11.3 Hot Weather Concreting Controls & Evaporation Rate Mitigation (ACI 305R)

11.3 Hot Weather Concreting Controls & Evaporation Rate Mitigation (ACI 305R)

ACI 305R-20 identifies high air temperature, high concrete temperature, low humidity and wind as conditions that alone or together can impair concrete quality. Solar radiation can heat exposed concrete. A windy, dry placement may need protection even when air temperature by itself appears moderate. Hot concrete can lose slump and set faster, reducing time for placement and finishing. Higher early temperatures can promote early strength gain and reduce later strength compared with more moderate curing conditions. The magnitude depends on the mixture and temperature history.

Confirm the concrete temperature limit

ACI 301-20 Section 4.2.2.5(b) sets 95°F as the default maximum as delivered, and Section 5.3.2.1(c) links the as-placed requirement to that limit unless otherwise specified. ACI 305.1-14 Section 3.2 sets 95°F at discharge and Section 3.3 describes qualification of a proposed higher maximum using suitable field experience or preconstruction testing. Confirm the actual project limit, location and temperature test method. Use ASTM C1064/C1064M for fresh concrete temperature. A temperature within the specified maximum is a separate check from evaporation control.

ACI 301-20 Section 5.3.2.1(c) calls for fine mist to cool and moisten reinforcement, embedments or forms above 120°F and removal of standing water before placement. Verify the receiving surfaces and protection before discharge. Mass concrete may have separate project thermal control limits.

Estimate free-water evaporation

As plastic concrete bleeds, water reaches the surface. If evaporation outpaces replenishment, drying and contraction of the surface can cause plastic shrinkage cracks before the concrete has developed sufficient tensile strength. Low-bleed mixtures can be vulnerable at lower estimated evaporation rates. A surface that looks dry can still be too soft to finish. Use the approved finishing and curing sequence.

Where ACI 305.1-14 applies, Section 3.1.3 uses the Uno equation with the following inch-pound inputs.

$$E=(T_c^{2.5}-rT_a^{2.5})(1+0.4V)\times10^{-6}$$

• $E$ estimates the rate in pounds per square foot per hour from a water-covered surface.
• $T_c$ is the concrete temperature in °F used for the evaporating surface and $T_a$ is the surrounding air temperature in °F.
• $r$ is relative humidity as a decimal, such as 0.35 for 35%.
• $V$ is average wind speed in mph, measured 20 inches above the evaporating surface.
• Measure air temperature and humidity about 4 to 6 feet above that surface, on the windward side and shielded from direct sunlight.

Under Section 3.1.3.1 start monitoring at least an hour before placement and continue at intervals no greater than 30 minutes until accepted curing is applied. Record the conditions at the placement, including where windbreaks change exposure. The Menzel-based nomograph is useful for an approximate field estimate. Read from air temperature to relative humidity, across to concrete temperature, down to wind speed and across to evaporation rate. ACI 305.1-14 Appendix B explains that the graph does not replace the Uno equation for the specification calculation.

For 90°F concrete and air, 35% humidity and 12 mph wind, the equation gives about 0.290 lb/ft²/h. If the windbreak reduces wind at the measuring height to 5 mph and all other conditions stay the same, it gives about 0.150 lb/ft²/h. The default action value under ACI 305.1-14 is 0.20 lb/ft²/h, with accepted evaporation control required while the estimated rate reaches or exceeds that value unless otherwise accepted. The initial estimate exceeds that value. The later estimate still exceeds an accepted project value of 0.10 lb/ft²/h, if specified for this mixture. The rate estimates atmospheric demand for water from the surface. It does not predict a universal crack threshold.

Manage concrete temperature and exposure

Verify the approved batching approach. Shading aggregates, using chilled mixing water or substituting ice can reduce the concrete temperature. If aggregate is wetted for cooling, account for the moisture in the water calculation. Where an approved liquid nitrogen process is used, check the resulting concrete temperature against the applicable limits.

ACI 305R-20 discusses using chipped or flaked ice as part of the mixing water. Ice at 32°F absorbs about 144 BTU per pound during melting before the resulting water warms. For 25 lb of ice, melting absorbs about 3600 BTU. This is the heat absorbed in melting alone. Check that 25 lb has been included in the mixing-water total and that mixing continues until the ice has melted completely.

Windbreaks, shade, scheduling and fogging can reduce exposure. Verify that initial fogging protects the surface without standing water. Do not add water to the surface to ease finishing. Under ACI 301-20 Section 5.3.6.2(b) an evaporation reducer is applied after strike-off or initial bull floating according to its manufacturer and may not be used to increase workability at the surface. Apply the specified final curing promptly after finishing. For a specified membrane compound, verify timing as the water sheen disappears and follow the product instructions. Where the ACI 301-20 provision for unformed silica-fume concrete applies, the final curing method supplies additional water for the entire curing period.

Check delivery time and adjustment

Read the applicable ASTM C94 edition and the order. Under C94/C94M-17 and -17a, the default 90-minute deadline applies to completing discharge, and the applicable drum-revolution limit separately applies to starting discharge. The purchaser may specify a shorter time for conditions that promote rapid stiffening. Beginning with C94/C94M-21, use the discharge deadline established for the order and recorded on the ticket. Check the applicable conditions and authority for any waiver.

A set-retarding admixture under ASTM C494, such as Type B or Type D, may be specified to help manage setting. Check the approved mixture and placement plan for admixture use and separate protection from surface moisture loss. An admixture does not by itself change the applicable discharge deadline.

If the applicable requirements permit a jobsite adjustment, verify the preliminary result and the authorized addition against the approved maximum total mixing water and delivery limits. Make permitted adjustments before discharge for placement. Record additions, remix with at least 30 revolutions at mixing speed, and then obtain the acceptance composite sample.

Model IBC Table 1705.3 calls for continuous special inspection of proper placement and periodic verification of specified curing techniques and temperatures. Document the ticket, discharge temperature, observed conditions, applicable evaporation limit, mitigation and transition to final curing. If conditions change, record updated measurements and check the protection against the approved procedure.