Clients ask why the trucks arrive at first light and the crew is standing around at seven. The heat is the whole answer.
A slab pour needs the concrete on site before eight, because by eleven the sun makes finishing it a fight. That is the short version of a question we get on nearly every job, usually from a client who has been woken up by a mixer.
Here is the long version. Concrete starts to stiffen from the moment it is mixed, and the rate depends on the temperature. On a Male roof at midday the surface of a fresh slab can be ten degrees hotter than the air, and water leaves it faster than the mix can bring more to the top. The finishing crew then has a slab that is closing under their trowels at one end while the far end is still soft. Whatever they do, the finish is uneven and the top few millimetres are weaker than the concrete underneath.
The visible result is a floor that dusts under traffic and crazing that shows up as a map of fine cracks in the first year. The invisible result is worse: a surface that has lost water too quickly has lost strength exactly where a floor gets the most wear, and no amount of tile adhesive on top puts that back.
Starting at seven changes the arithmetic. The first truck goes in while the slab is in shade, the pour is finished by ten, and the crew is power floating in the last of the cool. Curing starts the same morning with sheeting and water rather than at the end of the day, which is the other half of the job and the half most often skipped.
There are jobs where an early start is not possible, usually because of where the plot is and what the neighbours have agreed to. On those we pour in smaller bays, add a retarder so the mix stays workable longer, and put more finishers on than the area really needs. It costs more and it works. What does not work is pouring a big slab at eleven with the same crew and hoping.
So if you are a client and you see the crew standing in the half light at ten to seven, they are not wasting your money. They are waiting for the truck so that the slab you are going to live on top of is finished in the two hours of the day when it can be finished properly. If you are a neighbour, we will have agreed the hours with you in writing before the first truck arrives, and we would rather hear a complaint from you than have you ring the council.
