Curing of Concrete
The curing of concrete shall be in accordance with ACI 308.1.
The contractor shall protect all concrete as required to maintain the surface temperature above 10°C (50°F) for the first five consecutive days after placement.
Curing shall be accomplished by preventing loss of moisture, rapid temperature change, and mechanical injury or injury from rain or flowing water for a period of five days. Curing shall commence immediately after the finishing operations have been completed and the concrete has set sufficiently to prevent marring the surface.
Curing of formed undersurfaces of beams, floor slabs, and other similar undersurfaces shall be accomplished by moist curing with forms in place for the full curing period or, if forms are removed before the end of the curing period, by other means as approved in writing by the Company representative.
Unformed surfaces shall be covered with burlap, cotton, or other fabric mats, as approved by the Air Products representative. Alternatively, the contractor may use curing compounds as set forth.
Burlap covers shall consist of two or more layers of burlap having a combined weight of 14 ounces or more per square yard in a dry condition. Burlap shall be new or shall have been used only for curing concrete. Cotton mats and burlap strips shall have a length, after shrinkage, at least 0.3 m (1 ft) greater than necessary to cover the entire width and edges of the surface to be covered.
The mats shall overlay each other at least 150 mm (6 in). The mats shall be thoroughly wetted before placing and shall be kept continuously and in intimate contact with the surface being cured for the duration of the required curing period.
Failure to provide sufficient cover material of the type selected, failure to maintain saturation in the moist-curing method, lack of adequate water for both curing and other requirements, or other failures to exercise reasonable and proper care consistent with good engineering practice shall be cause for immediate suspension of concreting operations.
Wetted covers shall not be used when concrete is placed under the provisions of ACI 306R. Polyethylene covers shall be used.
Removal of Forms
The contractor shall not remove forms without the written approval of the Air Products representative. The following tables shall be used to determine the minimum curing time required before the removal of forms:
| Structural Classification | Minimum Concrete Strength (psi) |
| 1. Concrete not subject to appreciable bending or direct stress, nor relying upon forms for vertical support, nor liable to injury from other construction activity. | 500 |
| 2. Concrete subject to appreciable bending and/or direct stress and relying on forms for vertical support:
a Subject to dead load only. b. Subject to dead load and live load. |
750 1500 |
| 3. Concrete subject to high bending stress and wholly relying on forms for vertical support. | 2000 |
Probable Strength Gain
| Age | Cements | |
| (moist cured @ 70°F) | Type I and II | Type III |
| 12 Hours | — | 500 |
| 18 Hours | — | 750 |
| 36 Hours | 500 | 1500 |
| 60 Hours | 750 | 2000 |
| 3 1/2 Days | 1500 | — |
| 5 1/2 Days | 2000 | — |
After the removal of forms, the contractor shall patch all voids, pockets, tie holes, cracks, or other surface imperfections with cement mortar in a neat and thorough manner. Where necessary, the contractor shall chip away defective areas to a depth not less than 25 mm (1 in) with the edges perpendicular to the surface.
The surface to be patched and an area at least 152 mm (6 in) wide entirely surrounding it shall be kept wet for at least four hours before patching and shall be kept moist for at least five days after patching.
The contractor shall make the patch of the same material proportions used for the concrete, except that the coarse aggregate shall be omitted. The mixing water shall be as little as is consistent with the requirements of handling and placing.