregate particles are not coated with clay and that lumps of clay are not mixed in the aggregate. The presence of clay indicates that the aggregate has not been washed adequately before delivery or that the aggregate has subsequently become contaminated.
Accurate determinations of the fines content in aggregates are made in laboratories using either sedimentation or (more usually) decantation or wet sieving methods in accordance with Standard procedures.
Organic and other impurities
Coarse aggregates from any source, and crushed-rock sands, are unlikely to contain organic impurities, though natural sands may do so. Current aggregate standards include tests for organic impurities. If there is reason to suspect the presence of organic impurities that could retard the hydration of the cement, the effects should be determined by performance tests on concrete made with the aggregate in question.
Where appearance is an essential feature of the concrete, aggregates should be selected from sources known to be free from materials such as iron pyrites or particles of coal that could mar the surface. The only guide is a knowledge of the source and of similar work that has been carried out with the aggregate in question.
Sieve analysis
The grading of an aggregate is found by passing a representative sample of dry aggregate through a series of sieves, starting with the largest mesh. If the sieving is carried out by hand, each sieve is shaken separately over a clean tray for not less than two minutes. For many routine purposes mechanical sieving is advantageous, but if this method is used care should be taken to ensure that sieving is complete.
The material retained on each sieve, together with any material cleaned from the mesh, is weighed and recorded. The amount passing each sieve is then calculated as a percentage by weight of the total. Table 20 gives an example of a method for recording a sieve analysis and calculating the percentage passing each sieve. Comparison with Table 6 on page 12 will show that this sample is a sand falling within grading limit M.
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Table : Example of the method of recording sieve analysis of sand.
Sieving will not be accurate if there is too much material left on any mesh after shaking. The maximum weights of aggregate to be retained on a sieve to avoid overloading are given in BS 812.
The size of the sample tested depends upon the maximum size of the aggregate. For nominal 40 mm aggregate the sample should weigh at least 5 kg, for 20 mm at least 2 kg, for 10 mm at least 0.5 kg and for sand at least 0.2 kg.
The test results can be plotted on a chart similar to that shown in Figure 2 on page 11 so that the specified gradings and the sample gradings can be more easily compared. It should be noted that the points representing the percentage of material passing the various sieve sizes are joined by straight lines and not by curves. Although the BS 812 sieve test should be made on dried samples, an approximate grading, accurate enough for routine site testing, may be obtained by sieving coarse aggregates in a damp condition.
Moisture content
The purpose of measuring the moisture content of aggregate is to enable an estimate to be made of the quantity of water contained within it so that the water added at the mixer can be adjusted to control the required free water in the mix; as mentioned on page 1 3 (under Storage of aggregates), the moisture content of aggregates, especially the sand, can vary considerably from load to load or in the stockpile. Ideally, the weight of aggregate in each batch of concrete should be adjusted to allow for changes in the moisture content of the aggregate, but this is seldom practicable; adjustments to the batch weight of dry aggregate are therefore usually based on an average moisture content. There are various methods of determining the moisture content, which are fully described in BS 812 : Part 109 and outlined below.
Drying methods.
Methods involving the drying of representative samples of aggregate are often used. On site the quickest and most direct way of measuring the moisture content of both coarse aggregate and sand is the 'frying pan' technique in which the aggregate is dried by heating it in an open pan. The sample of aggregate to be tested should weigh between 1.8 and 2.2 kg for coarse aggregate. The sample size for sand may be reduced to not less than 0.5 kg if an accurate balance is used for weighing.
The aggregate is first weighed (W1), then dried and re-weighed (W2). The moisture content is then calculated as:
W1 - W2 x 100%
w2
When the 'saturated surface-dry' (SSD) moisture content is measured, coarse aggregate should be dried until surface moisture has evaporated (this is often accompanied by a slight change in colour), but any further heating should be avoided. Sand should be dried until it just fails to adhere to a glass rod when stirred.
If an open source of heat is used it is important not to overheat the aggregate or heat it too rapidly, which could cause the particles to break up and spit out of the pan. A microwave oven may be more convenient.
This method gives the water content as a percentage of the SSD weight of the aggregate, which is the measure of moisture content generally used on site. It takes into account water on the surface of all particles of aggregate, but does not include water absorbed into the pores of the aggregate. The 'total' moisture content, which includes water absorbed into the pores of the aggregate particles, is sometimes used instead of the SSD moisture content. When the total moisture content is to be measured, the aggregate should be dried thoroughly; it is necessary to heat the aggregate at 105±5°C in an oven overnight. An important point to note is that the SSD moisture content must be used in conjunction with a specified saturated surface-dry water/cement ratio referred to as the 'free' water/cement ratio; similarly, the total moisture content must be used with a 'total' water/cement ratio.
Other methods.
Most of the other methods of determining the moisture content of an aggregate are proprietary methods and instructions for carrying out the test are supplied with the apparatus.
In the calcium carbide method, using the 'Speedy' apparatus, a sample of aggregate is mixed with an excess of calcium carbide in a sealed metal flask.
The pressure produced by the acetylene liberated in the reaction between water and carbide is related to the moisture content, which can be read off a dial. This test is very quick, but the size of sample is small and two or three samples may be advisable to give a reliable measure of the moisture content in a stockpile of aggregate.
Care must be taken when releasing the pressure to avoid any possible source of flame as acetylene is flammable and can be explosive. There must be no smoking during this test.
Mechanical properties of aggregates
Water
BS 3148 / BS EN 1008* describes the testing of water for concrete by comparing the properties of concrete made with any particular sample of water with those of an otherwise similar concrete made with distilled water.
The Standards specify acceptance limits for these tests, and guidance is given on the interpretation of the results. The tests will usually be performed in a laboratory.