Concrete for basement, foundation or other walls not exposed to severe weather should have a design compressive strength of 3000 pis. Walls exposed to severe weather require a design compressive strength of 3500 psi and an entrained air content adequate to resist freezing and thawing. This is usually around 5 to 1.5 percent, but higher if the coarse aggregate has a minimum size of less than 1 inch. Slumps should be kept low to assure good strength and minimize segregation; 3 inches is adequate for normal weight concrete and 2 to 3 inches for structural lightweight aggregate concrete but efforts should be made to consolidate the concrete well. A higher slump is permissible if a special mix is being used. Placement by pumping is advantageous where access for the mixer truck is difficult. It also helps avoid entrapment of air during placement. For this purpose the discharge end of the hose is kept below the concrete surface. When handling the hose from a concrete pump consideration should be given to the formwork. Stresses on the forms can be doubled locally merely by supporting the pump hoses on the walkway. Much advantage is gained from the use of vibrators in consolidating the concrete in the wall. Good vibration breaks up entrapped air bubbles; blends the coarse aggregate into the surface; and leaves the surface flat and glistening. After the forms are exposed to heating from the sun and after the concrete has attained initial set, they must be kept wet to minimize moisture loss from the concrete. Curing can be done by any of the approved methods: soaking, burlap kept continuously moist, plastic film, waterproof paper, and curing compound sprayed on the surface.