Hot Weather Concreting Design and Control of Concrete Mixtures CHAPTER 19

Overview When to Take Precautions Effects of High Concrete Temperatures Cooling Concrete Materials Supplementary Cementitious Materials Chemical Admixtures Preparation Before Concreting Transporting, Placing, and Finishing Plastic Shrinkage Cracking Curing and Protection Heat of Hydration

Weather Conditions Weather conditions at a jobsite may be vastly different from optimum concrete placement conditions assumed at the time a concrete mixture is specified, designed, or selected. Longer duration projects will require changes to the concrete mixture as the seasonal weather changes. Hot weather conditions can adversely influence concrete quality primarily by accelerating the rate of evaporation/moisture loss and rate of cement hydration.

Weather Conditions Detrimental hot weather conditions can adversely influence concrete quality primarily by accelerating the rate of evaporation/moisture loss and rate of cement hydration Many difficulties with hot weather concreting including increased water demand and accelerated slump loss lead to the addition of water on site Adding water to the concrete at the job site can adversely affect properties and serviceability of the hardened concrete

When to Take Precautions During hot weather, the most favorable temperature for achieving high quality freshly mixed concrete is usually lower than can be obtained without artificial cooling. Advanced planning is required for concrete placed in ambient conditions that are somewhere between 24°C and 38°C (75°F and 100°F). Setting a maximum concrete temperature is not a guarantee of strength or durability. Atmospheric conditions including air temperature, relative humidity and wind speed, in conjunction with site conditions influence the precautions needed.

When to Take Precautions Which precautions to use and when to use them will depend on: The type of member or construction Properties of the materials being used The experience of the placing and finishing crew in dealing with the atmospheric conditions on site

Effects of High Temperature At higher temperatures, a greater amount of water is required to hold slump constant. The addition of water lowers the strength at all stages and adversely affects other desirable properties. Adjusting mixture proportions for higher water demand while maintaining w/cm will improve concrete strength, however the durability and resistance to cracking will be still be impacted. Bureau of Reclamation (1981)

Effects of High Temperature As a general rule of thumb, the setting time changes by about 33% for every 5°C (10°F) change in the initial concrete temperature. Different cements behave differently and do not always follow this generalization. This shows that the setting time can be reduced by 2 or more hours with a 10°C (18°F) increase in concrete temperature. Concrete should remain plastic long enough so that each layer can be placed without development of cold joints in the concrete. Burg 1996

Effects of High Temperature This graph shows the effect of high initial concrete temperatures on compressive strength. The concrete temperatures at the time of mixing, casting, and curing were 23°C (73°F), 32°C (90°F), 41°C (105°F), and 49°C (120°F). The tests, using identical concretes of the same water-cement ratio, show that while higher concrete temperatures give high early strength than concrete at 23°C (73°F), at later ages concrete strengths are lower. Klieger 1958

Cooling Concrete The aggregates and mixing water have a greater influence on concrete temperature after mixing than other ingredients. Figure 19-5 graphically shows the effect of material temperature on the temperature of fresh concrete. It is evident that although concrete temperature is primarily dependent upon the aggregate temperature, cooling the mixing water can also be effective.

Calculating Temperature of Concrete The approximate temperature of concrete can be calculated from the temperatures if its ingredients by using the following equation: T= 0.22(TaMa + TcMc) + TwMw + TwaMwa 0.22(Ma + Mc) + Mw + Mwa Where: T= temperature of the freshly mixed concrete, °C(°F) Ta, Tc, Tw, and Twa = temperature in °C(°F) of aggregates, cement, added mixing water, and free water on aggregates Ma, Mc, Mw, and Mwa = mass, kg (lb), of aggregates, cementing materials, added mixing water, and free water on aggregates

Example calculations for initial concrete temperature are shown in these tables. (Top:Metric, Bottom:Inch-Pound)

Effects on Concrete Temperatures Aggregates: keep shaded and moist when stored Water: store in tanks away from sun or cool by refrigeration, liquid nitrogen, or ice Ice: must be completely melted by the time mixing is completed Liquid Nitrogen: best method for a greater temperature reduction Cement: has minor effect on temperature of freshly mixed concrete

Substituting ice for part of the mixing water will substantially lower concrete temperature. A crusher delivers finely crushed ice to a truck mixer reliably and quickly.

Heat of Fusion of Ice The temperature of concrete can be calculated from the approximate temperatures of its individual ingredients using the following equation. This equation accounts for the heat of fusion of ice: T(°C)= 0.22(TaMa + TcMc) + TwMw + TwaMwa-80Mi 0.22(Ma + Mc) + Mw + Mwa+Mi T(°F) = 0.22(TaMa + TcMc) + TwMw + TwaMwa-112Mi Where Mi is the mass in kg (lb) of ice (NRMCA 1962)

Effect of Ice on Temperature of Concrete The heat of fusion of ice in metric units is 335 kJ per kg (in British thermal units, 144 Btu per pound). Calculations in these tables show the effect of 44 kg (75 lb) of ice in reducing the temperature of concrete. Crushed or flaked ice is more effective than chilled water in reducing concrete temperature. The amount of water and ice must not exceed the total mixing-water requirements.

Supplementary Cementitious Materials Many concrete producers consider the use of supplementary cementitious materials (SCMs) to be essential in hot weather conditions. The materials of choice are fly ash and other pozzolans These materials generally slow both the rate of setting and slump loss The rate of bleeding can be slower than the rate of evaporation and plastic shrinkage cracking

Chemical Admixtures A set retarding admixture may be beneficial in delaying the setting time in hot weather concreting, despite the potential for increased rate of slump loss resulting from their use A hydration control admixture or set stabilizer can be used to stop cement hydration and setting Hydration is resumed with the addition of a special accelerator

Preparation Before Concreting Before concrete is placed, certain precautions should be taken during hot weather to maintain or reduce concrete temperature Mixers, chutes, conveyor belts, hoppers, pump lines, and other equipment for handling concrete should be shaded, painted white, or covered with wet burlap to reduce the effect of solar heating Forms, reinforcing steel, and subgrade should be wetted with cool water just before the concrete is placed Fogging cools the contact surfaces and increases humidity to minimize rate of evaporation Moisten subgrade for slabs on ground There should be no standing water or puddles when concrete is placed During extremely hot periods, concrete placements may be improved by restricting pouring times to early morning, late evening, or nighttime hours

Transportation and Placing Transporting and placing concrete should be completed as quickly as practical during hot weather. Delays contribute to slump loss and an increase in concrete temperatures. Prolonged mixing should be avoided. To avoid cold joints, care must be taken with placement techniques since setting is more rapid in hot weather. Overvibration with internal vibrators should be avoided. Burlingame 2004

Plastic Shrinkage Cracking Plastic shrinkage cracks sometimes occur in the surface of freshly mixed concrete soon after placement, during finishing or shortly thereafter. These cracks which appear mostly on horizontal surfaces can be substantially eliminated using preventive measures. Plastic shrinkage cracking is usually associated with hot-weather concreting; however, it can occur any time ambient conditions produce rapid evaporation of moisture from the concrete surface. These cracks occur when water evaporates from the surface faster than it can travel to the surface during the bleeding process. This condition creates rapid drying shrinkage and tensile stresses in the surface that often result in short, irregular cracks.

Plastic Shrinkage Cracking The following conditions, individually or collectively, increase evaporation of surface moisture and also increase the possibility of plastic shrinkage cracking: High cementitious materials content 2. Low w/cm 3. High concrete temperature 4. High air temperature 5. Low humidity 6. Wind The crack length is generally 50 mm to 1000 mm (a few inches to 3 ft) in length and they are usually spaced in a somewhat regular pattern with an irregular spacing from 50 mm to 700 mm (a few inches to 2 ft) apart.

Plastic Shrinkage Cracking Kohler 1952, Menzel 1954, and NRMCA 1960 The nomograph shown here is a graphical solution borrowed directly from hydrologic studies sponsored by the U.S. Navy on the shores of Lake Hefner in Oklahoma. The nomograph is useful for determining when precautionary measures should be taken. However, there is no sure or absolute predictor for plastic shrinkage cracking. An important note is that the nomograph evaluates the evaporative potential of the environment, not the rate of water loss from the concrete. However, the difference between the two may be small when the concrete surface is covered with bleed water.

Plastic Shrinkage Cracking Menzel (1954) adopted the Kohler (1952) equations, simply converting the unites used to express vapor pressure and wind speed. The values for the saturation vapor pressure of water are themselves temperature dependent: W= 0.315(eo-ea)(0.253 +0.060V) (for pressure in SI unit of kPa) W= 0.44(eo-ea)(0.253 +0.060V) (in.-lb units) Uno (1998) built-in regression equations for saturation vapor pressure and combined them with the Kohler/Menzel equation to produce a unified equation that takes vapor pressure into account E=5([Tc+18]2.5 – r •[Ta + 18]2.5)(V+4)x10-6 (SI units) E=(Tc2.5 – r • Ta2.5)(1+0.4V)x10-6 (in.-lb units) Where: W = mass of water evaporated in kg (lb) per m2 (ft2) of water-covered surface per hour eo = saturation water vapor pressure in mm HH (psi) in the air immediately over the concrete surface, at the concrete temperature ea = water vapor pressure in mm hg (psi) in the air surrounding the concrete V = average wind speed in km/h (mph),measured at 0.5 m (20 in.) above the concrete surface E = evaporation rate, lb/ft2/h (kg/m2/h) Tc = concrete (water surface) temperature, °F(°C) r = (relative humidity percent)/100 Ta = air temperature, °F(°C)

Plastic Shrinkage Cracking One or more of the precautions listed below can minimize the occurrence of plastic shrinkage cracking. They should be considered while planning for hot-weather concrete construction or while dealing with the problem after construction has started. Keep the concrete temperature low by cooling aggregates and mixing water. Add fibers to the concrete mixture Moisten concrete aggregates that are dry and absorptive. Dampen the subgrade and fog forms prior to placing concrete. Erect temporary windbreaks to reduce wind velocity over the concrete surface. Erect temporary sunshades to reduce concrete surface temperatures. Fog the slab immediately after placing and before finishing, taking care to prevent the accumulation of water that may increase the w/cm at the surface and reduce the quality of the cement paste in the slab surface. Tooling a wet surface will supply the mixing energy to increase the water-cement ratio. Protect the concrete with temporary coverings, such as reflective (white) polyethylene sheeting, during any appreciable delay between placing and finishing.

Plastic Shrinkage Cracking Fogging the concrete surface before and after final finishing is the most effective way to minimize evaporation and reduce plastic shrinkage cracking (left). Fog nozzles atomize water using air pressure (right) to create a fog blanket. They should not be confused with garden-hose nozzles, which leave an excess amount of water on the slab. Fogging should be continued until a suitable curing material such as a curing compound, wet burlap, or curing paper can be applied.

Curing and Protection The need for moist curing is greatest during placement and the first few hours after finishing. Concrete surfaces should dry out slowly after the curing period to reduce the possibility of surface crazing and cracking. Crazing, a network pattern of fine cracks (shown) that do not penetrate much below the surface, is caused by minor surface shrinkage. Crazing cracks are very fine and barely visible except when the concrete is drying after the surface has been wet.

Heat of Hydration Heat generated during cement hydration raises the temperature of concrete to a greater or lesser extent depending on the size of the concrete placement, its surrounding environment, and the amount of cement in the concrete There may be instances in hot-weather concrete work and massive concrete placements when measures must be taken to cope with the generation of heat from cement hydration and attendant thermal volume changes to control cracking.

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