Holdfast Training Services Trial Blog

Two Fifty One – Curing Methods

On Wednesday we did our first large(ish) concrete pour (225m3). This was for the basement raft and 2 pile caps. The aim was to fill the 2.1m deep raft (large pile cap) to underside of slab which will be formed across the entire site (450mm thick, to be cast in waterproof concrete). The setup was a mobile pump at the end of our pit lane which involved having concrete wagons back onto the pump. Luckily the front of the vehicle was far enough away from the road to avoid any road traffic accidents.

Pump positioned within the site, concrete wagons backing onto it, just into site. The pipe line then ran around along the capping beam and down to the pour location.

F5 consistence – therefore flow table tests. Interestingly the test was repeated after 4hours and it achieved just a 20mm lower radius.

Pour was to underside of slab level, which is about 400mm below the height of the reinforcement shown. Before the concrete is fully cured it will need to be removed from the rebar.

Overview of raft pour – see wall starters to negotiate when curing.

Next day thermal shrinkage cracks. The hope is that when the slab is cast on top it will have fewer cracks because it is only 450mm. Additionally, it will contain a waterproofing additive to prevent water ingress.

Quality Assurance

The quality assurance process involved taking flow tests of the wagons every 5 vehicles and cubes every 50m3. Each delivery was checked for the mix and the details recorded on the pour history sheet (mix, batch times, location of where the concrete ended up). The idea being that should a random concrete wagon arrive at our site we would spot it. Amusingly, one of our wagons ended up somewhere else who were not using quite such a stringent checking process!

There was a drained cavity sump within the pour which had to be in waterproof concrete and so this was poured using a hyrib stop end with the last pour of the day/night.

When we come to do the slab the final finish will be an epoxy floor coating, the concrete slab will be shot blasted to give it grip because it is a car park. Coming to our part in this plan, the slab will need to be cured. There are two common methods:

Polythene and spray on.

Polythene

Advantages: Quick to apply, logistically not too difficult to put in place (polythene rolls are not heavy), 100% moisture barrier (assuming all sides are fastened down),

Disadvantages: Protrusions such as wall starters, polythene marking the slab, mitigated by keeping the polythene sheets as flat as possible, and tearing is likely.

Curing agent

Advantages: Pretty cheap and not difficult to apply.

Disadvantages: It is time consuming to apply, a sprayer is needed, ensuring complete coverage is difficult because it is not easy to see where it is applied. It is not an impermeable membrane so there will be some evaporation.

Recommendations

Polythene, if applied correctly is likely to give a better method of curing because of the 100% moisture barrier. The difficulties in our case of fitting it around wall starter bars can probably be overcome by fastening it either side of the protrusion.

Anyone any experience/views on the matter? What do they do in the US/ Australia for curing slabs?