Question:

What can we do to prevent the problems we are experiencing with our concrete in hot weather?

Answer:

Concrete is a building material that can be used in all types of weather, throughout the year. In hot weather the craftsmen must handle the concrete with enough care and protection as to imitate "ideal" conditions.

During hot weather concreting, the temperature is one of problem, relative humidity and wind speed are the other components to the problems in hot weather.

History and experience have alerted us to the effects of hot weather on fresh concrete, 1. Increased water demand in the mix. 2. Rapid slump or workability loss. 3. Rapid stiffening or actual setting . 4. Development of surface plastic shrinkage cracks. 5. Fluctuation in a downward trend of air content.

Approaching this from a contractor's point of view, the water demand and slump loss can be addressed in the preconstruction meeting at the start of the project. Discuss these issues with the engineer and the concrete producer. Request proportioning the concrete as specified to meet the changing environment. Use of ice in the mix is possible, but it is an extreme measure and brings its own burden of cost.

Hot weather concreting may have a higher water requirement to maintain the workability. Mainly due to the rapid hydration and evaporation which occurs between batching in hot weather and on site delivery. A mix design performing well in moderate temperatures will need to be proportioned differently for hot temperatures. Remember, if more water is needed, more cement or cementious material will be needed to balance the increase in water.

Speaking to the issue I think you are really asking about, the control of surface plastic shrinkage cracks and surface crusting from rapid surface evaporation, consult ACI 305. It will direct you to a lot of helpful procedures to protect your concrete in hot weather. The one item to focus upon is the use of a monomolecular film on the surface of the fresh concrete. L&M's monomolecular film product is called E-CON.

Your crew would apply a repeat spray application of this easy to use liquid. The purpose of the monomolecular film is to reduce early evaporation of the surface water from sun and wind. A simple pump sprayer is the normal device to apply the E-CON. Finishing and tooling are not interfered with and the tools can travel right over the liquid with no consequence.

Summarizing, be proactive and alert the owner and the engineer of your crew are not working in Lab conditions, promote the proportioning changes to the concrete for the hot weather conditions. Finally, use the finisher's friend E-Con as an insurance to prevent early crusting of the surface and to resist the formation of surface plastic shrinkage cracks. Finally, consider working at a time of the day when the sun and wind are less a problem, early evening or nighttime.