Self-compacting concrete (SCC)
Self-compacting or self-consolidating concrete is a development of conventional concrete with unique characteristics. This special type of concrete was developed in Japan by professor H.Okamura in 1986.
According to ACI terminology, fresh concrete that has the ability to flow around reinforcement and consolidate within formwork under its own weight without any help of vibration can be categorized as self-compacting concrete.
Advantages of self – compacting concrete
The development of self-compacted concrete made it possible to achieve efficiencies in structures that were not possible with conventional concrete. Mostly highlighted advantages of this concrete type are,
- Faster construction and reduce manpower requirement: No vibration is required. The mix will consolidate itself.
- Easy to place and achieve better finish: Just pump the mix to the exact location. No risk of honeycombs due to high flowability
- Improved durability: High powder content will improve resistance to chemical attack
- Greater freedom for the designer: It’s easy to achieve complex shapes and thin sections with self-compacting concrete. Therefore the designers don’t have to worry about casting difficulties.
- Less noise in construction: No vibration noise with self-compacting concrete constructions. You can even work at night in urban areas
Uses of self-compacting concrete
A common use of self-compacting concrete is, using as a secondary concrete. When there are steel embedded parts or steel plates that need to seat perfectly with concrete structure SCC is used. You can see SCC is using for thin or dense reinforced precast sections. Surely, the high flowing ability and the self-consolidating ability make SCC the ideal material.
Usually, SCC contains high powder content. This helps the mix to maintain the required viscosity. However high fine content will increase shrinkage and creep in the concrete.
There are 3 key characteristics in SCC,
- Filling ability: Should be able to fill around the reinforcements
- Passing ability: Should pass through without any blockage
- Segregation resistance: Mix should be viscous enough to resist segregation
Several tests are available to check the above characteristics and at least doing 2 of them is enough for site quality control.
|Filling ability||Slump flow test||ASTM C1611|
|Passing ability||J ring test||ASTM C1621|
|Segregation resistance||Penetration test||ASTM C1712|
Another important parameter to look at is workability retention time. The mix tends to reduce workability with hydration. Any self-consolidating concrete mix should have the ability to maintain its workability for a period of time. Within this time, the contractor must deliver the mix to the site, test it at the site, and pump it to the desired location.
If the workability was lost before placing, the whole purpose of using self-consolidating concrete is wasted. Normally, maintaining the flowing workability at least 1 hr. time is acceptable for general use.
Quality assurance in SCC
Two of the common test for self-consolidating concrete is the slump flow test and the J ring test. These tests evaluate the filling & passing abilities of the mix. Other than them technician is responsible for testing strength parameters similar to conventional concrete.
Slump flow test
The test required a normal slump cone, base plate, a scoop, steel tape, and a stopwatch. Mark 500 mm diameter circle on the plate and place it on level ground. Prepare enough concrete mix for the slump cone before starting the test. Continue following steps and measure the slump flow.
- Place the slump cone on the center of the base plate circle
- Fill the cone using a scoop. Do not tamp
- Remove excess concrete at the top and wipe the base plate with a piece of cloth
- Raise the cone vertically while starting the stopwatch
- Let the mix to flow on the plate and check the time taken to cover the 500 mm diameter circle. This is the T50 time, a secondary indication of the flow. Typical values for T50 is 3 – 5 seconds. If it is below the range it may be due to lack of paste volume or too much fine aggregate using in the mix. High T50 times may result in too much water content or high superplasticizer dosage.
When the flowing is stopped, measure the diameter of the concrete patch in perpendicular directions. The mean diameter is known as the slump flow (SF). Typical slump flow of self-compacting concrete should be at least 650 mm.
J Ring test
The test measures the passing ability of the mix. The same equipment you used in the slump flow test and 300 mm diameter J ring is required.
- Place the J ring on the base plate and center the slump cone.
- Fill the concrete using the scoop and do not tamp or vibrate
- Remove excess concrete and wipe the base plate
- Raise the slump cone vertically without disturbing the J ring
- Let the mix to spread through the J ring
- Then, measure the concrete height inside and outside the J ring, calculate the difference. This should be within 2 – 10 mm range for acceptance. A high gradient means less passing ability. Increasing paste volume, reducing fine aggregate content, and reducing viscosity modifying agents may improve the mix.
When all the parameters are ok, the workability criteria are satisfied. Compressive and tensile strengths should be tested separately. Deliver the mix and pump within the workability retention time for the best results. Try to avoid more than 3 m free falling height and lengthy pump lines during the placement.