UNIT 7 TESTS ON WORKABILITY OF CONCRETE

Transcription

1 UNIT 7 TESTS ON WORKABILITY OF CONCRETE Structure 7.1 Introduction Objectives 7.2 Theory 7.3 Apparatus 7.4 Procedure Slump Test Compacting Factor Test 7.5 Observations and Calculations Slump Test Compacting Factor Test 7.6 Results 7.7 Comments 7.8 Precautions 7.9 Reference 7.1 INTRODUCTION Workability is a property of concrete which determines the amount of useful internal work necessary to produce complete compaction. The methods of measurement of workability bear no resemblance to any of the common methods of placing and compacting concrete and so the results are only relative and do not give any absolute measurement. Objectives To determine the workability of cement concrete by a) Slump test, and b) Compacting factor test. 7.2 THEORY This method specifies the procedure to be adopted, either in the laboratory or during the progress of work in the field, for determining the consistency of concrete where the nominal maximum size of aggregate is less than 38 mm. Where larger size aggregates are used the concrete is wet sieved through a 38 rnm screen to remove the larger particles of the aggregate and greater than 38 rnm size. This test is designed to give a certain amount of work on a given volume of concrete and to reduce to a minimum the work required to overcome the friction between the concrete and the surfaces in contact. 7.3 APPARATUS The apparatus needed for the experiment include : 1) Weigh balance 2) Mixing pan

2 ConcreteTechndogy 3) Trowel 4) Scale 5) Tamping rod 6) Slump test apparatus 1) Weigh balance 2) Mixing pan 3) Trowel 4) Compacting factor test apparatus Apparatus The mould for the test specimen is in the form of the frustum of a cone having the following internal dimensions. Bottom diameter Top diameter Height = 20-cm = 10cm = 30cm The mould is constructed of metal (brass or aluminium should not be used) of at least 1.6 mrn thickness and the top and bottom should be open and at right angles to the axis of the cone. The mould should have a smooth internal surface. It is provided with suitable foot pieces and also handles to facilitate lifting it from the moulded concrete test specimen in a vertical direction as required by the test. A mould provided with a suitable guide attachment may be used (Figure 7.1). All dimensions are in mm Figure 7.1 : Slump Test Apparatus The tamping rod is of steel or other suitable material, 16 mm in diameter, 60 cm long and rounded at one end. b) Compacting Factor Apparatus The apparatus (Figure 7.2) consists of two conical hoppers (A and B) mounted above a cylindrical mould (C). The hoppers and the cylinder are of rigid construction, true to shape and smooth on the inside. They are made of cast brass or bronze, but stout sheet brass or

3 steel may be considered satisfactory provided the inside surfaces of the joints are smooth and flush. The lower ends of the hoppets should be closed with tightly fitting hinged trap doors having quick release catches. Metal plate 3 rnm thick is suitable for the trap doors. The frame in which the hoppers and the cylinder are mounted is of rigid construction and finnly located in the relative positions as per specifications. The cylinder and hoppers are easily detached from the frame. Tesb on WorkPbility of Concrete View of trap-door portly open Hopper clamp Hinge Cylinder clamp each srde Figure 7.2 : Compacting Factor Apparatus 7.4 PROCEDURE The procedures for the two tests are as follows : Slump Test, The inner surface of the slump test mould is thoroughly cleaned and freed from excess moisture and any set concrete before starting the test. The mould is placed on a smooth, level, rigid and non-absorbent surface, such as a metal plate, the mould being firmly held in place while it is being filled. The mould is filled in four layers, each about onequarter of the height of the mould. Each layer is tamped with 25 strokes of the rounded end of the tamping rod. The strokes should be distributed in a uniform manner over the cross-section of the mould and for the second and subsequent layers should penetrate into the underlying layer. The bottom layer should be tamped throughout its depth. After the top layer has been rodded, the concrete is struck off level with a trowel or the tamping rod, so that the mould is exactly filled. Any mortar that may have leaked out between the mould and the base plate should be cleaned away. The mould should be removed from the concrete immediately by raising it slowly and carefully in a vertical direction. This allows the concrete to subside and the slump is measured immediately by determining the difference between the height of the mould and that of the highest point of the specimen being tested. The above operations should be carried out at a place free from vibrations or shock, and within a period of two minutes after sampling. The slump measured is recorded ip terms of millimettes of subsidence of the specimen during he test. Any slump specimen that collapses or shears off laterally gives incorrect result and if this occurs the test shall be repeated with another sample. If, in the repeat test also, the specimen shears, the slump is measured and the fact that the specimen sheared, should be recorded.

4 Concrete Technology Compacting Factor Test The sample of concrete to be tested is placed gently in the upper hopper, using a hand scoop. The hopper is filled level with its brim and the trap-door is opened so that the concrete falls into the lower hopper. Certain mixes have a tendency to stick in one or both of the hoppers. If this occurs, the concrete may be helped through by pushing the rod gently into the concrete from the top. During this process, the cylinder should be covered by the trowels. Immediately after thcconcrete has come to rest, the cylinder should be uncovered, the trapdoor of the lower hopper opened, and the concrete allowed to fall into the cylinder. The excess concrete remaining above the level of the top of the cylinder is then cut off by holding a pair of trowels with one trowels in each hand, with the blades held horizontal, and moving them simultaneously one from each side across the top of the cylinder. The outside of the cylinder is then wiped clean..the above operations are carried out at a place free from vibration or shock. The weight of the concrete in the cylinder is then determined to the nearest 10 g. This weight is known as "the weight of partially compacted concrete". The cylinder is then refilled with concrete from the same sample in layers approximately 5 cm deep, the layers being heavily rammed or preferably vibrated so as to obtain full compaction. The top surface of the fully compacted concrete is carefully struck off level with the top of the cylinder. The outside of the cylinder is then wiped clean. The weight of the concrete in the cylinder is determined to the nearest 10 g. This weight is known as "the weight of fully compacted concrete". The compacting factor is defined as the ratio of "the weight of partially compacted concrete" to "the weight of fully compacted concrete". It is stated to the nearest second place of decimal. 7.5 OBSERVATIONS AND CALCULATIONS Slump Test Height of slump test apparatus (HI) - cm Height of concrete mass after removal of mould (Hz) = Slump = (Hz) - (HI) = cm, Compacting Factor Test Weight of empty cylinder (W1) = kg Weight of cylinder + partially compacted concrete (W2) = kg Weight of cylinder + fully compacted concrete (W3) = kg Compaction Factor, (CF) = 7.6 RESULTS (W2)- O"Y' 1) (W3) - (Wl) a) The slump of the concrete sample is cm b) The compacting factor (CF) of the concrete is.... SAQ 1 i) How is workability of concrete defined? i~) Does workability as measured in the laboratory or field have any resemblance to the method of compactiot~ employedd? cm S.4Q 2 i) What are the factors affecting the workability of a concrete mix'? ii) What are the li~nitat.ions of the slump test?

5 7.7 COMMENTS Tests on Workability of Concrete The workability of a concrete mix depends on the amount of cement, the fineness of cement and its chemical composition, the amount of water, the grading and shape of the fine aggregates, the grading, shape and surface texture of the coarse aggregate, the ratio of fine to coarse aggregates and the presence or absence of entrained or other admixture. Comment on the workability of the concrete in the light of these factors. 7.8 PRECAUTIONS 1) The tamping of the four layers of concrete should be uniform. The tamping rod should penetrate the second and subsequent layers. 2) There should be no leakage of slurry from the base of the mould. 3) The mould should be lifted vertically off the support. 4) The concrete should slump evenly with the top surface remaining level. 1) The trap-doors should be opened as soon as the concrete comes to rest inside the hopper. 2) The test should be carried out at a place that is free from vibration or shock. 7.9 REFERENCE IS:l : "Methods of Sampling andanalysis of Concrete", (reaffumed in 1991), Bureau of Indian Standards, New Delhi.