More Water? Why Not?
When delivering concrete throughout the year, especially during the summer, customers may request extra water to the mix for easier concrete placement.
What many don’t realize is excessive water in the mix design can result in a lesser strength and quality final product. The strength and durability depend on factors such as:
- Aggregate quality;
- Air content;
- Overall mix design; and
- Cement and water content.
Water Plus Cement Equals Hydration
When water meets cement, the hydration process begins. This chemical reaction, and the amount of water used relating to cement, is what causes the sand and stone to hold or stay bonded together. Too much water dilutes the process, resulting in a weaker product.
How Does The Water/Cement Ratio Affect Concrete?
Water/cement ratio has more influence on strength than cement content. A low slump five-bag mix can yield a higher strength than a seven-bag mix which has a high slump as a result of more water being used.
Water/cement ratio is expressed in decimal. In general, the minimum water/cement ratio required to hydration is 0.22 to 0.25. This means there are 22 pounds to 25 pounds of water in the mix for every 100 pounds of cement. In typical mixes, a water/cement ratio of 0.30 yields a zero slump. This ratio isn’t an absolute indicator of the slump because of the different mix designs — most concrete has a ratio of 0.55 or more. Water reducing agents let you maintain the workability of concrete without increasing the ratio.
Adding Water Changes Concrete Composition, Quality
Consequently, when delivering concrete to customers, keep the following points in mind when a customer says, “add a couple of gallons.”
The addition of one gallon of water to a properly designed, 3,000 psi cubic yard of concrete will:
- Increase the slump about one inch;
- Cut the compressive strength up to 200 psi;
- Waste the effect of 1/4 bag of cement;
- Increase the shrinkage potential about 10 percent;
- Increase seepage potential by up to 50 percent;
- Decrease the freeze/thaw resistance by 20 percent; and
- Decrease the resistance to de-icing salts.
Consider using a water-reducing admixture when possible to increase the slump. Also, check, and test if necessary, the moisture content in the aggregates before mixing. The free water will have an impact on the water/cement ratio.
Adding water per a customer’s request is providing what they want but take the opportunity to point out the potential consequences. You may want to note it on the customer ticket if they’re not happy with their decision to go with ease of placement over concrete quality.