Quality Control of Self Compacting Concrete

QC Aspects of Self Compacting Concrete

Due to the lower water/cementitious material ratios used in Self Compacting Concrete mixes and higher dosages of dispersive superplasticisers which are key to achieving the requiredrheological characteristics for ?ow and self-compaction, the properties of fresh SCC are more sensitive to variations in the quality and consistency of the mix constituents, particularly variations in water contents.  As the fresh properties of SCC are fundamental to both its short and long term performance (i.e. they can not be compensated with further compaction) tighter controls on materials and production consistent with the stipulated requirements are therefore required.

Important Considerations in producing Self Compacting Concrete

-SCC Production: Production of Self Compacting Concrete requires more experience and care than the conventional vibrated concrete. The plant personnel would need training and experience to successfully produce and handle SCC. In the beginning, it may be necessary to carry out more tests than usual to learn how to handle SCC and gain the experience.

Before any Self Compacting Concrete is produced at the plant and used at the job site, the mix must be properly designed and tested to assure compliance with the project specifications. The ingredients and the equipment used in developing the mix and testing should be the same ingredients and equipment to be used in the final mix for the project.

Most common concrete mixers can be used for producing SCC. However, the mixing time may be longer than that for the conventional vibrated concrete. SCC is more sensitive to the total water content in the mix. It is necessary to take into account the moisture/water content in the aggregates and the admixtures before adding the remaining water in the mix. The mixer must be clean and moist, and contains no free water.

Admixtures for the Self Compacting Concrete may be added at the plant or at the site. There is cost benefit in adding the admixtures at the site. Conventional ready-mix concrete can be bought at a lower cost than the cost of SCC bought from a ready-mix supplier.

- Transportation: The truck drivers should be given oral and written instructions for handling SCC. The truck drivers must check the concrete drum before filling with SCC to make sure that the drum is clean and moist, but with no free water. Extra care must be taken for long deliveries. In addition to the usual information, the delivery note should show the following information:

• Slump flow – target value and acceptable range.
• Production time – time when it was produced.
• Instruction for adding admixtures at the site, if allowed.

The truck drivers should not be allowed to add water and/or admixtures during transit.

- Form System: All commonly used form materials are suitable for Self Compacting Concrete. For surface quality of SCC, wood is better than plywood, and plywood is better than steel. More pores seem to form on the surface when the form skin is colder than the SCC. During cold weather placement of SCC, it may be necessary to insulate the formwork to maintain temperature and normal setting time. SCC is more sensitive to temperature during the hardening process than the conventional vibrated concrete.

Due to the cohesiveness of SCC, the formwork does not need to be tighter than that for conventional vibrated concrete.

Higher form pressures than normal were not observed even at high rate of concrete placement. However, it is recommended that the formwork be designed for hydrostatic pressure, unless testing has shown otherwise.

- Casting on Site: A pre-SCC placement meeting with all personnel involved in the SCC placement would be beneficial. The SCC placement plan, including QC/QA, and the roles and responsibilities of the field personnel should be explained and understood.

In addition to the normal testing, the slump flow, T50 and L-box tests are useful to check SCC at the job site before placement.

Self Compacting Concrete can flow horizontally a distance of 15 to 20m without segregation. A well-designed SCC may have a free fall of as much as 8m without segregation. However, it is recommended that the distance of horizontal flow be limited to 10m and the vertical free fall distance be limited to 5m.

For deck slab of a bridge, it would be difficult for the SCC to flow too far. This could be handled by designing an SCC with a lower slump flow. With a lower slump flow, a bridge deck with a slope of 2% could also be accomplished.

If an SCC placement is interrupted and the concrete has started to harden, it would be necessary to "wake up" the placed concrete by striking a stick or board into the concrete several times before starting the placement again.

SCC takes some time before the hardening starts, especially during cold weather conditions. When it starts to harden, the process is very rapid, which can cause problems in leveling and treating large surface areas.

- Surface Finishing and Curing: Finishing and curing of SCC can follow the good practices of superplasticized high performance concrete. Surface of SCC should be roughly leveled to the specified dimensions, and the final finishing applied as necessary before the concrete hardens.

SCC tends to dry faster than conventional vibrated concrete, because there is little or no bleeding water at the surface. SCC should be cured as soon as practicable after placement to prevent surface shrinkage cracking.

- Cold Joint: When placing a new layer of SCC on old SCC, the bond between the old and new SCC is equal to or better than in the case of conventional vibrated concrete. Normal vibration will not destroy the concrete, such as in the case of placing conventional vibrated concrete on fresh Self Compacting Concrete. This may be necessary when the surface slope is greater than practicable for SCC.