Slump Loss in Construction Concrete

Slump Loss in Construction
Slump Loss in Construction

What is Slump loss in concrete?

Fresh concrete is well known to lose its workability with time. This phenomenon is called 'slump loss'.

What causes slump loss?

Slump loss occurs when the free water from a concrete mixture is removed by hydration reactions, by adsorption on the surfaces of hydration products and by evaporation.

One of the most important nagging site problem is the loss of slump. Slump at mixing point is not of much importance, but the slump at placing point is of primary importance. Often there is delay between mixing and placing. Achieving high slump at the mixer, only to be lost with time, before placing is a bad economy. Loss of slump is natural even with unplasticized concrete, but rate of loss slump is little more in case of superplasticized concrete. Fig. 5.8 indicates the slump loss with time. 

Many users demand the slump value at mixing or batching plant and also specify the slump value after a delay of 1 or 2 or 3 hours period at placing point. It is not a correct specification. User should only specify the slump value at placing point after a delay of 1 or 2 of 3 hours. It should be left to the superplasticizer manufacturers or concrete supplier to supply concrete of slump value as demanded by user at the time of placing of concrete. 

Steps for Reducing Slump Loss / Methods to Control Slump Loss in Concrete

The slump loss can be managed by taking any one or more of the following actions: 
  1. Initial high slump.
  2. Using retarders.
  3. Using retarding plasticizer or superplasticizer.
  4. By repetitive dose. 
  5. By dosing at final point.
  6. By keeping temperature low.
  7. By using compatible superplasticizer with cement.

 When very high slump is managed at the mixing point, even if loss of slump takes place, still the residual slump will be good enough for satisfactory placing of concrete. Although this method is not a good and economical method some time this method is adopted.

slump loss definition

Pure retarders are used at the time of mixing. This will keep the concrete in a plastic condition over a long time. Just before adding an appropriate dose of plasticizer or superplasticizer which will give desirable slump value for placing requirements. This is possible only when concrete is conveyed by transit mixers. Some time instead of using pure retarders and plasticizer separately, a retarding plasticizer, or retarding superplasticizer is used in an appropriate dose in the initial stage itself. The retarding plasticizer or superplasticizers retains the slump for longer periods which may be sufficient for placing.

concrete slump loss

 One of the common methods to combat the slump loss is to give repetitive doses at intervals and thereby boosting the slump so that required slump is maintained for long time. The time interval should be chosen in such a way that the concrete will have such a residual slump value which can be boosted up. 

workability of concrete

Sometimes a small dose of superplasticizer is added at the beginning and the slump is boosted up. When the concrete arrives at the pouring point, it will still have some residual slump but not good enough for placing by pump or by tremie. For pumping concrete you need a slump of around 100 mm and tremie placing the desirable slump is 150 mm. At this point an appropriate dose of superplasticizer is added to boost up the slump to required level.

It is a common knowledge that hydration process can be retarded by keeping the temperature of the concrete low. At low temperature the slump loss is also slow. Use of ice flakes instead of water is resorted to reduce the slump loss. Often the use of ice flakes is an additional step to reduce the slump loss. 

Use of highly compatible admixture with the given cement or viceversa will also reduce the problem of slump loss. A cement with low C3A content will be of use in this regard. In one of the limited trial conducted, 43 grade cement has shown better compatibility and performance than 33 or 53 grade cement. 

how to reduce slump in concrete

Other Potential Problems 

Sometimes, it is possible that a strong retardation and excessive air-entrainment may take place when lignosulphonates are used in large dose, particularly when the undesirable components in commercial lignosulphonates are not removed. In some superplasticizers, problems like low fluidification, rapid slump loss, severe segregation, have been reported. The problem of incompatibility seems to be one of the common problems generally met with in the field. The practical approach to solve these problems is to cross test with other plasticizers or other cement samples, and practical solutions arrived. In a overdosed mix, cement paste may become too fluid and no longer retain the coarse or even fine aggregates in suspension, causing severe segregation. In such cases either the dose could be reduced or aggregate content, particularly the sand content may be increased.

 When concrete pump and placer boom are used for placing concrete, the slump requirement is around 100 mm. Suppose 100 mm slump concrete is used for a roof slab casting, such a high slump which is undesirable for roof casting, causes problems by way of segregation and bleeding particularly in the hands of inexperienced workers. Such concrete will have to be handled with care and understanding.

Similarly, while casting cubes using highly plasticized concrete, special care and understanding of concrete is required. Compaction of cubes can not be done in the usual method of vibrating or even tamping. If the casting of the cube is done blindly without understanding the behaviour of such plastic concrete, serious segregation occurs in the cube mould. Top half of the cube mould consists only of mortar and is devoid of coarse aggregates, with the result, that such segregated concrete cubes show very low strength. They blame the plasticizers or cement for such low strength. Often such anomalous situations have come to the notice even with major contractors.

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