After concrete is in place it should be protected against freezing. Where several cycles of freezing and thawing occur at an early age, strength and other qualities are permanently affected. Most problems can be minimized by the following precautions. One, plan in advance. Have equipment and materials ready before cold weather arrives and use a high early strength concrete where job conditions make it desirable. Two, heat the material. The temperature of the concrete as it is placed in the forms should be between 50 and 70 degrees F. for slabs. When air temperatures are below 30 degrees F. the mixing water and sand (and sometimes coarse aggregate) should be heated. But to prevent flash set, materials should not be overheated. Maximum allowable water temperature is about 140 degrees F. Also, fresh excavations should be covered with straw or other insulating material to prevent the ground from freezing until concrete can be placed. Three, use accelerators carefully. Use about 1 lb. of calcium chloride per sack of cement to hasten hardening. Not more than 2 lb. should ever be used because of the danger of flash set. Remember also not to use calcium chloride with other admixtures which accelerate hardening. Four, provide suitable curing temperatures. Maintain temperature of concrete, when using normal portland cement, at 70 degrees F. for 3 days or 50 degree F. for 5 days and don't allow the concrete to freeze during the next four days. Five, keep job condition records recording date, hours, weather conditions and temperature (both of the air surrounding the concrete and the surface of the concrete) at least twice daily. Finally, six, protect the concrete. Remember insulation, such as with a thick blanket of straw, without artificial heat is often sufficient protection for slabs on ground. For heating purposes, housings can be made of wood, insulation board, waterproofed paper or tarpaulins over wood frames. Avoid the risk of fire by placing coke or oil-fired heating units away from flammable material and vent them to the outside. And keep concrete moist, especially near heating units. In cold weather when artificial heat is applied, moisture for curing is still very important. First wet the slab well with water and cover with waterproof paper. Then apply heat to keep from freezing. This water treatment, along with the covering, prevents the surface of the slab from drying out.