slump test for fresh concrete
Slump test is one of fresh concrete tests. Slump test is made to measure the consistency of fresh concrete mix , It's the most commonly used all over the world according to (ASTM c143-78) . The main purpose of measuring consistency by slump test is to achieve acceptable fresh concrete workability.
Also slump test is one of the simple and important tests which helps us to get a homogenous fresh concrete mix before casting.
Slump test has a direct effect on compressive strength of the hardened concrete as slump test shows the water/cementing material ratio. In other words, when slump is big then water/cement ratio is big and the compressive strength of concrete is low.
Factors effect on fresh concrete consistency
Concrete components ratios: water, coarse aggregate, fine aggregate and cement.
Cement fineness (cement surface area): slump is directly proportional to cement surface area.
Maximum nominal size of coarse aggregate: when maximum nominal size increases, slump will also increase.
Time between finishing mixing and making slump test: slump decreases when that time increases.
Air temperature: slump decreases when air temperature increases due to evaporation of mixing water.
Admixtures: admixtures are used to improve fresh concrete consistency with different degrees. The most important admixture is Superplasticizers which added to concrete mix by (1-3) % of cement weight.
The purpose of slump test
Define the consistency of concrete mix by measuring the slump after deformed as a frustum cone in the site or laboratory to assure that the concrete mix components ratios are accurate. It is the simplest test to achieve concrete quality control in mixing stations and in the sites.
* Frustum cone made of
strong metal with 1.5mm thickness at least, opened from above and down, the
above hole has 10cm diameter and the down hole has 20cm diameter. The cone
height is 30cm.
* Compacting steel bar with 15mm diameter and 60cm height.
Slump test instruments
Steps of slump test
1- Make sure that all
instruments are clean.
2- Fill the cone with concrete to 3 layers, every layer is 1/3 the cone.
3- Compact every layer by 25 times by using the compacting bar with equal way then, surface is resolved.
4-Raise the cone vertically slowly without any lateral or torsional movement.
5-All this steps must be done in 150 second without stopping.
6-Measure the slump for the concrete by standard ruler.
The slump = The height of the cone – The height of the concrete cone after the slump.
*Slump value of this concrete will be compared with the acceptable values which the CODE mentioned.
You can get more information about the limits of slump test and other Concrete Workability Test Methods,