Holdfast Training Services Trial Blog

A RC beam for a radial arm coal stacker was recently completed on site as part of a £2M variation order to the original contract. The beam effectively supports a radial arm connected to a conveyor that will distribute coal evenly onto the newly created coal yard slab.

Radial beam for coal yard stacking arm.

The stepped design allows the radial arm to move laterally and distribute the coal. The top concrete surface on the main running section of the radial beam has a series of small cracks tracing the position of the rebar and depressions inbetween as seen in the following photos.

Radial beam top surface.

Radial beam top surface in detail.

This has occurred previously across the site (although not as severe) and seems to happen when there is a large volume of concrete poured into reasonably congested rebar.

My initial assumption was that the vertical cover was insufficient (specified 50mm) and the top reinforcement was too close to the surface of the concrete. However I saw the cover being checked prior to the pour and know that it was closer to 60mm than 50mm in most areas.

My next hypothesis was that the concrete was over vibrated creating the problem of segregation where the denser aggregates settle to the bottom while the lighter cement paste tends to move upwards allowing the lattice pattern of the rebar to be seen. This would be further accentuated by the practice of wetting up the concrete pre-pour as I have highlighted on previous blogs.

After further thought and reflection after watching the pour I had another theory and have put together a couple of slides to try and explain what I thought was happening. My theory had been that due to the depth of the pour (>2m) concrete was hardening on the top layer of reinforcing bar as it was splashed over it by the pump during the initial stages of the pour. If this was not cleared away it would create an area of hardened concrete above the top rebar that would eventually be covered during the final stages of the pour. As the concrete shrinks during the curing process it would arch over these small triangles of hardened concrete on top of the rebar, creating a surface tension and the eventual cracking and apparent depressions in-between the rebar. I know this was the case in certain areas as I spoke to the engineer in charge and he had to instructed the concreters mid pour to start cleaning off the top rebar as the pour continued due to bad practice from the sub-contractor.

Stage 1.

Stage 2.

Stage 3.

Stage 4.

However I have done some more detailed research and believe that the problem may be plastic settlement cracking. I found the following diagram on the concrete society website.

Plastic settlement cracking.

This suggests that the cracks could be caused when the settlement of fresh concrete is restrained by the reinforcement. Plastic settlement cracks can form in young concrete, within the first few hours after placing. As water moves upward through the mixture, the denser constituents move downward accentuated by any over-vibration. This downward movement may be obstructed by the top layer of reinforcement.

The plastic concrete may arch over the top of individual reinforcing bars, bringing the surface into tension. Cracks may develop at regular spacing and usually follow the line of the uppermost bars, giving a series of parallel cracks; there may also be shorter cracks at right angles over the bars running in the opposite direction as seen on the radial beam.

It is sometimes possible for plastic settlement cracks to form on a vertical face where reinforcement has restricted the free flow of concrete within the formwork. In such cases it is possible that the cracks are formed between the lines of the reinforcement.

The concrete can also be supported by the shuttering, causing restraint to the concrete in connected members. This typically happens at mushroom-heads on columns but can also occur at other locations, such as under spacer blocks. Cracks at mushroom heads of columns are generally horizontal. They are also typically 1 mm wide and can cross the full section.

So in conclusion I now believe that a combination of these factors have led to the formation of the cracks.

The radial beam was made up of large (22mm) diameter rebar along the top layer and was reasonably congested (top rebar and links at approx. 150mm spacing). The C32/40 concrete is made using 20mm aggregate and has a slump of consistence S3 100-150mm for use with the pump but is often wetted up even more prior to pouring. The concrete was pumped into place and there was no doubt that some was left on the top rebar due to splashing over it and not cleaned off. It would all be furthered by the tendency to over vibrate during the pour due to the volume.

So what’s the remedial action?

I believe the client is currently unaware of the finish as the beam has been completed well ahead of schedule. This means that remedial action can take place. It is likely that the top layer will be removed by high pressure water (hydro-blasting) down to the top layer of rebar before the surface is re-applied and finished correctly. Contractually I am unsure of where the liability currently lies, I suspect with the concrete subcontractor but I will try and dig into the subcontract to find out.

Why am I concerned?

The reason I have investigated this particular issue, that is outside of my section and responsibility, is 2-fold. Firstly it has happened on site before, including the previous sub-station ground slab and although it was not as severe when I enquired as to why it was occurring, no-one offered a solution.

Secondly, and most importantly, as I am likely to be building a significantly larger substation I do not want the same to happen to the slab I will be required to pour. Therefore any mitigation measures I can take during the pour will hopefully limit my blushes.

So what’s the preventative action?

The tendency for plastic settlement cracks to form may be reduced by adjusting the concrete mix, for example by avoiding gap-graded fine aggregate and reducing the water content, and by appropriate workmanship and control of vibration. Particular care will be required for tall elements especially the 2m+ concrete cable pits required in the sub-station base. In some cases, plastic settlement cracks can be eliminated, rather than prevented, by careful re-vibration of the concrete after they have formed. However, it is important to ensure that the concrete is not over-vibrated as may have been the case in the radial beam.

Are my conclusions correct? Thoughts???