Adding Water to Concrete: A Common Site Practice or Common Site Mistake?

Expert guidance: Jason Chandler is a recognised leader in high-performance, sustainable cements and concretes, bringing more than 22 years of construction industry experience in technical, operational, project management and innovation roles.

Making concrete is much like making a cake: the supplier must provide the specified consistency—usually measured as slump or flow—using a mix designed with the correct proportions of cement, aggregates, admixtures and water. Too much or too little of an ingredient changes the finished product.

Excess water can reduce strength and durability and contribute to dusting and cracking. Adding 20 litres per cubic metre can reduce 28-day strength by up to 6 MPa—about the difference between N25 and N20 concrete.

Why Excess Water Weakens Concrete

“The use of too much mixing water is probably the most common abuse of concrete, and is often very costly.” (Boral Australian Concrete Guide, 2023 Edition 2.1, Ch 3)

For typical 20–40 MPa concrete, only about 50–60% of the mixing water hydrates the cement. The balance is “water of convenience” that makes the concrete workable enough to mix, transport and place.

The water-to-cement or water-to-binder ratio is one of the most important parts of the mix design. Adding water increases that ratio, reducing the density, strength and durability of the hardened concrete.

Other effects of excess water

• Surface dusting: too much water at the surface while concrete sets can form a weak, powdery layer that fails under foot or vehicle traffic.
• Cracking: water used or lost through evaporation causes volume change and shrinkage. More water usually means greater shrinkage and a higher cracking risk.
• Reduced durability: voids left by excess water make it easier for chlorides, sulphates and carbon dioxide to enter the concrete and attack reinforcement.

When Water May Be Added

Water should never be added indiscriminately. A controlled addition may be made only when all three of these conditions are satisfied:

• Less than 75 minutes have elapsed since batching, or less than 80% of an agreed extended delivery time has elapsed.
• Discharge has not commenced.
• The concrete remains within the specified consistency tolerances.

Add only enough water to bring the concrete to the consistency that was ordered. If you are unsure, ask the driver or the concrete tester before authorising an adjustment.

Effect on Strength & Slump

The chart shows indicative changes in slump and 28-day strength as water content changes for a typical N25 mix design in metropolitan Sydney. Results can vary by region and strength grade.

What Happens When Water Is Added on Site

A truck arrives at the requested slump, but the placers find the concrete stiffer than they expected and ask for water. It flows more easily immediately, while the consequences—lower strength, a greater risk of dusting and cracking, and poorer durability—may not become visible until much later.

Repair or rectification can be costly. If the purchaser or a representative such as a placer, pump operator or site supervisor requests extra water, that party is responsible for any detrimental effects associated with the addition.

Order the Slump You Need

The best way to avoid on-site water adjustment is to order concrete at the maximum slump that still allows sound placement for the job. Kerb, slabs and wall fill have different placement needs, so discuss the element and placing method with your placer and concrete supplier before ordering.

If you are new to specifying concrete, see our guide to concrete slump and our first-time ordering guide.

AS 1379 Rules on Water Addition

AS 1379: Specification and supply of concrete governs ready-mixed concrete in Australia. Its controls include:

• Unless otherwise specified, the supplier may add water to the mixed batch, followed by mixing at full speed for at least 30 revolutions.
• Any water added on site must be documented on the delivery ticket.
• The total water-cement ratio must remain within the design mix.
• The purchaser or site supervisor—not the driver—authorises extra water.

If a problem is associated with site-added water, liability shifts to the person who authorised it. Record what was added, how much, when it was added and who authorised it.