Adding Water to Concrete After 1 Hour (Don't Do It)

The Danger of "Retempering" Old Concrete

Concrete is a living, breathing chemical reaction. From the moment the water touches the cement at the batch plant, a clock starts ticking. Usually, you have about 90 minutes to get that concrete out of the truck and onto the ground. But as the concrete sits in the hot sun, it begins to "stiffen up." The temptation for a struggling crew is to add a few gallons of water to the truck to "loosen it back up." This is called retempering, and if it's done too late, it's one of the worst things you can do to a slab.

Why the 1-Hour Mark Matters

In the first hour, the chemical crystals (calcium silicate hydrates) are just starting to form. Adding a little water then is usually okay. But after 60 to 90 minutes, those crystals are beginning to interlock and create the "bridge" that gives concrete its strength. When you add water and spin the drum at this stage, you are literally **snapping the chemical bonds** that have already formed. You aren't "loosening" the concrete; you are destroying its internal structure.

The Result of Late Water Addition

• Permanent Strength Loss: Retempered concrete can lose up to 50% of its designed PSI. A 4000 PSI mix can easily become a 2000 PSI mix if water is added after it has already begun to set.
• Color Blotching: Adding water at different stages of the pour creates different shades of gray. Your driveway will look like a patchwork quilt of dark and light spots.
• Surface Scaling: The added water doesn't integrate well with the "old" mix, often sitting on top and creating a weak surface layer that will peel off within 24 months.

The Professional Solution: Superplasticizers

If the concrete is getting too stiff to work with, a professional contractor doesn't reach for the hose—they reach for a bottle of **Superplasticizer** (also known as a high-range water reducer). This chemical additive breaks the surface tension of the cement particles, making the mix flow again without breaking any chemical bonds and without adding a single drop of strength-robbing water.

How We Manage "Hot" Concrete

At Local Concrete Contractor, we plan our pours to avoid the need for water. We use **set retarders** in the summer to slow down the reaction, and we ensure we have enough manpower to get the concrete on the ground within the 90-minute window. If a truck is delayed and the concrete is too old, we reject the load. We'd rather lose a truckload of concrete than lose our reputation by pouring a compromised slab.

The Verdict

If your concrete crew is adding water to the truck after it's been on-site for over an hour, they are compromising your project to save themselves work. Demand a "dry" pour or the use of chemical additives. At Local Concrete Contractor, we follow the science, not the easy path. Contact us today for a quote on a slab that's built for strength, not for speed.