In this instance, the relationship between water content and consistence is established in the laboratory or early in the site work. Then, by maintaining the correct proportions of cement, aggregate and any admixture, the free water/cement ratio is controlled by adding water to maintain the consistence. It is essential that all test results are based on representative samples of concrete. Tests on fresh concrete are described in BS 1881 and BS EN 12350.
Slump test
The slump test is suitable for normal cohesive concretes of low to high consistence and is the test most commonly used. Changes in the value of the slump may indicate changes in materials, in the water content or in the proportions of the mix, so it is useful in controlling the quality of the concrete as produced.
The apparatus consists of a truncated conical mould 100 mm diameter at the top, 200 mm diameter at the bottom and 300 mm high, with a steel tamping rod 16 mm diameter and 600 mm long with both ends hemispherical. The inside of the mould should be clean and damp before each test and it should be placed on a smooth, horizontal and rigid impervious surface. The mould is held down using the foot rests and filled with three layers of approximately equal depth. Each layer is tamped with 25 strokes of the tamping rod, the strokes being uniformly distributed over the cross-section of the layer. When tamping the first layer, the rod should be inclined slightly, and about half of the 25 strokes should spiral towards the centre. Each layer should be tamped to its full depth, allowing the rod to penetrate through into the layer below. The concrete should be heaped above the mould before the top layer is tamped.
off level with the top of the mould by a sawing and rolling motion of the tamping rod. Any spillage is cleaned away from around the base of the mould and, taking 5 to 10 seconds, the mould is then slowly lifted vertically from the concrete.
The slump is the difference between the height of the mould and of the highest point of the concrete being tested. If it collapses or shears off laterally, the test should be repeated with another sample of the same concrete and the type of slump noted. The slump should be recorded to the nearest 10 mm (Figure 38).
After the slump measurement has been completed, if the side of the concrete is tapped gently with the tamping rod, a well- proportioned cohesive concrete will gradually slump further, but if it is a harsh or uncohesive it is likely to shear or collapse.
A formal certificate should be completed for every slump test. See Appendix 3.
Compactability
The compacting factor test of BS 1881: Part 103 has been replaced by a new test, described in BS EN 12350-4, which represents the way in which concrete may respond to vibration on site. A simple rectangular metal mould (Figure 39), is filled to the top using a special trowel, struck off with a straightedge and compacted using a small poker vibrator. The extent to which the concrete consolidates after complete vibration is measured and the compactability is calculated from the expression:
Degree of compactability = 400
400-S
where S is the distance (in millimetres) from the top of the mould to the surface of the concrete after vibration.
Flow table test
The growing use of concrete that flows into place and is self- compacting calls for a test method that is more sensitive than the slump test. The slump test should not be used for any concrete whose slump could exceed 210 mm as the test results would simply be recorded as 'collapse' every time. The flow table test enables quite fine distinctions to be made between batches of concrete with differing degrees of very high consistence.
The main item of apparatus, shown in Figure 40, is a 700 mm square table that is attached by a hinge along one edge to a firm base. A handle is fixed to the edge opposite the hinge and the table is fitted with stops, which prevent it rising off the base by more than 40 mm. A truncated cone is used but it is quite different from the standard slump cone by being only 200 mm high. A wooden tamping bar 40 mm square completes the apparatus.
It is very important to set the base plate on a level surface to avoid the sample running off one side during the test.
Before starting the test, any dry surfaces of the table top, cone or tamping bar are moistened with a clean damp cloth. The person doing the test presses down on the foot pieces of the cone so that no movement or leakage of concrete occurs.
A representative sample of fresh concrete is remixed and placed into the cone in two layers of equal depth, each layer being tamped exactly ten times. The tamping is rather different from that of other tests such as the slump test, because the bar should not penetrate far into the concrete and more of a leveling action is used for this fluid concrete. After the second layer has been tamped, the surface is ruled off with a suitable straightedge, and the surcharge removed from the table. Thirty seconds are allowed to pass between striking off the surface and starting to lift the cone, which is done in 3 to 6 seconds.
With one foot on the base plate and one hand on the handle fixed to the table top, the tester then raises the table top as far as the stops and then lets it drop onto the base. This is repeated every four seconds until the table has been given 15 drops. The concrete spreads across the table as a result and the maximum diameter is measured in two directions parallel to the table edges. The average of the two dimensions is calculated and reported as the flow of the concrete, recorded to the nearest 5 mm.
If any aggregate particles shake loose during the test or the final shape of the concrete is markedly asymmetrical, the fact should be reported because these are indications of possible problems with cohesion that could lead to segregation in the concrete. Also it is a good idea to leave the concrete on the table for a few minutes after the test is completed: any tendency to bleeding will become apparent. The test may, therefore, be regarded as a useful means of giving early warning of potential problems with placing and compacting concrete on site but it should be remembered that the flow table test is suitable only for mixes with very high and flowing consistence classes.
The normal tolerance for flow, according to BS 5328 : Part 4, is ±50 mm.
It is permissible, in the case of extreme consistence - where the concrete might be in danger of flowing off the table - to reduce the number of drops from 15 to a number determined by trials and agreed by all the interested parties.
Full details of the flow table test procedure are given in BS 1881 : Part 105 and BS EN 12350-5
Air content test
If an air-entrained concrete is used, the air content of the fresh concrete should be determined in accordance with either of the two methods given in BS 1881: Part 106 and BS EN 12350-7. Typical air meters as used for method A and method B are shown in Figure 41 . exceed 75 mm. The object of tamping or vibrating the concrete is to attain full compaction without removing an appreciable proportion of any deliberately entrained air.
The concrete should be compacted until it just makes good contact with the sides of the container; prolonged working should be avoided. After compacting each layer, the side of the apparatus is tapped using a 250 g soft mallet to remove any large air voids entrapped in the concrete. Provided that the air has been correctly entrained, the amount of air removed by excessive compaction is likely to be small.
The correct operating pressure (which is predetermined by calibration) is applied to the concrete and the volume of entrained air is read off the water column (method A) or the pressure gauge (method B).
An aggregate correction factor is necessary and will vary with different aggregates. This can be determined only by test, since it is not directly related to the water absorption of the particles. Ordinarily the factor will remain reasonably constant for a particular aggregate, but an occasional check is recommended.
Full details of the air meter test are given in the Standard and a proper certificate should be completed. See Appendix 4.
Testing hardened concrete
The strength of hardened concrete is usually measured on specimens that are tested in compression. Other tests are non- destructive, such as the rebound hammer, ultrasonic pulse, or various pull-off/break-off techniques, that can recognize variations in concrete strength and quality.
Manufacture of test cubes
Compressive strength tests for UK concretes with maximum aggregate sizes (Dmax) of 10, 14 or 20 mm are usually made on 100 mm cubes. For aggregate with a Dmax greater than 20 mm, 150 mm cubes are used. Details of the making and curing of test cubes are given in Parts 108 and 111 respectively of BS 1881, and BSEN 12390-2.
A summary of the procedure on site is given below. It should be emphasized, however, that cubes should always be made by personnel trained in the work and that it is preferable for the same personnel to make all the cubes throughout the job.
The test involves measuring the reduction in volume of air resulting from an increase in the applied air pressure. The air meter should be of a type in which the air content is read off while the concrete is under an operating pressure of approximatelym 1 bar. The container of the meter should have a nominal capacity of at least 5 litres, and should be filled with concrete in three approximately equal layers. Each layer is compacted with at least 25 strokes of a steel tamping bar weighing 1.8 kg, and having a tamping face 25 mm square (as used for cube making).
Alternatively, vibration may be used, provided the slump does not The moulds for test cubes are traditionally made of steel or cast iron, with very close tolerances for dimensions, flatness, squareness, parallelism, and surface texture and hardness. Each mould should have a removable steel base plate with a true surface to support the mould and prevent leakage. It is essential to keep the mould and base plate clean, and both should be thinly coated with release agent to prevent the concrete sticking to them. No undue force should be used during assembly.
Hard plastic cube moulds may be permitted where equal levels of precision can be assured and, if plastic moulds are deemed to be acceptable, hand tamping may be done with a slump rod to avoid the damage likely to be caused by the traditional square bar.
It is essential that the concrete in the cubes should be fully compacted. A 100 mm cube mould should be filled in two layers and a 150 mm mould in three layers. When compaction is by hand each layer should be tamped with at least 25 strokes for 100 mm cubes and at least 35 strokes for 150 mm cubes, with a steel bar, weighing 1.8 kg and having a tamping face 25 mm square. More strokes should be used if required to ensure full compaction. The tamping of the concrete should be carried out methodically, the strokes being evenly distributed over the surface of the concrete in a regular pattern and not concentrated in one particular spot.
Alternatively, the concrete can be compacted by vibration, again in layers, using either an electric or pneumatic hammer or a suitable table vibrator. In all cases the amount of compaction is recorded either as the number of strokes per layer or as the duration of vibration. After compaction, the surface of the concrete should be trowelled as smooth as practicable, level with the top of the mould.
Normal curing of test cubes
Immediately after making, cubes should be stored under damp matting or similar material in a place free from vibration, and wrapped completely with plastic sheeting to prevent loss of moisture. Cubes to be tested at an age of seven days or more should be kept at a temperature of 20 ±5°C; cubes to be tested at earlier age should be kept at a temperature of 20 ±2°C, during this initial moist-air curing period.
Cubes to be tested at 24 h should be demoulded just before testing; cubes for testing at greater ages should be demoulded within the period 16 - 28 h after the time of making. Each cube should be clearly and indelibly marked for later identification and immediately submerged in a tank of water maintained at a temperature of 20 ±2°C until the time of testing. Cubes that have to be transported to another location for testing should be removed from their moulds or from the curing tank and packed in such a way that they do not become damaged or dry. This can be done by enclosing them in plastic bags and transporting them in purpose-made boxes. Cubes can be transferred any time after demoulding, but they must arrive at the place of testing at least 24 hours before the time of testing, where they must be stored in water at 20 ±2°C.
A record should be kept of maximum and minimum temperatures, both for the initial moist-air curing period and for the subsequent water curing. A formal certificate should be completed for every set of test cubes made, see Appendices 5 and 6.
Testing cubes
Details of the testing of cubes are given in BS 1881 : Part 116 and BS EN 12390-3. Specimens will usually be sent to a laboratory for testing and it is recommended that the laboratory is one which is accredited for cube testing by the United Kingdom Accreditation Service ( UKAS). For site testing, reference should be made to the Standard for full details of requirements for the testing procedure, but some of the more important points relating to testing procedure for cubes are as follows:
n The cube should be stored in water, as described above, and tested immediately on removal from the water. Surface water, grit and projecting fins should be removed and the dimensions and weight recorded, noting any unusual features, such as honeycombing. Cubes that are clearly misshapen should not be tested
n The bearing surfaces of the testing machine should be wiped clean and the cube should be placed in the machine in such a way that the load is applied to faces other than the top and bottom of the cube as cast. The cube must be carefully centred on the lower platen
n The load must be applied without shock and increased continuously at a rate within the range of 0.2 - 0.4 N/mm2 per second until no greater load can be sustained. The maximum load applied to the cube is recorded. Any unusual features in the type of failure should be noted. The cube should be retained for a minimum period of one month
n The compressive strength is recorded to the nearest 0.5 N/mm2
n It is important to maintain testing machines in good working condition ensuring, for example, that the spherical seating can move correctly. The seating must move freely as the slack in the machine is taken up but then must lock and remain rigid until the cube fails; otherwise low failure loads will be recorded, and the shape of the failure will be one-sided. The type of failure should always be noted, because an unusual shape of failure surface may indicate a defective machine. Compression testing machine requirements are specified in BS 1881: Part 115 and BS EN 12390-4. It is important that machines are regularly calibrated.