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  • Writer's pictureSi Shen

Self-compacting concrete

If normal concrete is like a stew, then self-compacting concrete is like a soup. I have some first-hand experience of using self-compacting concrete when I worked on site at Crossrail Bond Street Station and I believe the fore-going analogy is an accurate representation. In this post I will explain some of the key features of self-compacting concrete from a practical point of view.

A quick Google search will tell you what self-compacting concrete looks like. The surface finish for self-compacting concrete is nice and smooth, and hence does not require any making good to be used as an architectural finish. It is easier to pump (less ‘grindy’) than normal concrete so there is less risk of blockage. But none of these are the primary advantage of self-compacting concrete. Its most useful feature is the ability to fill spaces in awkward shapes without the need for vibration. From my experience, its best use are three:

1, Pours in areas with awkward bends and corners.

2, Pours in areas with high reinforcement congestion.

3, Deep pours where vibrating pokers are hard to reach with accuracy.

Let me explain them in more details.

Normal concrete is like a stew. It has got solid chunky bits that are not flowy. If normal concrete is poured on flat ground from a bucket, unlike water, it would form a little mound, because it has lots of chunky bits and is much thicker than water. Without any human intervention, naturally poured concrete also has lots of voids in between the solid chunky bits (aggregates), especially around rebars where aggregates can get stuck around the rebars or bridge between the rebars. These voids are called ‘honeycombing’ and is often unacceptable quality as they seriously compromise structural performance including compressive/shear strength and bond with rebars. It takes a lot of manual handling to form a flatter surface as most finishes would require, and a lot of vibration to get rid of the voids, which is a process called ‘compaction’. Anyone who has poured salt, sugar, or sand from a bag into a jar would know that after you poured some, and to make more room, you need to bang the jar lightly a few times on the table, and that makes the salt, sugar or sand firmer and a bit smaller in size. This is the essence of ‘compaction’. For concrete, you need a skilled worker to stick a poker into the concrete and vibrate it. Normal concrete would very likely fail to compact where the poker can not reach.

Self-compacting concrete is like a soup. It is much more fluid than normal concrete. If you pour self-compacting concrete into a container, the top will stay level, just like water. It is for this reason also called ‘self-levelling’ concrete. Since it is very fluid, it is able to rely on its own gravity to compact, not requiring vibration. Although so the theory says, my practical experience of using of it on site shows that for some high reinforcement congestion areas or sharp bends and corners, a bit of vibration would be very prudent, to prevent remedial works, which is laborious, hazardous and untidy. Especially when the concrete is supplied ready-mixed from a remote site and traffic congestion could delay the delivery which makes the concrete go off before use. Nevertheless, overall speaking, it substantially reduces the labour cost during pour.

One of the risks with self-compacting concrete is segregation. Anyone who has cooked or eaten soups will know this. When the soup is too ‘watery’, i.e. not thick enough, all the solid bits like carrots and beef sink to the bottom of the pot. Same goes for the concrete. Stones are heavier than water. The heavy aggregates may sink to the bottom of the pour if there is insufficient suspending solid particles in the concrete mix, i.e. not ‘thick’ enough. Segregation can be prevented by adjusting the concrete mix to ensure that it is ‘thick’ enough to suspend the heavy aggregates. A structural designer or contractor can leave this issue wholly to the concrete supplier to sort out as it is a specialist’s job.

The optimal viscosity (degree of ‘thickness’) of the self-compacting concrete should be achieved. If the concrete is too watery, segregation risk is too high; if it is too ‘starchy’, it loses the point of being self-compacting. The method for verifying this on site is the ‘slump flow test’. The test for normal concrete is ‘slump’ test, where you fill a cone with concrete and topple it over on to a flat board and see how much the top of it slumps down from its initial position. The test for self-compacting concrete is ‘slump flow test’, where you fill the same cone and see how much a diameter of a circle it forms.

Lastly, some pointers on self-compacting concrete.

• Self-compacting concrete has exactly the same structural functionality as normal concrete, so a designer does not need to differentiate between them. Their difference exists almost purely in constructability.

• For areas with highly congested reinforcement, aggregate size should normally be 10mm or less. Smaller aggregates make the concrete more flowy, and able to pass through smaller gaps in between bars. Having said that, it is always good practice for the designer to avoid rebar congestion and make good design details in the first place. Self-compacting concrete or smaller aggregates should always be the last resort and do not guarantee their effectiveness as a risk mitigation.

• Self-compacting concrete is considerably more expensive than normal concrete, as it is usually more bespoke thus requiring a lot of tests to confirm a mix. But some argue that the save in labour cost sufficiently offsets its incremental cost. I think whether you can realise this depends on how the job is managed – maybe those workers need to be there no matter using self-compacting concrete or not, because they are employed on a termed basis and can not be sent away and called back any time you wish.

Picture below: self-compacting concrete being pumped into a slab pour. You can see how wide it is able to spread without any manual handling

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