Concrete’s cobra effect: unintended results of embodied carbon reduction

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Plan for concrete protection and curing

Planning for concrete curing is essential to maintaining lower carbon concrete properties, whether that is achieved through internally cured concrete mix design or conventional external curing methods. Protection and curing of concrete have always been required, but often reduced or neglected to speed up placement of successive concrete pours, or to reduce the amount of time when adding additional suspended slabs. Conventional concrete has suffered by the lack of attention to protection and curing during the early stages of strength gain, the new lower carbon concrete is more susceptible to lapses in administration of a curing plan and requires a more extensive process to maintain specified concrete properties.

Curing plans must account for active protection and curing strategies during the initial strength gain stage and need to account for passive curing strategies once the initial strength gain has been achieved. This helps reduce undesirable cracking, warping, curling, and delamination which can result when moisture and temperature are not controlled throughout the remaining construction phases. ACI 308R, Guide to Curing Concrete²¹ is an excellent resource for all types of concrete.

Speed of placement and provisions for an unencumbered worksite are often used as reasons to reduce protection measures when delivering concrete to the project. Weather conditions have always affected conventional concrete, and lower carbon concrete are even more susceptible to a lack of protection during placement activities.

• Cold weather protection must account for temperature control, and temperature of the concrete will substantially affect strength gain. Compressive strength of concrete cured at 10 C (50 F) will gain strength at half the rate as concrete cured at 21.1 C (70 F).
• Hot weather protection must account for rapid moisture loss, and will include suns and wind screens.
• Protection must also account for contingencies during the time concrete is placed. Warm weather can turn cold within a couple of hours in northern latitudes and torrential rains can occur regardless of where the project is located.
• Concrete curing is essential to attain specified design strength. A shorter wet curing period leaves the concrete to “dry out,” and typically results in achieving only 50 per cent of the design strength, severely compromising durability.
• Concrete curing has been under specified by architects and engineers in the past, and seldom address appropriate curing solutions described in concrete reference standards.
• Most specifications will defer to the contractor’s construction schedule and their need for rapid formwork deployment and removal, which is detrimental to the wet curing process.
• Concrete mix design and curing are integrally related, particularly for low carbon concrete compositions. Wet curing and concrete quality must be seen as a priority over speed of construction.
• Special procedures can be used to accelerate strength gain where speed of construction is critical, and will require incorporating heat and additional moisture to maintain concrete quality, such as introduction of live steam, radiant heating coils, electrically heated formwork, or insulating blankets.