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
- 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.