5.1 Rebar Clear Cover Requirements & Environmental Exposures (ACI 318 Table 20.5.1.3.1)

5.1 Rebar Clear Cover Requirements & Environmental Exposures

Cover protects reinforcement from exposure and can affect bond and fire resistance. ACI 318-19 Section 20.5.1.1 requires using the applicable specified cover, with greater cover if required for fire protection by the general building code. A drawing can also require a larger value for development or other project conditions. Verify the approved drawing and apply placement tolerances as a separate check.

1. Identify the surface and reinforcement

Measure from the concrete surface perpendicular to the applicable outer steel surface. Before placement, the inside face of the form can represent the finished concrete surface. Commentary R20.5.1.1 illustrates that a member cover requirement is measured to the outside of the enclosing stirrup, tie or spiral. Where a table row applies to a bar size, also verify the clear distance to the bars of that size. A member can have different cover requirements on different faces.

These are the values in Table 20.5.1.3.1. A footing bottom placed directly on earth and a formed side that will be backfilled fit different rows. For No. 6 bars on a formed side in contact with the ground, the table gives 2 inches. At the unformed bottom cast directly against the soil and left in permanent contact, it gives 3 inches. If the footing is cast on a mud mat, read the approved detail for its cover datum and design requirement. A mud mat is a separate placement and does not authorize changing the shown cover.

Worked example: Outside stirrup and main bar

An approved exterior beam detail calls for 1-1/2 inches to the outside of a No. 4 stirrup. The 1/2-inch-diameter stirrup lies between the concrete surface and a No. 8 main bar. Where the round bars are tangent along the measurement line, clear cover to the main bar is $1.5+0.5=2.0$ inches. The No. 8 bar center is another 1/2 inch inward, at 2.5 inches from the same face. Check actual geometry. A dimension to a bar center cannot be used as its clear cover.

2. Check whether a different cover table applies

Table 20.5.1.3.2 covers cast-in-place prestressed members. Table 20.5.1.3.3 covers precast members manufactured under plant conditions. For example, the plant-conditions table gives 5/8 inch for a nonexposed precast slab with No. 11 bars and smaller, compared with 3/4 inch in the nonprestressed cast-in-place table. For an exposed precast wall manufactured under plant conditions with No. 8 bars, the plant table gives 3/4 inch. Read the member and reinforcement row before using a value.

Table 20.5.1.3.4 specifically covers deep foundation members. For a cast-in-place member cast against and permanently in contact with ground and without the table’s enclosure, it specifies 3 inches. For cast-in-place reinforcement enclosed by steel pipe, tube, permanent casing or a stable rock socket, the table gives 1-1/2 inches. Use the approved foundation detail for the actual support and exposure. For aggressive exposure, verify the specified cover and mixture requirements from the approved documents rather than adding a generic cover increment.

3. Check bundled bars and placement

Under Section 20.5.1.3.5, the bundle-cover criterion is the smaller of the equivalent bundle diameter and 2 inches, with 3 inches for concrete cast against and permanently in contact with the ground. Use the combined bar area to compute equivalent diameter. For a sample bundle of two bars with a specified nominal area of 0.79 in² each, the total area is 1.58 in² and the equivalent diameter is $\sqrt{4(1.58)/\pi}\approx1.42$ inches. Where the drawing calls for 2 inches, verify the shown dimension.

Check the installed supports at forms, corners and changes in elevation. Record the face, bar or tie, specified cover and observed distance. Check the approved placement tolerance for the actual member and location. If cover is deficient, bring the discrepancy to the contractor, verify the correction and escalate uncorrected conditions under the applicable model IBC reporting provision.