Tests for Fresh Concrete | Building Construction

Tests for Concrete to be performed on site

Tests are held at 28 days, 7 days and 3 days. Constituent of mix should be such that concrete can be transported, placed and finished easily without segregation.

• Workability – it is a property of fresh concrete and affects the finished product as it affects compaction.
• Need for workability – Imparted by the amount of useful internal work necessary to provide full compaction.

Optimum water varies for different methods of compaction

• i.e., more water – hand compaction – lower density
• less water – mechanized compaction – higher density

If we add more water, the workability may increase may increase but core has more voids

• 5% of more voids – reduces strength by 30%
• 2% of more voids – reduces strength by 10%

The voids result due to –

The entrapped air voids are caused by grading of fine aggregate.

Excess water evaporating later excess – (water) voids are due to increase in workability

Aim should be to have maximum density – therefore for maximum compaction, you will have to have optimum water cement ratio, at which both the type voids are at minimum.

Factors affecting workability

• Main factor is water content (kg/cm of cone)

• For a given type (M15, M20) etc, grading, and workability, the water content does not depend on,
• Aggregate – cement ratio (This is the basic approximation on which mix design is done)

• If air is entrained “workability increases and water can be reduced” (the tiny air bubbles are much smaller than voids)

• If water and mix proportions are fixed, workability depends upon maximum size of Ag, its grading, shape and texture.

• For a given water-cement ratio, there is one value of coarse aggregate/fine aggregate ratio that is given highest workability that requires lowest content.

• The influence of properties of aggregate on workability reduces as richness of the mix increases and disappears when Aggregate – Cement ratio approaches 2.5 or 2.

• Testing for workability – Measurement

Slump test

• Not a direct test but measure changes in reduction in proportions

• Cone is placed on 3 layers, each layer tapped 25 times with standard 16mm steel rod (rolled at the end).

• Immediately after filling, cone is shortly lifted.

• Decrease in the height of the centre of the slumped mass is slump measured nearest to 5mm. Shear slump indicates harsh concrete with lack of cohesion.

• The change in workability depends upon moisture content of Aggregate., ambient (dry Aggregate – more loss of water)

• Temperature, concrete temperature, relative humidity, elapsed time after mixing.

• Increase water content for hot days.

• As cone temperature increases, the percentage change in water requirements per 25mm slump increase up to 40%. With, relative humidity between 20 degrees – 70 %, the effect of temperature on slump is not much. But of temperature is more than 50 degrees Celsius or relative humidity less than 20%, slump falls off rapidly.

• Loss of slump in hot and dry air reduces the ease the ease of placing, not until 20 minutes of mixing and upto 40 degrees Celsius ambient temperature.

• Over a long haul as in case of ready mixed cone, loss of slump means that, it requires more water for a given workability.

In my next article, we will discuss “Tests for Hardened Concrete” in detail.