A ‘Dirty Secret’ of the Construction Industry is; adding of extra water to concrete mixes.
The ready mix concrete suppliers carefully design mixes with appropriate water cement ratios, which are loaded onto the delivery truck by computer controlled batching systems. . . . . . . All this effort on getting the mix right goes out of the window when the truck arrives on site and a hose comes out to arbitrarily add water to the mix.
Here are some of the problems adding extra water causes:
- Too much water will cause settling and segregation of the aggregate to the bottom of the slab (with more sand at the top) which results in a lower strength slab.
- Water that is not consumed by the chemical reaction will eventually leave the concrete as it hardens, resulting in holes that will reduce the final strength of the concrete.
- As the excess water leaves there will be more shrinkage, resulting in larger internal cracks and visible fractures.
Reasons Why Extra Water Is Added
- Easier To Lay A ‘wetter’ mix is said to be more ‘Workable’, in other words it can be spread and a top surface formed with little or no vibration. (Fully vibrated concrete will minimise voids in the concrete without the need to add water) Adding water saves the concretor time, effort, and hire costs for a vibrator
- Delivery Drivers Time A ‘wetter’ mix comes out of the drum faster allowing the driver, who is paid per load, to fit an extra delivery in his day.
- Material Costs Improved workability can be achieved by adding a plasticiser, rather than adding water, but this adds significantly to the cost.
I have worked on big construction projects where every concrete delivery has been been tested before pouring. Any load that was too wet, or any driver seen adding water, and the load was sent straight to the tip. ( I once saw 5 consecutive deliveries sent to the tip)
For you, organising your own concreting, the best advice I can give make sure that anyone you ask for a price knows that are not prepared to accept added water. Be prepared to pay extra for a plasticiser added to the mix.
For more posts on on getting your paths and driveways correct see Concreting