Short answer
Some water can be added on site, but only up to the mix design limits. NRMCA says water beyond the design amount delays setting, reduces strength, hurts durability and increases cracking, and if the buyer asks for it, the buyer owns the result. Water-reducing admixtures can usually raise slump without those problems.
Key facts
- Water beyond the design amount delays setting, reduces strength, hurts durability and increases cracking (NRMCA CIP 26)
- If the purchaser requests extra water, the purchaser assumes responsibility for the result (NRMCA CIP 26)
- Place most slab concrete at a slump of 3 to 5 inches (NRMCA CIP 2)
- Never sprinkle water on a slab to help finishing (NRMCA CIP 12)
When water is allowed
NRMCA explains that water can be added when the truck arrives if the concrete is stiffer than specified, as long as the specified slump and water to cement ratio are not exceeded. ASTM C94, the ready mixed concrete specification, permits that.
What extra water does
Beyond the design amount, NRMCA says added water delays setting, reduces strength, adversely affects durability and increases the potential for cracking. Its scaling and crazing guides add that high slump from added water can leave a weak, paste-rich layer at the surface.
If the buyer asks for extra water, NRMCA says the buyer takes responsibility for the concrete quality that results.
The better fix
NRMCA says a water-reducing admixture or superplasticizer can raise slump at the jobsite and usually will not change the concrete’s properties, as long as it is not overdosed and the mix does not segregate. For most slab work its scaling guide recommends a slump of 3 to 5 inches.
From Tim’s jobs
