Concrete Curing Methods and Maturity Flashcards

According to CSA A23.1, what is the minimum curing period required for concrete exposed to freezing temperatures when using Type GU (General Use) cement without supplementary cementing materials?

Answer: 7 days

CSA A23.1 requires a minimum curing period of 7 days for concrete made with Type GU cement when exposed to freezing conditions. During this period, the concrete must be protected and maintained at adequate temperature (typically ≥10°C) to allow sufficient hydration and strength development before exposure to freezing, which can permanently damage immature concrete.

What is the concrete maturity method used to estimate?

Answer: The in-place strength of concrete based on its time-temperature history during curing

The concrete maturity method (ASTM C1074 / CSA A23.2-24C) uses embedded temperature sensors to calculate a maturity index (either temperature-time factor in °C·hours, or equivalent age in hours at a reference temperature). A previously established strength-maturity relationship for the specific mix allows estimation of in-place compressive strength at any time during curing, without destructive testing.

Which of the following curing methods provides the most effective protection against moisture loss in freshly placed concrete flatwork?

Answer: Applying a liquid membrane-forming curing compound immediately after finishing

Liquid membrane-forming curing compounds (ASTM C309 / CSA A23.1) are applied immediately after the bleeding water sheen disappears from the surface. They form a continuous film that retains the mix water needed for cement hydration. They are particularly effective for flatwork where wet burlap or ponding is impractical, provided they are applied uniformly at the specified application rate.

In cold weather concreting, what minimum concrete temperature must be maintained at the point of delivery according to CSA A23.1 for sections up to 300 mm thick?

Answer: 10°C

CSA A23.1 specifies that fresh concrete must be delivered and placed at a minimum temperature of 10°C for sections with a least dimension up to 300 mm. Thicker sections (which retain heat better) allow slightly lower minimum temperatures. Maintaining ≥10°C prevents freezing of the mix water before sufficient hydration occurs and ensures adequate early strength development.

What is the primary risk of premature drying (plastic shrinkage cracking) in freshly placed concrete, and which condition most promotes it?

Answer: Rapid evaporation of surface moisture exceeding the bleed water supply rate; caused by high temperature, low humidity, or high wind speed

Plastic shrinkage cracking occurs when the rate of evaporation from the concrete surface exceeds the rate at which bleed water reaches the surface. The ACI 305R evaporation nomograph (and CSA A23.1 guidance) shows that high air temperature, low relative humidity, high concrete temperature, and high wind velocity all increase evaporation rate. When evaporation exceeds ~1.0 kg/m²/h, preventive measures (windbreaks, evaporation retarders, fogging) are required.

A maturity index is calculated using the Nurse-Saul function. If concrete has been curing for 24 hours at an average temperature of 20°C and the datum temperature is -10°C, what is the maturity index in °C·hours?

Answer: 720 °C·h

The Nurse-Saul maturity index M = Σ(T − T₀)×Δt, where T is the average concrete temperature during interval Δt, and T₀ is the datum temperature. Here: M = (20°C − (−10°C)) × 24 h = 30°C × 24 h = 720 °C·h. The datum temperature of −10°C is the temperature below which hydration effectively ceases and is commonly used per ASTM C1074.