Measures to avoid cracking in fresh concrete

Generally, the contractor shall allow for all necessary measures to monitor and avoid cracking in fresh hydrating concrete, regardless the size or volume of the pour. Such measures shall be to the satisfaction of the Engineer and shall be such that maximum surface crack width on hardened concrete measure immediately after the pour does not exceed 0.004 times the nominal cover of the main reinforcement.

The contractor shall allow for and provide approved instrumentation for the measurement of internal temperature changes in large pours. The maximum concrete temperature at the point of delivery shall not in general exceed the lower of either 37 degree C, or 6 degree C above the prevailing shade temperature in accordance with the recommendations of ACI. The limiting internal temperature differential measured across the extreme faces of concrete mass shall not exceed 25 degrees C at any time.

Curing of hardened concrete shall be executed in accordance with the curing specification. Generally, the element surface shall not be cooled to dissipate heat from the concrete. Curing methods, such as the wetting of heated concrete elements exposed to prolonged and direct radiation, which induce temperature gradients within the concrete mass are strictly prohibited.

For large pours, the contractor shall allow for and take extra precautions to reduce concrete temperature gradient and to prevent the loss of surface moisture. Such measures include but are not limited to:

• Keeping all mix constituents shaded where possible to reduce their temperatures in the stockpile

• Cooling of mixing water and/or replacing part or whole of the added water with ice.

• Reducing the cement content by the use of admixtures (but not below that required for the durability)

• Using a cement with a lower heat of hydration

• Injecting liquid nitrogen after mixing of concrete

• Restring the time between mixing and placing of the concrete to not more than 2 hours

• Providing approved surface insulation continuously over all exposed surfaces to prevent draughts and to maintain uniform temperature through the concrete mass

• Initiating curing immediately after final tamping and continue until the approved surface insulation system is fully in place

• Providing shade to the concrete surface to prevent heat gain from direct radiation.

If the surface exhibits crack after compaction, it shall be retamped to close the cracks while the concrete is still in plastic stage.