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Figure 1. Chart. Major points of preconstruction planning.

This flowchart summarizes the major points for preconstruction planning outlined in chapter 2. Under materials and mixture proportions, the box warns to be aware of factors affecting curing to avoid problems such as 1) water/cementitious materials ratio, 2) fineness of the cementitious materials, 3) cement content, 4) large amount of pozzolans, and 5) effects of admixtures. The next step is to measure bleeding whose patterns determine the amount of water evaporation without damage and affect the quality of the curing membrane formation. The basic test method is to use AASHTO T158 that can be modified into a field test measured at 30-minute intervals to show time dependency. The next step is to measure the setting time. Initial setting time determines the end of bleeding and the optimal time to apply curing procedures that determine water evaporation without damage. The test used is AASHTO T197 that can be approximated in the field and adjusted for in-place temperature. Following this, it is important to anticipate drying and thermal conditions because drying affects the probability of plastic shrinkage cracks. Early concrete temperature affects evaporation rates, and maximum concrete temperature affects thermal stress and cracking. The procedures to measure drying include estimate the drying conditions using ACI 308, figure 1. Then using ACI 305, calculate the probable concrete temperature. Using HIPERPAV software, anticipate thermal stress problems. Lastly, plan to cool the concrete, if needed. The section on analysis covers the net balance between water evaporation loss and bleed water formation to estimate when plastic shrinkage cracking will occur. The analysis procedure includes plotting bleeding versus time and plotting the maximum evaporation versus time. The critical points include soon after placing and/or immediately before setting time. Finally, describe the plan for corrective action if critical conditions occur. Corrective action can include evaporation reducers, reducing the concrete placing temperature, wind barriers, misting, and adjusting the placement time of day.

Return to Figure 1

Figure 2. Equation. Bleeding rate from water-cement ratio.

The bleed rate equals pavement thickness in centimeters, T, times the following: 0.051 times the water-cementitious materials ratio, W/CM, minus 0.015.

Figure 3. Equation. Time-averaged bleeding rate.

The average bleed rate equals the amount of bleed water in kilograms, V, divided by the surface area of the specimen in meters squared, A, times time in hours, T.

Figure 4. Graph. Plot of bleed water formation versus time for a typical paving mixture.

The graph shows the relationship of time and concrete bleeding or water loss. The horizontal axis is time in hours ranging from 0 to 4 with a time of setting (TOS) arrow pointing to a little less than 4. The vertical axis is bleed water in kilograms per meter squared per hour, ranging from 0.0 to 0.5. The graph shows that the highest bleed rate of 0.45 occurs at about an hour and a quarter.

Figure 5. Equation. Time of setting-adjustment for concrete temperature.

Time of setting, TOS, equals TOS subscript STD standard conditions, any unit, times E superscript the following: constant, R times the sum of the quotient of 1 divided by temperature of in-place concrete, CT minus 1 divided by temperature during laboratory test, STD Temp.

Figure 6. Equation. Evaporation of bleed water-effect of environmental conditions.

Evaporation rate, ER, equals 4.88 times the sum of 0.1113 plus 0.04224 times the wind speed, WS, divided by 0.447, times 0.0443 times E raised to the 0.0302 times concrete temperature, CT, times 1.8 plus 32, minus the following: relative humidity, RH, divided by 100 times E raised to the 0.0302 times air temperature, AT, times 1.8 plus 32.

Figure 7. Chart. Evaporation rate nomograph from ACI 308.

This complex chart uses many variables to estimate evaporation rates. The first section shows air temperature in both Celsius and Fahrenheit on the horizontal scale compared to relative humidity curves. The next section is concrete temperature in both Celsius and Fahrenheit, and the last section plots wind velocity in kilometers per hour (or miles per hour) against kilograms per meter squared per hour (from 1.0 to 4.0) to determine the rate of evaporation in pounds squared feet per hour. The steps to follow are 1) enter the air temperature and find where it intersects with relative humidity, 2) move right to the concrete temperature, 3) move down to the wind velocity, and 4) move left; read the approximate evaporation rate.

Figure 8. Graph. Plot of cumulative bleed and cumulative evaporation versus time.

This figure shows the cumulative bleed rate from the data in figure 2 plotted with a cumulative evaporation rate of 0.30 kilograms per meter squared per hour. The horizontal axis is time in hours ranging from 0.0 to 6.0 with the time of setting arrow pointing to 5 hours. The vertical axis is cumulative bleeding or evaporation in kilograms per meter squared ranging from 0.0 to 5.0. The bleed plot is a solid curved line and the evaporation rate is a straight dashed line, both trending upward nearly proportionately. In this example, evaporation rates exceed bleeding rates for the first hour after placing and again after about 3.5 hours where the setting time is 5.2 hours. These two periods represent critical times for the potential for plastic shrinkage cracking.

Figure 9. Graph. Effect of reducing concrete placing temperature from 30 degrees Celsius to 25 degrees Celsius.

This figure shows the effect of reducing concrete placing temperature. The horizontal axis is time in hours ranging from 0.0 to 6.0 with the time of setting arrow pointing to 5 hours. The vertical axis is cumulative bleeding or evaporation in kilograms per meter squared ranging from 0.0 to 5.0. The bleed plot is a solid curved line and the evaporation rate at 25 degrees Celsius is a straight short dashed line while the evaporation rate at 30 degrees Celsius is a straight long dashed line. The higher temperature shows a cumulative bleeding of 4.0 while the lower temperature shows a less critical rate of 2.75 kilograms per meter squared that is less likely to crack.

Figure 10. Graph. Effect of reducing evaporation 50 percent by using evaporation reducer.

This figure shows the effect of using concrete evaporation reducers. The horizontal axis is time in hours ranging from 0.0 to 6.0 with the time of setting arrow pointing to 5 hours. The vertical axis is cumulative bleeding or evaporation in kilograms per meter squared ranging from 0.0 to 5.0. The bleed plot is a solid curved line and the evaporation rate with no evaporation reducer is a straight long dashed line and the rate with a 50 percent reduction in evaporation is a straight short dashed line. The evaporation reducer cuts the bleeding rate in half from 4 to 2 kilograms per meter squared, a less critical rate where the concrete is less likely to crack.

Figure 11. Chart. Major items requiring attention during construction-initial curing period.

This chapter covers the initial curing period as the time between placing the concrete and application of final curing. Construction activities include verifying the environmental conditions by preconstruction planning to identify the probable conditions and then determining the actual concrete temperature, setting time (corrected for concrete temperature), air temperature, wind speed and relative humidity. The next step is to make onsite adjustments such as the concrete temperature or evaporation reducers if critical drying develops. Methods to achieve low concrete temperature include cooling the aggregate stockpiles, using ice for mixing water, calculating the application rate and frequency of using evaporation reducer, misting, wind breaks or possibly alternate curing compound practices.

Figure 12. Equation. Temperature of fresh concrete from ingredients.

The concrete placing temperature, T, equals 0.22 times the following: temperature of coarse aggregate TCA times dry mass of coarse aggregate WCA plus temperature of fine aggregate TFA times dry mass of fine aggregate WFA plus temperature of pozzolan TP times WP mass of pozzolan plus temperature of TC times mass of cement WC. Then add temperature of mixing water excluding ice TW times mass of mixing water WW, plus TCA times mass of free and absorbed moisture in coarse aggregate WCAM, plus TFA times mass of free and absorbed moisture in fine aggregate WFAM. Then subtract mass of ice WI times the sum of 79.6 minus 0.5 times temperature of ice TI. Take this result and divide it by the following: 0.22 times the sum of WCA plus WFA plus WC plus WP, then add to this WW plus WI plus WCAM plus WFAM.

Figure 13. Equation. Frequency of application of evaporation reducer.

Frequency of application, F, equals the application rate, AR, divided by the following: evaporation rate of bleed water, ER, times the sum of 1 minus 0.4, minus the bleed rate of concrete, BR.

Figure 14. Chart. Major items requiring attention during construction-final curing period.

The final curing period is the time between application of curing procedures and the end of deliberate curing. Under final curing methods, this figure lists that they are economically driven in conventional large paving operations but that high performance concrete might need technical considerations. Curing compound methods are usually the most economical for large operations, but are characterized by relatively complex application and acceptance issues. Water-added methods are good for low water-concrete ratio concretes with a simple but messy application. The acceptance criteria and QC are also simple. The third section of this figure outlines the sheet material methods that are practical for small areas with simple acceptance tests and QC. The fourth section details temperature control methods that are complicated with many variables. This method can control the temperature of fresh concrete and the HIPERPAV software can calculate the many variables. The caution is that temperature control methods can cause rapid evaporative cooling.

Figure 15. Chart. Major features in curing compound practice.

This figure lists the many variables in using curing compounds. First are the specifications in selecting a curing compound that include water retention (most critical), pigments (monitor application and reflect sun), drying time (influenced by rain washes if not dry), type and amount of solids (some agencies distinguish), volatile organics (monitored by environmental and OSHA regulations), compatibility with coatings (paints and adhesives) and low viscosity (important if texturing). The next section describes the factors for application time: after initial setting is best for performance because early application can cause problems if not validated. The next section lists application rate parameters that must account for texturing/grooving. The application is best in two coats with the guidance variable 3.5 to 5 meters squared per liter. The next section covers the application verification. Steps include calculating the volume applied and the area covered, from the measured delivery applicator rate, the application speed along the pavement, a visual estimation if white pigmented, and measurement by the portable reflectometer.

Figure 16. Equation. Drying time for curing compound-temperature correction.

Drying time equals evaporation rate, ER, raised to the negative 0.67.

Figure 17. Equation. Application rate for curing compound-correction for texturing.

Adjusted application rate, AR, equals AR subscript ungrooved times the quotient of the space between grooves, S, plus the width of the grooves, W, all divided by S plus 2 times the depth of the grooves, D, plus W.

Figure 18. Chart. Major features of curing with added water.

This figure lists the features of water-added methods of concrete curing. The material requirements include temperature no more than 10 degrees Celsius cooler than the concrete to prevent cracking and a limit on dissolved materials so staining doesn't occur. The next section outlines the time of application, after initial setting unless applied as a mist with the warning that early water application causes erosion of the paste. The application methods include wet, absorbent materials like burlap, straw or dirt; misting; using sprinklers; or ponding. The next section describes the visual method to verify application. It includes comments such as water curing is recommended for low water to concrete ratios (less than 0.40), the method is messy, and it depends on the availability of water. The warning is that thermal shock can occur when evaporation rates are high.

Figure 19. Chart. Major features of curing with water-retention methods.

Sheet materials are impervious, simple and usually don't involve specification compliance issues. This method is impractical for large areas of paving or windy conditions and can cause a mottled pattern on the concrete. The material requirements include compliance with ASTM C171 that emphasizes sheet overlap. The major material is olyethylene but white-pigmented and plastic sheet-burlap products are available. The next section outlines the time of application as after initial setting but warns that early application of water will mar the surface. Finally, verification of application is described as visual.

Figure 20. Chart. Thermal effects.

This thermal effects chart covers the time within the first few days of placement when problems occur with volume changes in hardened concrete. The first section describes the problem with temperature gradients that can cause volume changes. Uniform cooling or simple contraction can cause cracking. More complicated cooling with complex gradients can also cause cracking. The next section lists the sources of heat that can cause rapid curing such as cement hydration, solar radiation and environmental conduction. Next, the development of temperature gradients is described: losses to the environment, cooling may not be uniform across thickness of pavement, warping and curling may occur, and it can by cyclic. The analysis describes the HIPERPAV program that generates a graph to estimate the heating phase, temperature gradients, strength development, and times of crack development. Simple expedients outlined in the section include limiting the heat of concrete hydration, cooler placing temperatures, time-of-day adjustments for placement and reflective curing materials.

Figure 21. Chart. Considerations pertinent to the termination of curing.

This figure summarizes the major considerations involved in curing length and curing verification for concrete. The first section mentions that the prescriptive curing time is usually 3 to 7 days with performance requirements expressed as a percent of field-cured cylinders or maturity (time-temperature history) now is used commonly. Next, curing verification is prescriptive; follow procedures and curing will be correct. Performance is based on physical tests on in-place concrete.