Guide for Curing Portland Cement Concrete Pavements, II

CHAPTER 3: DRAFT GUIDELINES

A draft guide for curing portland cement concrete pavements is presented in figures 2 through 9. Figure 2 gives an overview of the entire guide. Figures 3 through 9 give details of subsections of the guide, with figures 3 through 7 covering choices among materials and methods, and figures 8 and 9 covering temperature management. The guide was developed principally from existing U.S. standards, with some features added for which U.S. standards provide little or no guidance.

As a result of review by the TAP, some features of the guide were identified as poorly developed, needing additional work to develop the information necessary. Text accompanying each figure covers pertinent details and subjects that needed more work in subsequent tasks in the project.

General Outline of the Guide

Figure 2 presents a general summary of the draft guide. The guide is organized around

three moist-curing procedures and a separate section on temperature management. The three moist-curing procedures are curing compound methods, water-added methods, and water-retention methods other than curing compounds.

Figure 2. General layout of the draft guide.

Selection Among Moist-Curing Methods

Figure 3 depicts the decision tree associated with selection among moist-curing methods. Use of curing compounds for curing large placements of highway paving is usually the most practical and economical method. This may also be true for smaller pavement projects due to unavailability of materials for other curing methods or because of the impracticality of maintaining inspection.

Concretes with water-cement ratios less than 0.40 are known to desiccate internally due to consumption of water during hydration. More water must be added if additional hydration is thought necessary and/or to prevent the shrinkage associated with internal desiccation. There is considerable doubt expressed in the literature about whether water-added curing is effective in penetrating low water-cement ratio concretes, and so it may be a wasted effort from the perspective of improving strength. However, it is plausible that the added water has positive effects on the near-surface concrete. There is not much in the literature on this detail. Expansive cements are also believed to benefit from added water, but these are rarely used in concrete paving.

Figure 3. Selection among moist-curing methods.

If the water-cement ratio is greater than 0.40, more than sufficient mixing water exists to completely hydrate the cement and to prevent internal desiccation, so no added water is required. This branch of the guide also includes guidance on water-retention methods to use when curing compounds are not suitable. The most likely reasons this might occur would be unavailability (small jobs, short notice), anticipated problems with bonding of paint or adhesives to the pavement surface, or a desire to dry the concrete as rapidly as possible after curing is completed, such as if some kind of covering is to be applied that is intolerant of residual moisture in the concrete. This method may not be suitable under windy conditions.

Selecting Among Curing Compound Methods

The outline of decision criteria pertinent to curing compound methods is shown in figure 4.

Figure 4. Decision criteria for curing-compound methods.

Choosing curing compounds. White pigmented curing compounds are normally required when exposure to sunlight is anticipated. Either white pigmented or fugitive dye compounds are required when visual inspection of the quality of the application is required.

Environmental regulations limiting the amount of VOCs released during construction activities are currently being implemented by some organizations. More information on these regulations needs to be obtained so additional details can be added to this branch. Also, information on any differences in practices required of low VOC versus conventional curing compounds needs to be obtained and incorporated into the guide.

AASHTO M 148⁽²⁴⁾ (ASTM C 309⁽²³⁾) is the most commonly referenced specification for curing compounds for highway paving. Some State transportation departments have developed their own standard, which limits moisture loss measured using AASHTO T 155⁽⁵⁵⁾ (ASTM C 156⁽¹⁸⁾) or similar test, to values lower than those found in M 148⁽²⁴⁾ (0.55 kg/m² at 72 h). ACI 305R⁽¹²⁾ recommends lower limits in hot weather (0.39 kg/m² at 72 h). ASTM C 1315⁽²⁴⁾ limits moisture loss to 0.40 kg/m² at 72 h, but is not referenced in any current paving guidance found. Additional information on the relationship between the moisture loss requirement and performance needs to be developed. The precision of test methods is also a problem with respect to user-producer agreement on acceptance.

Application rate. ACI 301⁽⁴³⁾ requires a curing compound application rate of no more than 5 meters squared per liter (m²/L), with two applications at right angles on rough surfaces, not to exceed 5 m²/L for the total application. The AASHTO Guide Specification⁽³³⁾ requires 3.5 m²/L, applied by fully atomized spray equipment with a tank agitator and wind guard. USACE requires two applications of 10 m²/L each.⁽³⁴⁾ Manufacturer's recommendations are typically 5-10 m²/L. This information needs to be supplemented with information on practices from State departments of transportation.

It seems plausible that an application rate could be based on the physical properties of the pavement surface, the performance properties of the curing compound, and the climatic conditions anticipated during use. Additional work needs to be done to explore this possibility.

Start of curing activity. Guidance typically directs application of final curing after finishing is complete and sheen has disappeared. If evaporation rates are high and the concrete does not bleed, protection against excessive evaporation must be provided until this point is reached. According to guidance in ACI 305R⁽¹²⁾ and ACI 308⁽³¹⁾ evaporation rates of ≥S1.0 kg/m²/h seem to be the upper limit for acceptable conditions for requiring no action. Other guidance recommends consideration of action at lower rates. ACI 305R⁽¹²⁾ says that precautions should be taken if evaporation rates approach this limit. Army TM 5-822⁽²⁵⁾ says that 0.75 kg/m²/h may be problematic. ACI 308⁽³¹⁾ says that precautions may be needed at 0.5 kg/m²/h. Additional work may be needed on this limit.

If concrete is bleeding, then a negative interaction between bleeding and early application of curing compound can result. ACI 308-92⁽³¹⁾, paragraph 2.3.3 cautions:

If ambient evaporation rate exceeds 0.2 lb/ft²/h (1.0 kg/m²/h)...the concrete may still be bleeding even though the surface water sheen has disappeared and steps must be taken to avoid excessive evaporation. If membrane-forming compound is applied to a dry-appearing surface, one or the other of two undesirable conditions may follow: a) evaporation will be effectively stopped but bleeding may continue, resulting in a layer of water forming below the layer of cement paste to which the membrane is attached (such a condition promotes scaling); b) evaporation will be temporarily stopped but bleeding may continue resulting in map cracking of the membrane film, requiring reapplication of the curing compound.

This problem can be avoided by maintaining the surface moisture, either by misting or applying evaporation retarding compounds.

Inspection. Current guidance is to inspect application equipment (Army TM 5-822⁽²⁵⁾, ACI 305R⁽¹²⁾) to ensure proper function and to estimate the rate of application by measuring the amount of curing compound used to cover a known surface area at the end of each operation (USACE Guide Specification, CWGS 03300⁽¹⁹⁾). In the case of pigmented curing compounds, visual inspection for uniformity is recommended (CWGS 03300⁽¹⁹⁾). No information was found on the reliability of visual inspections to detect poor application of curing compound. Work may need to be done here.

No standard test methods were found that are suitable for determining in place rates of curing compound application. Additional work is required.

Verify curing. The basic approach to verifying that proper curing has occurred is to ensure that proper curing procedures have been applied (ACI 308⁽³¹⁾). No standard procedures for performance testing currently exist. Potential performance measures of the adequacy of curing include measuring near-surface properties of concrete and/or measuring the amount of moisture on the surface of the concrete. Work needs to be done here.

Water-Added Methods

Figure 5 shows the decision tree and criteria for water-added methods.

Specifications on water. Temperature of curing water must be no more than 10 °C cooler than the surface of the concrete (ACI 308⁽³¹⁾, ACI 308.1⁽⁴⁵⁾, ACI 305R⁽¹²⁾). Requirements on dissolved materials mostly pertain to the staining potential of curing water. ACI 308.1⁽⁴⁵⁾ directs that curing water be either potable or meet requirements of ASTM C 94⁽¹⁵⁾. The requirements in this standard purport to address mixing water, not curing water. USACE standard CRD-C 400 addresses staining in curing water, but in a relatively qualitative way.⁽¹⁶⁾

Figure 5. Decision criteria for water-added methods.

Specifications on absorbent materials. Absorbent materials are useful for holding water on the concrete surface, either vertical or horizontal. They are required to be nonstaining (ACI 308.1⁽⁴⁵⁾) or free of sugar or fertilizer (ACI 308⁽³¹⁾), but no quantitative criteria are cited. Earth materials must be free of organic matter and free of particles larger than 25 mm. Straw or hay should be at least 150 mm thick (ACI 308).⁽³¹⁾ Burlap must meet AASHTO M 182-91⁽⁵⁶⁾.

Start of curing. Guidance on delaying application of water until after final finishing is taken from ACI 308.1.⁽⁴⁵⁾ ACI 308⁽³¹⁾ directs that even more strength is required for heavier mats, but no quantitative guidance is given. As with curing compounds, excessive evaporation must be avoided before application of curing to avoid plastic shrinkage cracking.

Inspection. Moist curing inspection shall occur once per shift or not less than two times per day, including nonworkdays. Moisture conditions of the concrete are recorded. If an area is found to be dry, corrective action is taken and curing is extended one day (USACE CWGS 03300).⁽¹⁹⁾ ACI 308.1 allows that the architect may specify the frequency of inspections.⁽⁴⁵⁾

Water-Retention Methods (other than curing compounds)

Figure 6 shows the decision tree and criteria for water-retentive methods other than those that use curing compounds.

TOS = time of initial setting

Figure 6. Decision criteria for water-retentive methods other than those using curing compounds.

Specifications. Sheet materials are covered by ASTM C 171.⁽¹⁷⁾ The sheet color requirement is in ACI 308.1.⁽⁴⁵⁾ The USACE Guide Specification (CWGS 03300) only allows plastic covered burlap sheeting.⁽¹⁹⁾

Start of curing. The same information applies as described above for application of water absorbent materials.

Inspection. Inspection must include verification that the concrete surface is wet under the sheet. Inspection frequency is the same as for water-added methods.

Verification of curing. Verification methods are the same as described under curing-compound methods.

Duration of Curing

Figure 7 shows the decision tree and criteria for duration of curing.

NDT = non-destructive testing

Figure 7. Decision criteria for duration of curing.

Prescriptive times. Additional work needs to be done here. There seems to be conflicting guidance when strength is the property on which curing duration is based. Durability may require even longer times.

Performance-based, strength. ACI 301 gives three alternative criteria:⁽⁴³⁾

(1) 70 percent f_c¢.

(2) Curing time equal to lab time required to achieve 85 percent f_c¢.

(3) 100 percent f_c¢ when using NDT methods for estimation.

Performance-based, durability. Work needs to be done here. There is no additional guidance on durability.

Temperature Management, Hot Weather

Figure 8 shows the decision tree and criteria for temperature management in hot weather.

Figure 8. Decision criteria for temperature management in hot weather.

Temperature rise. Information on calculating heat rise in concrete pavements based on heat of hydration and environmental conditions exists. HIPERPAV⁽¹³⁾ software (McCullough and Rasmussen (1999)^{(57, 58, 59)}), for example, uses these kinds of input data as part of a calculation of strains in pavement concrete. The temperatures of the concrete are not currently part of the output of the program. Kapila, Falkowsky, and Plawsky (1997) have developed a computer program that executes this calculation.⁽⁶⁰⁾ The program is reported to be personal computer compatible. This paper also references other such programs. Predicting concrete temperature rise is reasonably outside of the scope of this work.

Thermal stress. Several items in current standard guidance apply to the problem of thermal stress that can result if cooling of concrete during curing is too severe. ACI 305R directs that water used in curing should not be more than 10 °C cooler than the concrete.⁽¹²⁾ The USACE Guide Specification (CWGS 03300) limits the temperature difference between the concrete surface and a depth of 50 mm to no more than 13 °C.⁽¹⁹⁾ A thermal stress analysis using finite element methods would be suitable for detailed analysis of this problem, but it is reasonably outside of the scope of this work.

Strength reductions at high temperatures. There does not seem to be a simple calculation for estimating the effect of high curing temperatures on ultimate strength, and this phenomenon probably does not lend itself to a simple calculation. This effect was not included in the literature search that was executed for this preliminary work, so an additional effort was required to identify this information (see chapter 4).

Adjustments to Curing Methods. Curing compound application rate or water-retention specification limits may need to be adjusted for hot weather conditions. There is a strong heat of hydration exotherm of portland cement-based concretes that starts at time of setting and continues for several hours. The exact timing and size of the exotherm depends on the cementitious materials (there is quite a bit of variation even among portland cements) and type and amount of chemical and mineral admixtures. If this exotherm occurs simultaneously with the peak in solar radiation, then there could potentially be a synergistic effect, resulting in a very large temperature rise. The timing of this exotherm does not seem to lend itself to prediction by simple calculation, but the effect is relatively easy to measure. At least one commercial manufacturer of equipment for measuring "heat signatures" exists. This information would probably be very useful in scheduling construction so as to avoid the confluence of autogenous heating and solar heating. HIPERPAV⁽¹³⁾ software (McCullough and Rasmussen, 1999)^{(57, 58, 59)} addresses these issues.

Temperature Management, Cold Weather

Figure 9 shows the decision tree and criteria for temperature management in cold weather.

Figure 9. Decision criteria for temperature management in cold weather.

Strength limits for exposure to freezing. Various guidance on freezing exists. AASHTO Guide Specification⁽³³⁾ directs that concrete be protected from freezing for 10 days or until strength reaches 15 MPa. ACI 306R⁽¹⁴⁾and 308⁽³¹⁾ direct that a single freezing event must be avoided until concrete has reached a strength of 3.5 MPa. The USACE Standard Practice (EM 1110-2-2000) cautions avoidance of freezing and thawing cycles until strength reaches 24 MPa.⁽⁸⁾

Insulation. ACI 306R gives insulation constants (R values) required to maintain a temperature of 10 °C for various ambient conditions.⁽¹⁴⁾ This concept could be extended to include insulation constants required to maintain higher temperatures. This should lend itself to standard engineering calculations on heat transfer.

Need for moisture retention measures in cold weather. Paragraph 8.2 of ACI 306R on curing during the protection period applies here.⁽¹⁴⁾ If concrete is warmer than 16 °C and air temperature is 10 °C or higher, then drying can be a problem. Use of steam for both heating and moisture maintenance is the preferred method, otherwise a curing compound is required. Water-added methods are not recommended because of the practical problems of ice forming outside of the enclosure. When the enclosure temperature is 10 °C or lower, then no moisture maintenance is required if the relative humidity is less than 40 percent.

Additional Information and Modifications Needed for Completion of the Guide

The following list summarizes items in the draft guide that need some additional attention, along with notes on how this will be accomplished.

Concrete materials and mixture proportions. A section needs to be added to the guide on concrete materials and mixture proportions. Concrete materials and mixture proportions are not normally chosen with respect to maximizing or even optimizing curing procedures. Specifications for materials tend to be rather monotonous. Cements, pozzolans, aggregates, admixtures, and mixing water are required to meet standard specifications. However, within the limits imposed by the standard specifications, there are a few places where some choice can be made that will have some impact on curing practices. Mixture proportions are an area in which quite a bit of latitude in choice exists. This information will be put together from existing knowledge of the relationship between materials and mixture properties and performance properties that are pertinent to facilitating curing. Agency guide specifications will be reviewed since these typically set outer boundaries on materials and mixture proportions.

Protection between placing and final curing. Details on options during the time between placing concrete and start of curing needs to be developed more fully. A considerable amount of information is currently under development in ACI 308Ron this period (referred to as "initial curing period" in their guides).⁽⁶⁾ Research literature on plastic shrinkage was examined in more detail for information that might be useful in developing guidance. State DOT guidance was reviewed to see how this period is handled in various localities (see chapter 4).

Evaporation rate nomograph. The draft guide contains little information on the use of the evaporation rates, as calculated from windspeed, air temperature, concrete temperature, and relative humidity. There has been some criticism of the information derived from this calculation. Considerably more information has been developed recently in the new ACI 308R and in recent literature.⁽⁶⁾ The limits of 0.5 and 1.0 kg/m²/h for caution and definite action (respectively) may need to be revised.

Time of setting. Knowledge of the time of setting of concrete is useful for planning curing operations. It is well known that time of setting has a strong temperature dependency, and it has been suggested that it should follow the same Arrhenius relationship that is the basis of one of the maturity methods. A calculation will be developed based on this concept.

Curing compound technical information. More information on the diversity of curing compounds that are available needs to be developed and worked into the selection of materials section of the Guide for Curing Compound procedures. There does not seem to be much information on this in the research literature, so we will have to rely on information that can be extracted from curing compound manufacturers. Some of this information is expected to be proprietary and not available, particularly details of the formulation. Also details on application practices need to be filled out and organized in one place. This was obtained later from manufacturers and by comparing experiences and guidance manuals of various agencies.

Effectiveness of Water Curing of Low Water-Cement Ratio Concretes. Water curing is not a major method for curing concrete pavements, but it is sometimes used in curing small sections. If concrete is low water-cement ratio (<0.40), then internal desiccation may be an important source of shrinkage strains. Water curing can, in theory, relieve this form of drying shrinkage. Additional information will be located that comments on the plausibility that water applied to the surface of a low water-cement ratio concrete will actually penetrate far enough into the concrete to be effective.

Evaporation reducers. More information on evaporation reducers will be developed. Extensive literature on this technology does not exist, so information will have to be located in nonstandard places, such as through anecdotal information. As part of the evaluation of the guide, some evaporation reducers will be evaluated in the laboratory.

Tined surfaces. Tining a concrete surface greatly increases the effective surface area. Information needs to be developed on how this affects application rates of curing compound. A section of the guide will be developed that provides the user with a way to calculate adjustments to application rate of curing compound required to compensate for this increased surface area.

Length of curing. More information needs to be gathered on length of curing, particularly when pozzolans or other non-portland cement items are included in the cementitious materials. There is substantial literature on this and existing practice. This needs to be reviewed again in more detail to arrive at reasonable guidance for pavements.

Test methods-verifying curing compound application. Plausible test methods for verifying curing compound application need to be developed. The preliminary report listed several plausible ones, but no details on how they might be executed were included. These will be developed and then evaluated in that section of the report.

Test method-verifying curing. Plausible test methods for evaluating the quality of curing of the near-surface concrete need to be developed. Several candidate procedures were mentioned in the preliminary report, but no details were included. Methods will be evaluated in that section of the report.

Emphasis on curing compound methods. It was emphasized during the review of the draft guide with the TAP and in comments from the contracting officer's technical representative (COTR) that the emphasis of the guide should be on concrete paving and that water-curing and sheet-curing methods would generally not be practical. These methods will be de-emphasized in the revised guide, although information will be presented in the section on technical notes on these topics.