determination of the workability of fresh concrete (slump test and making test cubes) practical


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determination of the workability of fresh concrete (slump test and making test cubes) practical

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determination of the workability of fresh concrete (slump test and making test cubes) practical

  1. 1. In the determination of the workability of fresh concrete (slump test and makingtest cubes) practical, the first objective of the practical is to determine the amount ofcollapse, or slump, as an indication of the workability of different type of fresh concretemixture. In the experiment, total six experiments are done with different of concretemixtures which is 1:3:6, 1:2:4, 1:1 :3 using inch aggregate with water/cement ratio (w/c)0.6 and 0.8 for the mixes. Slump test is suitable for the site used as it determines the workability of freshconcrete in the specific batch. In the experiments, six different results are obtained. Theslump measured is recorded using a truncated cone with a height of 300mm using theformula,The slump = the height of the cone – The height of the concrete cone after the slump.Slump Shape is divided into three types which are true, shear and collapse. When a ratio of concrete mixture of 1:3:6 with w/c ratio of0.6, the height of theconcrete cone drops from 300mm to 299mm, thus a 1mm slump is formed. A true slumpshape is obtained. Meanwhile, with a w/c ratio f 0.8, a 20mm slump is obtained and ashear slump shape is formed. In the concrete mixture of 1:2:4 with a w/c ratio of 0.6, a90mm slump shape is formed with a shear shape and a collapse of 95mm slump isformed in w/c ratio of 0.8. In the fifth concrete mixture 1:1 :3, w/c ratio of 0.6 gets a80mm slump in collapse form and a w/c ratio of 0.8 gets a 80mm slump with a shearshape is formed. The slumps of 1:2:4 with 0.6 and 0.8 ratio water is lower compared to the slumpsof 1:1 :3 with 0.6 and 0.8 ratio water as some errors occurred. One of the possiblereasons for errors to occur is due to the leaking of water from the from the 1:2:4 concretemixture slump cone causing the slump of 1:1 :3 is higher as the water is decrease due tothe leaking. The slump shape of the 1:1 :3 concrete mixture is also affected due to thewater leaking as the mixture become dry and it won’t collapse. Thus, error occurs due tothe water leaking from the cone.
  2. 2. Another possible reason is that of the insufficient compression on the concretemixture. This causes the mixing of concrete mixture to be less compact. The concretemixture is more workable thus not strong. Hence, it tends to collapse easily, forminghigher slumps. In this experiment, 25 strokes using the tamping rod are needed tocompress the concrete mixture. An error will occur is probably due to the inadequate ofstrokes or the insufficient force. Limitations of slump test are the compacting factor which will affect theworkability of concrete mixtures and the slump formed. One of the limitation is it is notsuitable for concrete with an aggregate size >4cm. Thus, sieve of stone has to be done toeliminate the bigger aggregate. Next, a wet mix will cause a higher slump to occur as it iswet and a dry mixture is not suitable as slump does not occur. It is difficult to decide theexact value for the shapes of slump. Temperature will also limit the slump test, and theweather is humid, the sand and aggregates will be wet, thus affecting the slump. A collapse slump will generally mean that the mixture is too wet or that it is ahigh workability mix. The ideal value of the slump for a very dry mix, a slump will be 0-25mm, low workability mixes. The concrete mixture having slump 50-90mm is amedium value of slump while slump value>100 is a wet concrete mixtures. A collapseslump is an indication of a wet mixture. Super-plasticizer effects the slump of concrete as the value of slump can beincreased by the addition of chemical admixtures like mid-range or high-range waterreducing agents (super-plasticizers) without changing the water/ cement ratio. Super-plasticizer enables the cement to become a more effective lubricant, thus increasing theslump without adding additional water. Slump can be seen as either a water slump or as aplasticized slump. If the slump is determined only by the water content, it is said that theslump is water slump, if the slump is determined by water content and the effects of achemical admixture, it is said to be a plasticized slump. Among the six mixtures, the concrete mix of 1:3:6 with 0.6 and 0.8 ratio ofwater/cement and 1:2:4 with 0.6 and 0.8 ratio of water/cement will produce the concretewith the highest strength. The concrete mixture of 1:3:6 with 0.6 ratio of water will has
  3. 3. the highest strength among all. It is because the compressive strength of the hardenedconcrete as slump test is affected by the water/cement ratio. When a slump is big thenwater/ cement ratio is big and the compressive strength of concrete is slow. A lowerwater-cement ratio leads to higher strength. The amount of water is enough to mix withall the cement causing the cement has a dramatic impact on its strength. Aggregateswhich contained within concrete consist of small rocks. The aggregates used in concretemix affect its strength based on how they interact with the water and cement mixture.Larger aggregates increase the cost effectiveness of concrete without decreasing itsstrength since they have a relatively smaller surface area, requiring less concrete to coat.Cement is the glue that binds concrete together. Increasing the proportion of cement inyour concrete mixture will increase its strength but it will also make the mixture stifferand more difficult to work with. The mixtures are then transferred to an oiled cube mould. The mould must beoiled in order to make it easier when we remove the cube. Precautions should be taken toensure standard cube specimens in the laboratory. While finishing off the surface of theconcrete, if the mould is too full, the excess concrete should not be removed by scrapingoff the top surface as this takes off the cement paste that has come to the top and leavesthe concrete short of cement. The correct way is to use a corner of the trowel and gig oura fair sample of the concrete as a whole, then finish the surface by trowelling. Once aspecimen has been compacted, it should not be left standing on the same bench as anotherspecimen that is being compacted.Graphs
  4. 4. a. Graph of strength against w/c ratio. b. Graph of workability against water. c. Graph of Cost effective against cement.ConclusionThe amount of collapse, or slump indicates the workability of different type of freshconcrete mixtures. The higher the amount of slump, the higher the workability ofconcrete mixtures. The amount of slump is affected by a few factors such as w/c ratio andcompacting factor.