Structural Concrete - Hassoun

19.2 Materials and Serviceability Requirements 735
Figure 19.4 Distribution of stresses in beams with varying amounts of prestressed and
nonprestressed reinforcement.
2. Class T (section in the transition zone), with 7.5 √ f ′ c < f t ≤ 12 √ f ′ c. This type of section has a
tensile stress in concrete higher than the modulus of rupture of concrete, f r = 7.5 √ f ′ c, producing
a case between uncracked and cracked sections. In this case, the gross section properties
are used to check stresses, while the cracked section bilinear section is used to calculate deflection.
No skin reinforcement is needed in the tension zone.
3. Class C (cracked section), with f t > 12 √ f ′ c. The tensile stress in the section exceeds 1.6 times
the modulus of rupture. Therefore, cracks will develop as in the case of partially prestressed
concrete members. In this case a cracked section properties should be used to check stresses,
cracking, and deflection. Crack control provisions and skin reinforcement should be used as
explained in Section 6.7 for reinforced concrete members with the effective depth of d > 36 in.
19.2 MATERIALS AND SERVICEABILITY REQUIREMENTS
19.2.1 Concrete
The physical properties of concrete were discussed in Chapter 2. Although reinforced concrete
members are frequently made of concrete with a compressive strength of 3 to 5 ksi (21 to 35 MPa),
prestressed concrete members are made of higher strength material, usually from 4 to 8 ksi (28 to
56 MPa). High-strength concrete may be adopted for precast, prestressed concrete members where
components are prepared under optimum control of mixing concrete, placing, vibrating, and curing.
The allowable stresses in concrete according to the ACI Code, Section 24.5.3, are as follows.
1. Stresses after prestress transfer and before prestress losses:
a. Maximum compressive stress of 0.6 f ′ ci
b. Maximum compressive stress at ends of simply supported 0.7 f ′ ci