Slump value is of great importance when concrete is in its fresh state. Slump value represents the workability property of concrete. Due to its importance, the value of slump is generally specified by the designer. And while working on site, the engineer’s responsibility is to make sure that the concrete should show the same slump value as specified. It is therefore required to check slump before placing concrete. But due to several reasons, the slump value gradually decreases as time elapsed. This reduction in value of slump with time is called slump loss in concrete.
METHODS TO CONTROL SLUMP LOSS IN CONCRETE
The following methods are helpful to manage slump loss in fresh concrete.
Initial high slump
Using retarders
Using plasticizers or super plasticizers
By repetitive dose
By dosing at final point
By keeping temperature low
By using compatible super plasticizers with cement
1. INITIAL HIGH SLUMP
In this method a very high slump is managed at the mixing point, so that after slump loss takes place, the residual slump is still satisfactory for placing concrete. This method is not economical & hence not good. But sometime this method is adopted.
2. USING RETARDERS
In this method retarders are used at the time of mixing. This will help concrete to remain in a plastic condition over a long period of time.
3. USING RETARDING PLASTICIZERS OR SUPER-PLASTICIZERS
Before placing the concrete, add an appropriate dose of plasticizers or super-plasticizers, which will give desirable value of slump. This method is adopted only when concrete is conveyed by transit mixers.
4. BY REPETITIVE DOSE
This is one of the common methods to manage slump loss in concrete. In this method repetitive doses is given in intervals and thereby maintaining the required slump 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.
5. BY DOSING AT FINAL POINT
Sometimes a small dose of super-plasticizers is added at the beginning and the slump is increased up. After concrete arrives at the pouring point, it will still have some residual slump but not good enough for placing by specified methods (such as pumping or by tremie). At this point an appropriate dose of super-plasticizers is added to boost up the slump to required level.
6. BY KEEPING TEMPERATURE LOW
Hydration process of cement generates heat, which is also a cause of slump loss. The process of hydration can be retarded by keeping the temperature of the concrete low. At low temperature the slump loss is also slow. Use of ice flakes by replacing some amount of water is resorted to reduce the slump loss.
7. BY USING COMPATIBLE SUPER-PLASTICIZERS WITH CEMENT
Use of highly compatible admixture with the given cement will also reduce the problem of slump loss. Cement with low C3A content will be of use in this regard.
WHAT IS WORKABILITY OF CONCRETE & WHAT IS ITS IMPORTANCE AND SIGNIFICANCE?
DEFINITIONS OF WORKABILITY OF CONCRETE
According to Granville “it is that property of the concrete which determines the amount of useful internal work necessary to produce full compaction.”
Powers defined it as “that property of plastic concrete mixture which determines the ease with which it can be placed and the degree to which it resists segregation”
ACI (American Concrete Institute) defines it as ‘that property of freshly mixed concrete or mortar which determines the ease and homogeneity with which it can be mixed, placed, consolidated and finished’.
ASTM (American Society for Testing and Materials) defines it as “that property determining the effort required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity”.
Workability of Concrete
IMPORTANCE AND SIGNIFICANCE OF WORKABILITY
From these above definition it is very clear that there exist no straightforward definition for the term “Workability”. In very simple words we can say that workability of concrete means the ability to work with concrete. A concrete is said to be workable if
It can be handled without segregation
It can be placed without loss of homogeneity
It can be compacted with specified effort
It can be finished easily
In every construction work we use different quantitative or qualitative terms to express workability. Before specifying workability for any work a concrete technologist must keep the following things in mind.
Type of construction work
Method of mixing
Thickness of section
Extent of reinforcement
Mode of compaction
Distance of transporting
Method of placing
Environmental condition
Concrete that can be placed readily without segregation or separation in a mass dam could be entirely unworkable in a thin structural member.
Workable concrete compacted by means of high frequency vibrators would be unworkable if vibrators could not be used and hand tamping and spading were required.
Concrete having suitable workability for a pavement might be unsuitable for use in a heavily reinforced section.
WHAT ARE THE ADVANTAGES AND LIMITATIONS OF SLUMP TEST?
ADVANTAGES OF SLUMP TEST
It grants the facility to easily detect the difference in water content of successive batches of concrete of the same identical mix.
The apparatus is cheap, portable and convenient to be used at site.
Slump Apparatus
LIMITATION OF SLUMP TEST
As such, there is no direct relationship between the workability and the value of slump.
It is not suitable for a concrete in which maximum size of aggregate exceeds 40 mm.
There are chances of many shapes of slump to occur and it is difficult to decide which is the correct value.
The slump occurs only in case of plastic mixes. It does not occur in case of dry mixes.
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