Precast Inc. - July/August 2008 - (Page 14) feet of cover. Minimum spacing of reinforcing steel The main reason for a “minimum spacing” is to allow the concrete to flow between reinforcing steel bars. This permits concrete to get into all areas to avoid honeycombing and ensure against concentration of bars on a line that may cause shear or shrinkage cracking. ACI 318- 08 Section 7.6.1 requires that the minimum spacing be the larger of: • Diameter of rebar used • 1 inch The minimum spacing in utility and wastewater structures becomes important in top slab design when multiple access openings require a narrow beam between the holes. Most of the ASTM standards refer back to ACI 318 for minimum spacing requirements. FIGURE 2 Maximum spacing of reinforcing steel It is important to identify the recommended standards to be followed when spacing rebar in slabs and walls, as these standards help control cracking in precast structural members. ACI 318-08 Section 7.6.5 requires that the spacing of load-carrying reinforcement in walls and slabs be no greater than the smaller of: • 3 times wall (slab) thickness • 18 inches Section 7.12.2.2 refers to non loadcarrying reinforcement such as shrinkage and temperature reinforcement. Reinforcement shall be no greater than the smaller of: • 5 times wall (slab) thickness • 18 inches FIGURE 3 required to provide protection from weathering and other effects. This concrete is normally called “minimum cover” (see Figure 1). ASTM standards C1227 and C1613 specify 1 inch cover because they are for water-retaining structures. ASTM C913 specifies 1 inch for water-retaining structures and ¾ inch cover for other structures. ASTM C858 is a specification for non-water-retaining units such as utility structures. The minimum cover requirement in this specification is ¾ inch. ASTM C1433, for box culverts, specifies 1 inch minimum cover except on outer top slabs when the box has less than 2 14 JULY/AUGUST 2008 | PRECAST INC. ASTM C913-08 requires that maximum bar spacing should not exceed 12 inches. This requirement helps to control cracking better than the minimum 18 inch requirement of ACI 318 and is recommended for underground structures. ASTM C478, C858 C1227 and C1613 refer to ACI 318 for maximum spacing requirements. Crack control (Z factor) The “Z factor” first appeared in ACI 318-71. It had been found that the use of reinforcing steel stronger than grade 40 (fy = 40,000 psi) was resulting in large flexural cracks. To insure protection of reinforcing steel, several fine hairline cracks are
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