Concrete has a notable capacity against compression stresses but very little capacity against tension stresses (it cracks easily). One strategy to counteract this weakness in tension involves using wires or rods to prestress the concrete beam in regions where flexural tension can be anticipated. Consider the simply-supported beam shown, which has a rectangular cross section of 18" x 12". If concrete has specific weight of 150 pcf (pounds per cubic foot) determine the required tension in rod AB, which runs through the beam so that no tensile stress is developed in the concrete at its center section a-a. Neglect the size of the rod and any deflection of the beam. 16 in. B÷2 in. 18 in. 6 in. 6 in. - 4 ft- - 4 ft

Concrete is required for a column that will be Severe exposed to freezing and thawing. The cross section of the column is 300 × 300 mm. The smallest spacing between reinforcing steel is 30 mm. The Required Average Compressive Strength of concrete at 28 days is 35 MPa with a slump of 80 to 100 mm. The properties of materials are as follows: ❖ Cement used is type I Portland cement with a specific gravity of 3.15. ◆ The available coarse aggregate has a maximum size of 23 mm, a dry-rodded unit weight of 1800 kg/m3, a bulk specific gravity (SSD) of 2.7, absorption capacity of 0.55%, and moisture content (SSD) of 0.25%. The fine aggregate has a bulk specific gravity (SSD) of 2.75, absorption capacity of 1.4%, a moisture content (SSD) of 2.5%, and a fineness modulus of 2.70. The aggregates conform to the ASTM C33-84 requirements for grading.

Concrete is required for a column that will be Severe Exposure to freezing and thawing. The cross section of the column is 300 × 300 mm. The smallest spacing between reinforcing steel is 40 mm. The specified compressive strength of concrete at 28 days is 40 MPa with a slump of 25 to 50 mm. The properties of materials are as follows: (a) Cement used is type I Portland cement with a specific gravity of 3.10. (b) The available coarse aggregate has a maximum size of 23 mm, a dry-rodded unit weight of 1800 kg/m3, a bulk specific gravity (SSD) of 2.68, absorption capacity of 0.65%, and moisture content (SSD) of 0.27%. (c) The fine aggregate has a bulk specific gravity (SSD) of 2.75, absorption capacity of 1.5%, a moisture content (SSD) of 2.5%, and a fineness modulus of 2.7,The aggregates conform to the ASTM C33-84 requirements for grading.