Guide for Curing Portland Cement Concrete Pavements, II

CHAPTER 2: SYNTHESIS OF INFORMATION

This section represents the synthesis of information that supported the development of the draft guide presented in the interim report. Additional information required to complete the guide was identified as a result of ideas and suggestions made by the TAP and will be added in chapter 4. A full literature review is in appendix B.

Terminology

ACI 308R⁽⁶⁾ defines two phases of curing. For purposes of this project, it will be useful to refer to these. The initial curing period is the time between when concrete is placed and when final curing operations can be initiated. Final curing operations typically cannot be applied during this period because one or more properties of the concrete are not suitably developed, and the concrete could be damaged. The objective of curing activity during the initial curing period is to prevent excessive loss of mixing water from the plastic concrete, which can lead to plastic shrinkage cracking. Deliberate activity during this period is only required if drying conditions are sufficient to cause excessive water loss. This will be discussed in chapter 5.

The final curing period is the time between application of final curing procedures and the cessation of deliberate curing activity. The objective of curing activity during this period is to insure that necessary water is either retained or added and that temperature is maintained within a range that the hydration of the cementitious materials can progress sufficiently for development of necessary physical properties and that temperature is controlled sufficiently to avoid damaging thermal gradients.

Selecting Curing Methods

Initial Curing Period. If deliberate curing activity is required during the initial curing period, two methods are commonly cited in guidance for reducing evaporation rates-wind breaks and water misting devices. These are often considered impractical for large pavement construction projects because of the large amount of hardware and labor required.

Figure 1. Canopy used to protect fresh concrete.

Practice in the United Kingdom (UK) (Carroll, 1988) at one time included use of a long canopy that could be towed behind the paving operation that would protect from sun, wind, and rain (figure 1).⁽⁷⁾ It is unknown if this practice is still used there. No mention of such a device was found in American literature or guidance.

Evaporation reducers (sometimes called evaporation retarders) are a relatively new product being marketed to protect concrete during the initial curing period. Mention of these is largely absent from standard guidance and the research literature. These products are being used in construction, but no information was found on the breadth of their use. No specifications or test methods are known to exist. Users are currently relying on manufacturer's instructions.

Evaporation reducers are gaining popularity because of the relative ease of use. Like curing compounds, their use requires a relatively small labor cost. Evaporation reducers can usually be applied with the same hardware used to apply curing compound or applied as a drip on a burlap drag, so the additional hardware costs are not high. This technology needs to be further investigated.

Final Curing Period. Standard guidance on choosing final curing methods for portland cement concrete allows for a number of options. Allowable methods can be broadly classified either as water-retention methods or as water-added methods. As the terms suggest, the objective of the former is simply to retain the mixing water in the concrete for the period of time necessary for curing to be completed. Water-added methods provide water in excess of the mixing water. Occasionally water-added methods are used as a matter of practicality-not because more water is needed.

For paving concrete, the choice of curing methods from among these is largely one of practicality and economics rather than quality of performance. However, a distinction is made in some guidance between water-added and water-retentive methods based on water-cement ratio, which may affect paving concrete is some cases.

Guidance featuring this distinction includes the USACE Standard Practice for Concrete (EM 1110-2-2000)⁽⁸⁾ and Standard 3420, as reported in Meeks and Carino (1999)⁽⁴⁾. This distinction is also evident in the research literature. At water-cement ratios below approximately 0.40 (the exact number varies with the information source), water-added methods are recommended. This guidance is based on the fact that concretes can internally desiccate from consumption of water by hydration reactions at water-cement ratios lower than about 0.40, and additional water must be added if additional hydration is required. The 0.40 threshold is an approximate value, varying some with chemical composition of cementitious materials. It probably derives from Philleo's (1986⁽⁹⁾, 1991⁽¹⁰⁾) calculations on the amount of water filled space in fresh cement paste and the volume occupied by reaction products in a fully hydrated paste. At a water-cement ratio greater than about 0.40 there is, initially, more mixing water filled space than can be filled with hydration products if all the cement hydrates. At a water-cement ratio less than about 0.40 there is initially insufficient mixing water filled space to allow all of the cement to hydrate. Hence at water-cement ratios greater than 0.40, some mixing water can be lost without jeopardizing maximum hydration of the cement, if this is desired. At a water-cement ratio less than 0.40, this is not the case.

Another, and probably more consequential, effect of internal desiccation due to complete consumption of mixing water is the development of shrinkage cracks. The same mechanism that causes drying (atmospheric) shrinkage cracks apparently causes cracking when the water is lost to hydration. This source of shrinkage strains is a form of autogenous shrinkage. This form of cracking is probably not a common problem in pavements since water-cement ratios are commonly above 0.40, but may be a substantial cause of cracking in bridge decks. Water-added methods are sometimes advised in this application.

One potential problem with water-added curing of low water-cement ratio concrete is that after a relatively short curing time, the capillary pores may be sufficiently disconnected that no appreciable amount of water from wet curing penetrates the concrete. Powers, Copeland, and Mann (1959)⁽¹¹⁾ report that capillary continuity of 0.40-water-cement ratio paste is lost at 3 days. For purposes of achieving adequate strength, it may not be very important that additional water get into low water-cement ratio concrete. Experience reported by the Iowa Department of Transportation showed that the concrete mixtures commonly used in paving achieve adequate strength even if all of the cement does not hydrate. However, for durability considerations, it may be important that additional water gets into the near-surface zone of the concrete. No research on the effectiveness of water-added methods on the near-surface properties of low water-cement ratio concretes was found.

With today's pavement technology, it is usually true that the most practical and economical method of curing large amounts of portland cement concrete pavements is by using water-retention methods, generally in the form of curing compounds. Water-cement ratios of paving mixtures are usually close to or greater than 0.40, so there is no important theoretical reason to recommend water-added methods, unless the economics are favorable.

Temperature management of large paving operations is difficult and mostly relies on selecting appropriate climatic conditions. The commonly used approach is to specify air temperature conditions considered suitable for paving. Allowable placing temperatures typically range from a minimum of about 5 degrees Celsius (°C) (with temperatures rising) to a maximum of about 30 °C. These limits on temperatures during placing are to minimize the possibility of damage due to early freezing events in the case of low temperatures or due to development of plastic shrinkage cracks in the case of high temperatures.

For the duration of the final curing period, protection from freezing is required and a minimum average concrete temperature of about 10 °C is typically required to insure strength development. If concrete temperatures fall below this, extended curing time is often required. No guidance was found that referenced a maximum curing temperature during the final curing period, although guidance does require use of reflective materials under hot weather conditions, as described in ACI 305R⁽¹²⁾. Not covered in standard guidance is the problem of thermal stresses that can develop within the first few days in pavements due to the combined effects of heat of hydration and environmental heating and cooling. This is the principal subject of new FHWA software called HIPERPAV^®(13).

Temperature management of small areas of pavement in cold weather can be practically accomplished by using insulation. ACI 306R⁽¹⁴⁾ gives extensive guidance on insulation required to maintain this temperature. A wide variety of guidance was found in State DOT requirements for protecting concrete from freezing. The major need here is to put this information into a format that is easily accessible to the user.

Selecting Curing Materials

Initial Curing Period. The major activity during the initial curing period is to control evaporation of bleed water. Bleed water can generally be lost without major detriment, but there is a fine line between losing only bleed water and loss of pore water, which causes shrinkage. For use in the initial curing period, options are limited to two materials: water (in the form of mist) and evaporation reducers. Water for application to prevent excessive loss of bleed water generally faces few serious specification compliance issues and may be a reasonable option when evaporation rates are such that one or two passes by the application equipment are sufficient to protect the concrete. At an application rate of 0.2 kilograms per meters squared (kg/m²) and an evaporation rate of 1 kilogram per meter squared per hour (kg/m²/h), water would need to be applied every 12 minutes (min) to avoid loss of mixing water. Evaporation reducers are a very practical option for extending this period between required applications. However, no published information or standard guidance was found on these products. The only information found was in the form of manufacturer's product literature. These products are potentially quite valuable for helping protect concrete pavements from excessive water under conditions of high evaporation rates. Their use fits easily into the construction practices already in common use and require little in the way of additional equipment. A major impediment to their use is the absence of test methods, specifications, and detailed guidance.

Final Curing Period. For final curing, several material options are available within each method of curing. Water-retentive methods include waterproof sheeting and curing compounds. Water-added methods include water, wet burlap, wet soil, and wet cotton mats. Waterproof sheeting, water, burlap, and curing compounds are covered by standard specifications. The specifications for water, waterproof sheeting, and burlap are relatively simple and compliance issues relatively easy to deal with. Compliance issues associated with the use of curing compounds are much more difficult, but, except for small areas, the economics of paving usually dictate using curing compounds as the material for final curing.

Curing water is limited only by staining materials or dissolved salts (American Society of Testing and Materials (ASTM)) ⁽¹⁵⁾, USACE CRD-C 400⁽¹⁶⁾). Sheet materials are specified by ASTM C 171⁽¹⁷⁾, which limits water loss (ASTM C 156⁽¹⁸⁾), physical properties, and reflectance. The USACE, in CWGS 03300⁽¹⁹⁾ limits use of impervious sheeting to plastic coated burlap. Simple plastic sheeting is not allowed because of the mottled appearance it can cause on the surface of the concrete. Burlap is specified by American Association of State Highway and Transportation Officials (AASHTO) M 182.⁽²⁰⁾ACI 325.9R further requires that the fabric be water absorptive.⁽²¹⁾ No other selection criteria among materials were found. Curing compounds are specified by AASHTO M 148⁽²²⁾ (ASTM C 309⁽²³⁾) and ASTM C 1315.⁽²⁴⁾

Curing compound selection, acceptance testing, verification of application and effectiveness probably present some of the greatest challenges in curing practice. The critical performance property limited by specifications is moisture loss, determined by ASTM C 156⁽¹⁸⁾ or a similar method. State DOTs commonly use their own variation of ASTM C 156⁽¹⁸⁾ for acceptance testing of curing compound, and some have more stringent moisture loss requirements than those in ASTM C 309.⁽²³⁾ For example, Minnesota DOT (Mn/DOT) allows a maximum of 0.15 kg/m² at 24 hours (h) and 0.40 kg/m² at 72 h. Virginia allows a maximum of 0.116 kg/m² at 24 h and 0.232 kg/m² at 72 h. Curing compounds are manufactured along several formats (waxes, resins, water-based, solvent-based), but there seems to be little guidance on selecting among these. Discriminating guidance that does exist is mostly based on practical problems. For example, the USACE (CWGS 03300⁽¹⁹⁾) directs use of styrene acrylate or chlorinated rubber curing compounds when paint or bituminous or waterproofing is to be applied to the cured surface later. USACE TM 5‑822-7⁽²⁵⁾ directs that wax-based products are to be avoided if the concrete surface is to be painted. White pigmented curing compounds are required if use is in direct sunlight. ACI 305R⁽¹²⁾ recommends that the moisture loss requirement for curing compounds be tightened in hot weather. A limit of 0.39 kg/m² is recommended.

ASTM C 1315⁽²⁴⁾ has more stringent requirements on curing compounds than does C 309.⁽²³⁾ Moisture loss is limited to 0.40 kg/m² (at a lower application rate than allowed in C 309⁽²³⁾), and a minimum solids content of 25 percent is required. The purpose of the solids content requirement is to insure a membrane thickness of at least 25 microns (mm). This standard has not become common practice in specifying materials for use in pavement construction, although ACI 308R⁽⁶⁾, a relatively new committee report, does recognize this specification.

Recent developments in environmental regulations governing volatile organic compounds (VOCs) are expected to have substantial effects on choice of curing compounds. Manufacturers' literature has already started to show a classification of VOC-compliant materials. VOC-compliant materials are largely water-based, which means their rate of drying could be affected by very humid conditions. Application of water-based compounds when relative humidity is near 100 percent would interfere with formation of the membrane, which is dependent on evaporation of the water from the curing compound. The application might then be susceptible to washing away or damage by rainfall for many hours after application. Essentially no information was found on practical matters associated with use of low-VOC curing compounds.

There is some variation among curing compounds from different manufacturers in the chemical composition and principle of action. Except for the guidance that cautions against some products if the concrete is to be painted or when surface adhesion is important, there is not much information available on the relative strengths and weaknesses of the competing technologies. The differences may not be important in the performance of the materials, but it would be useful to have some information gathered on the logic of the competing materials.

The current status of acceptance testing of curing compounds is, at best, inadequate. The between-laboratory precision in ASTM C 156⁽¹⁸⁾ is so poor that buyer/seller agreement or disagreement on the acceptability of a product is strongly affected by testing error. Either this test method needs substantial improvement or a more precise test method needs to be developed.

Improving this test method would be the preferable solution, given the degree to which it is embedded in current practice.

As published in the precision and bias statement of the test method, the between-laboratory standard deviation is 0.30 kg/m². This equates to a coefficient of variation of 54.5 percent, at a water loss rate of 0.55 kg/m² at 72 h (the specification limit of ASTM C 309⁽²³⁾). Two laboratories might reasonably be expected to differ in comparing single test results by as much as 0.84 kg/m² in 95 percent of 2-laboratory comparisons. (This statistic is called the D2S limit, as described in ASTM C 670⁽²⁶⁾). This level of uncertainty makes it almost impossible for a manufacturer to be assured that a material meeting the moisture loss requirement by a seemingly comfortable margin will be found to comply consistently with specification requirements by a user's testing laboratory. The small difference between ASTM C 309⁽²³⁾ and C 1315⁽²⁴⁾ requirements for moisture loss (0.55 kg/m² versus 0.40 kg/m²) would be very difficult to verify with this test method.

The within-laboratory precision of the method is considerably better than the between-laboratory precision, and other methods have been presented that claim to have considerably better within-laboratory precision than C 156⁽¹⁸⁾. ASTM C 1151⁽²⁷⁾(now withdrawn), BS 7542⁽²⁸⁾, and a method published by Dhir, Levitt, and Wang (1989)⁽²⁹⁾ are examples. But, it is the between-laboratory precision that controls acceptance of a material and ultimately has the most effect on it use. The practice of some State DOTs noted above, of setting more restrictive specifications on this property, may be partially attributable to the precision issue.

It is difficult to know how much of the between-laboratory precision problem is due to poorly described details of the test method and how much is lack of operator attention to the details of the test method. A formal ruggedness analysis of this procedure (ASTM C 1067⁽³⁰⁾) would probably contribute significantly to an understanding of the sources of variation and their relative contribution to the current precision. The major variables either are known or can reasonably be anticipated. These include temperature, relative humidity, wind velocity, time of application of curing compound, and surface finish of cement specimen. A between-laboratory coefficient of variation of about 10 percent is probably required for the method to be effective in distinguishing materials.

Accounting for Climatic Conditions

Climatic conditions, particularly when at extremes, can have significant impacts on moisture balance, rates of hydration of cementitious materials, damage due to freezing and thawing, and can contribute to damage due to excessive thermal gradients. These effects constitute the principal need for deliberate curing activity.

Initial Curing Period. Concrete is particularly sensitive to the effects of excessive losses of mixing water, although it is not usually necessary to conserve all mixing water. Mixing water that appears as bleed water on the surface of concrete is generally considered to be in excess of water needed for sufficient hydration and for protection from shrinkage effects. However, as a

practical measure, it is best not to push too close to the limit of the bleed water and to accidentally lose critical pore water.

The rate of evaporation of bleed water is commonly estimated using the nomograph in ACI 308⁽³¹⁾ (also 305R⁽¹²⁾, 308R⁽⁶⁾, and other sources). This nomograph provides a graphical calculator of evaporation from a free-water surface given inputs for concrete temperature, air temperature, relative humidity, and windspeed. ACI 308R addresses the issue of agreement between rates of evaporation predicted by the nomograph and rates actually measured and concludes that the nomograph is accurate to within approximately 25 percent for actual evaporation rates near 1.0 kg/m²/h or lower, but that it tends to over represent the evaporation rate at higher levels.⁽⁶⁾ The committee report also describes in detail the measurement of evaporation rates used to collect the data on which the nomograph is based. Wind velocity appears to be the most critical variable.

Losses of water to evaporation in excess of bleed water during the initial curing period are particularly critical. Under climatic conditions particularly favorable to drying, evaporation of bleed water can be quite rapid because a free-water surface is exposed to the environment. When evaporation exceeds bleeding, then the near-surface zone of the cement paste starts to dry, which results in shrinkage and development of tensile strains. Tensile strength at such early ages is very low, and the fresh concrete easily then develops plastic shrinkage cracks.

The single most critical activity during the initial curing period is in accurately anticipating this evaporation-to-bleeding water balance and taking adequate steps to shift it to a more favorable position. The current guidance suggests either setting limits on the length of time concrete can be left in an unprotected condition or setting upper limits on evaporation rates.

Some State highway departments set a maximum time that concrete can be left unprotected during the initial curing period of 30 min. This guidance assumes that evaporation of bleed water cannot reasonably exceed formation of bleed water in that time frame, at least to the point of being critical. No information was found that comments on this assumption.

Until recently, the major guidance on maximum tolerable evaporation rates was to be found in the caption to the ACI 308 nomograph.⁽³¹⁾ The values returned from the calculation do not apply to evaporation rates when a free-water surface is not present, although the values are sometimes used as an indicator of drying potential in such situations. The guidance in the caption directs that when evaporation rates exceed 0.5 kg/m²/h precautions are recommended to reduce evaporation rates, and when evaporation rates exceed 1.0 kg/m²/h, action to reduce evaporation rates should be required. USACE TM 5-822 directs that paving operations be avoided if evaporation rates exceed 1.0 kg/m²/h and perhaps even at 0.75 kg/m²/h if plastic-shrinkage cracks must be prevented.⁽²⁵⁾

Al-Fadhala and Hover (2001)⁽³²⁾ comment on the history of the ACI 308⁽³¹⁾ guidance. When these limits were established, typical bleeding rates for concrete were believed to usually fall between 0.5 and 1.5 kg/m²/h. It can be deduced that most bleeding rates exceeded 1.0 kg/m2/h since the prescriptive limit in ACI 308 for required action was set at this level. Recognizing that this range may not represent some modern concretes, the recently published ACI 308R⁽⁶⁾ contains the following guidance. For concretes that show little tendency to bleed, it may be necessary to set a requirement on evaporation rate below 1.0 kg/m²/h. Limits of 0.25 kg/m²/h for silica fume concrete and 0.50 to 0.75 kg/m²/h for other low-bleed concretes are recommended.

Air temperature is an important climatic variable during the initial curing period beyond its direct effect on evaporation rates. High air temperatures tend to result in higher fresh concrete temperatures unless deliberate steps are taken to cool concrete materials stockpiles. However, temperature of fresh concrete and wind are more important variable driving evaporation of bleed water than is air temperature. High temperatures of fresh concrete also accelerate hydration of cement, affecting working times and the length of the initial curing period.

Final Curing Period. Losses of water from concrete due to evaporation after the end of the initial curing period are much lower regardless of curing activities. After bleeding has stopped and any residual bleed water is lost, evaporation rates slow down considerably because the free-water surface exposed to the atmosphere is now below the surface of the concrete. The near-surface zone acts to protect the bulk of the concrete by reducing windspeed at the water surface and maintaining a relatively high humidity near the surface of the concrete. ACI 308R reports that actual evaporation rates from concrete that has no bleed water are 10 to 50 percent less than evaporation rates from a free-water surface.⁽⁶⁾ However, even though rates are lower, total water loss can accumulate over a period of several days to the point of being significant and affecting cement hydration, possibly causing damaging shrinkage. The purpose of curing in the final curing period is to conserve this water.

Limits on the amount of water that can be lost during the final curing period without detriment were determined early in the development of curing compounds. Information in the older literature seems to converge on a loss figure of about 0.6 kg/m² at 3 days as being a useful upper limit, although other work suggests that losses as high as 1.0 kg/m² can be tolerated. A loss of 0.6 kg/m² in 72 h equates to an average rate of water loss rate of 0.008 kg/m²/h during the time interval. This rate is two orders of magnitude lower than the limits on evaporation of bleed water allowed during the initial curing period.

There does not seem to be a simple way to determine what levels of evaporation, as estimated by the nomograph, would result in this kind of water loss potential when no free-water surface exists. The evaporation rate probably is not a constant, but plausibly varies considerably with concrete materials and mixture proportions and is a declining function with time as more of the mixing water becomes chemically combined during hydration. ACI 305R recommends using a high performance curing compound in hot weather conditions.⁽¹²⁾ This report does not precisely define hot weather conditions, but it might be reasonably deduced from other information in 305R to be conditions that equate to a drying condition of 1.0 kg/m²/h or more.⁽¹²⁾ ASTM C 156 (method for testing water loss through a curing compound) uses a set of test conditions that result in an evaporation rate in the range of 0.65 to 1.2 kg/m²/h.⁽¹⁸⁾ It can be deduced from this information that drying conditions considered dangerous to fresh concrete would also be dangerous to hardened concrete within the first few days after placing.

Establishing an exact limit on drying conditions that require attention for hardened concrete is not critical in most practices since the standard practice in construction for most concrete pavements is to execute curing actions, regardless of the anticipated weather conditions.

Air temperature is an important climatic variable during the final curing period beyond its direct effect on evaporation rates. High temperatures affect strength development and can contribute to peak concrete temperatures, particularly in the first 24 h after placing. High peak temperatures can lead to a thermal stress problem. Low air temperatures are primarily a matter of concern because of the reduced rate of strength gain and because of the potential for damage due to freezing.

Upper limits on air temperature are common in standard guidance. For example, the AASHTO Guide Specification⁽³³⁾ recommends against concreting at temperatures higher than 30 °C. Other agencies have lower limits. For example, Arizona requirements limit placing to temperatures of 29 °C or lower. A bridge deck replacement in Nebraska was limited to 27 °C maximum temperatures (Beacham, 1999).⁽³⁴⁾ ACI 305R does not give any maximum allowable placing temperatures.⁽¹²⁾ Army TM 5-822⁽²⁵⁾ directs that the precautions in ACI 305R⁽¹²⁾ should be invoked if temperatures exceed 32 °C, but also advises that concreting should be avoided above this temperature, if possible. This limit is based on concerns for high evaporation rates.

Regarding temperature gradients, ACI 305R directs that curing water must be within 10 °C of concrete temperature.⁽¹²⁾ The USACE Guide Specification for Structural Concretelimits the temperature difference between a point 50 mm into the concrete and the ambient temperature to be less than 13 °C.⁽³⁵⁾

Three reports in the literature identified temperature rise in summer placements as potentially important even when ambient conditions did not involve high temperatures (Okamoto and Whiting, 1994⁽³⁶⁾; Healy and Lawrie, 1998⁽³⁷⁾; Mohsen, 1999⁽³⁸⁾). Portland cement hydration characteristically has a major heat of hydration peak that starts at about the same time as time of setting and continues to be a significant source of heat during the first 24 h after placing. The peak rate of heating normally occurs 8-10 h after placing, but exact timing and duration are cement specific and are strongly affected by temperature and admixtures. This heat can accumulate in thick pavements and, together with heating from solar radiation, can result in a significant temperature rise, which can further accelerate the hydration of the cement, contributing more heat, and thus setting up a self-stoking cycle of temperature rise. Peak temperatures of 60 °C can develop. Determining the early age heat of hydration patterns may be very useful in specifying a construction schedule that would avoid this synergistic effect of autogenous and climatic heating, thus reducing the peak temperature that a pavement experiences.

ACI 306R⁽⁽¹⁴⁾ covers the slow strength-gain effect of low temperatures and gives guidance on managing the problem, as discussed above in the section on "Selecting Curing Methods." The maturity method (ASTM C 1074⁽³⁹⁾) is an effective way to anticipate the rate of strength gain in low temperature conditions.

The guidance on avoiding freezing is inconsistent. The AASHTO Guide Specification⁽³³⁾ directs that concrete be protected from freezing for 10 days or until compressive strength reaches 15 megapascal (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⁾ Avoidance of freezing conditions and use of insulation are the two major features of standard guidance. There is no mention of using antifreezing admixtures. The AASHTO Guide Specification recommends avoiding concreting at ambient conditions less than 10 °C and directs that concrete be protected with blankets if ambient temperatures are expected to fall below 2 °C.⁽³³⁾ ACI 306R gives guidance on how to use insulation to maintain a minimum temperature of 10 °C in concrete for ambient temperatures below 10 °C.⁽¹⁴⁾ Moisture retention is often not a serious issue because of the low evaporation rates associated with cold weather, unless heated enclosures are used, in which case moisture retention practices for warmer conditions are applied.

Both the British standard (BS 8110: Part 1⁽⁴⁰⁾) and the European standards (CEB-FIP⁽⁴¹⁾ and EN 206⁽⁴²⁾) account for climatic conditions during curing, but neither use evaporation rate. Both use relatively nonquantitative measures, such as amount of direct sun (none, medium, and strong) and wind (low, medium, and high), but there is quantitative guidance on relative humidity (RH), with RH less than 50 percent being severe, RH between 50 and 80 percent being moderate, and RH greater than 80 percent being mild.

Start of Curing

Typical guidance states that curing should start after concrete is placed, finishing is complete, and the surface sheen has disappeared. In conventional concrete construction, finishing is typically not completed until sometime near time of initial setting. In pavement construction, finishing is essentially completed when the paving machine passes, although there may be some touch-up work executed within a few minutes. Surface sheen may disappear soon after that, given the tendency of slip-form mixes to bleed very little.

Using the concepts of initial and final curing period, as described in ACI 308R⁽⁽⁶⁾, the start and duration of these periods is quite different for slip-form paving and for more conventional concrete placements. In the case of conventional concrete, the initial curing period would start immediately after placing, then would end, and the final curing period would start at approximately the time of initial setting. This would normally be 2-4 h, although it could extend for several more h if admixtures cause retardation. In the case of slip-form paving, the initial curing period would start immediately after the paving machine passes, and the transition point to the final curing period is unclear.

In the case of the slip-form paving, the concrete probably would not have reached time of initial setting when final curing procedures are applied. If water added and/or sheet materials were used for final curing, a problem might develop because the concrete is not strong enough to resist washing out or marring. Since these techniques are not commonly used in large slip-form applications, the problem really does not present itself. Curing compounds can be applied at almost any time without either washing out cement paste or causing mechanical damage. However, the question of curing compound performance is unresolved. Do curing compounds applied before time of setting perform as well as when applied after time of setting?

ACI 308 contains a short section cautioning application of curing compounds before bleeding has stopped.⁽³¹⁾ Interviews with project engineers indicate that the concrete used in slip-form paving does not bleed, or if it does, it is so slight as to be insignificant. If this is true, then it appears that curing compounds could be applied very early without detriment. This detail needs to be verified.

Curing Duration

Requirements on duration of curing are mostly based either directly or indirectly on strength-gain rate, which is determined by temperature, properties of cementitious materials, and concrete mixture proportions. This is simplified in most U.S. guidance (ACI, FHWA, and DOT) to specifying curing duration as a required fixed-time interval, with some adjustment for rate of expected strength gain of the concrete (e.g., high early strength versus normal strength). There is some inconsistency in the prescribed curing times, even among the various ACI guidance. As a secondary position, many standards allow curing duration requirements to be based on time required for strength to reach some minimum fraction of the specified strength (f_c'), as measured by c specimens.

The AASHTO Guide Specification⁽⁽³³⁾ requires 3-day curing for pavements and 7 days for structures, including bridge decks. The latter is increased to 10 days if more than 10 percent pozzolan is used. There is a provision on structures that curing may be terminated if 70 percent of f_c¢ is reached. This is the only standard guidance that requires extra curing for concrete containing a slowly reacting cementitious material, such as pozzolan.

A limited survey of State DOTs indicates that curing duration largely follows the AASHTO guidance.

ACI guidance typically sets a fixed time period provided concrete temperatures are maintained at greater than or equal to 10 °C. Sometimes there is the caveat that earlier termination can be allowed if testing shows a prescribed percentage of f_c' (typically 70 percent) at an earlier age.

The guidance in the ACI Building Code (ACI 318⁽³⁾) of 3 days for high-early strength concrete and 7 days for normal strength concrete at temperatures greater than or equal to 10 °C is an example of the simplest form of this kind of guidance. High-early strength and normal strength are not defined. The Standard Specification for Structural Concrete (ACI 301⁽⁴³⁾) prescribes 7 days curing (3 days for high-early strength concrete) with the provision that curing can be terminated if 70 percent f_c¢ is reached in field-cured specimens; or if temperatures are greater than or equal to 10 °C (50 °F ) for the length of time required to reach 85 percent of f_c¢ in laboratory-cured specimens; or if strength reaches f_c¢, as determined by nondestructive methods, which are not specified. The Standard Practice for Curing Concrete (ACI 308⁽³¹⁾) bases duration of curing on the types of cement (ASTM C 150⁽⁴⁴⁾) used: 3 days for Type III, 7 days for Type I, and 14 days for Type II. The most recent ACI guidance, Standard Specification for Curing Concrete (ACI 308.1-98⁽⁴⁵⁾), gives a default duration of 7 days, but allows for curing to be terminated early if strength reaches 70 percent of f_c¢ or if desired levels of durability are reached early. There is no additional guidance on the latter criterion. ACI 325.9R⁽²¹⁾ on concrete pavements requires 7 days at temperatures above 4 °C or 70 percent f_c¢. ACI 330R on concrete parking lots requires 7 days at temperatures above 15 °C or 21 MPa compressive strength.⁽⁴⁶⁾ ACI 306R accounts for the longer curing times needed in cold weather with a sliding scale, depending on the extent of cold.⁽¹⁴⁾

The ACI 308 guidance that is based on cement type is obsolete in the case of Type II cement.⁽³¹⁾ When ACI 308 was first written, most Type II cements on the market were slow to hydrate compared to Type I cement, because they are formulated to meet moderate heat of hydration cements.⁽³¹⁾ Type II cement is now almost undistinguishable from Type I cement in its rate of hydration, except in the rare instance when the cement is manufactured to meet the optional heat of hydration requirement in ASTM C 150⁽⁴⁴⁾ (Poole, 1998).⁽⁴⁷⁾

The British Standards Institution (BSI) and the European Committee for Standardization (CEN) guidance is very prescriptive in nature and considerably more complicated than the guidance in U.S. standards. BSI 8110, Part 1, bases curing duration on specific lengths of time, but these times are determined as a function of the cement's rate of strength gain, climatic conditions after casting, and surface temperature of concrete during curing (details are reproduced in Appendix B).⁽⁴⁰⁾ A maturity function is used to account for concrete temperature effects. EN 206 accounts for strength gain of concrete, water-cement ratio, ambient temperature, and climatic conditions to give variable curing times, but maturity does not appear to be used.⁽⁴²⁾

Australian guidance (AS 3600⁽⁴⁸⁾) is based strictly on durability considerations. Climatic zones and environmental aggressiveness within the zones are the major variables. Curing duration ranges from 3 days for the mildest exposures to 7 days for the most aggressive, with some adjustments made for accelerated curing.

Very little of the U.S. guidance accounts for the slower rates of hydration of concretes containing cementitious materials other than portland cement, particularly in paving concrete. Although it is well documented that pozzolans and slag, two of the most commonly used supplemental cementing materials, can cause strength gain to be significantly retarded, the phenomenon is so variable that a prescriptive specification would need to be extremely conservative to account for all of the possibilities.

Perhaps a better approach would be to rely totally on performance testing. Once a concrete has reached 70 percent of its required strength, then deliberate curing efforts could cease. Maturity (ASTM C 1074⁽³⁹⁾), temperature-matched curing, and field-cured specimen methods are sufficiently well understood to serve as reasonably practical ways of determining when to cease curing operations. If, as a matter of practicality, a user wanted to develop and follow a prescriptive time-based specification, then laboratory testing could be used to determine conservative time-base limits for the job concrete.

Some concern is occasionally expressed over use of strength as an indicator of development of other physical properties critical to paving, such as abrasion resistance and water permeability. Strength, however, is a good measure of degree of hydration of the cement, and represents development of other physical properties reasonably well. So, using strength as an indicator criterion is a reasonable practice.

Procedures for Verification of Curing

Initial Curing Period. Procedures for verifying application of curing procedures during the initial curing period are not well developed. Several State departments of transportation direct that if cracking starts to appear during this period, then measures to reduce evaporation or to compensate the concrete for water lost to evaporation be implemented. If possible, it would be preferable to identify or develop a method that would detect deficiencies before damage starts to develop.

A plausible approach would be to monitor the surface sheen until the end of the initial curing period. Loss of sheen during this period is an indication that evaporation is exceeding bleeding, potentially conditioning plastic shrinkage cracking. Observation of laboratory specimens indicates that in the absence of surface evaporation, a surface sheen is present until approximate time of initial setting even if there is essentially no detectable bleeding.

It would be a very useful feature of this guide if it could give the user a way to anticipate that this condition is possible under expected project conditions. Construction crews could then be prepared with equipment and materials in case critical conditions develop.

Final Curing Period. Verifying water-added and methods that use waterproof sheets is mostly a matter of inspecting at prescribed intervals for evidence of dry spots or areas uncovered by sheet material. Verification procedures for the curing compound application are not well developed.

The basic approach to verifying effectiveness of curing is prescriptive. All of the standard guidance prescribes curing compound application rate, either as the manufacturer's recommended rate or according to a prescriptive value. USACE guidance has additional prescriptive items. TM 5-822-7 prescribes equipment for applying curing compound for pavements.⁽²⁵⁾ It directs that equipment must be power driven, straddle the newly paved lane, and give uniform coverage. Nozzles must be surrounded by hoods to prevent wind blowing curing compound. There is no guidance on how to ensure that uniform coverage is actually achieved. Guide Specification CWGS 03300 directs that curing compound must be applied in a two-coat operation at a minimum pressure of 500 kilopascals (kPa).⁽¹⁹⁾ If it rains within 3 h of application, then the application must be repeated. Application rate of curing compound is verified by documenting the amount of curing compound used and estimating the covered area.

Another concept in use by at least one State for estimating the amount of application is to relate flow rate through nozzles (by direct measurement) to pressure, then to monitor pressure and rate of travel of the application equipment. From this information, a calculation of the rate of application is possible.

Another common practice is direct visual inspection when pigmented curing compounds are used. This is based on the concept that an uneven appearance of color or whiteness will indicate problems in curing compound application. No information was found on the sensitivity of visual inspections to variation in application rates. Reflectance measurements may also be a plausible way to estimate coverage of white pigmented curing compounds. Portable instruments developed for measuring reflectance of paint are available and may have application for evaluating pavements.

ACI 308R cautions that estimating application rates on highly textured surfaces from using calculation methods can be misleading because the actual area of a highly textured surface can be grossly underestimated by a calculation based on simple dimensions.⁽⁶⁾ No information was found either in standard guidance or in the literature on direct verification of application of curing compounds.

In theory, a pavement on which curing compound has been improperly applied or on which it is not functioning properly should be cooler than a well-covered pavement, due to evaporative cooling. Infrared imaging of the surface could be effective here. This approach could give results quickly for quality control purposes.

Verification of Effectiveness of Curing

There are a number of test methods for verifying that expected curing levels have occurred, including measuring strength, both directly and or by use of nondestructive methods, and by measuring surface or near-surface physical properties. Strength methods are covered in ACI 228⁽⁴⁹⁾ and include in place curing of test cylinders, ultrasonic pulse velocity, rebound hammer, pull out, and penetration resistance. These are all covered by ASTM standard test methods. Tests of near-surface properties that develop with curing are covered in Kropp and Hilsdorf (1995)⁽⁵⁰⁾ and include methods that measure permeability, water absorption, relative humidity, abrasion resistance, and hardness.

Standard guidance still heavily relies on strength or strength correlates. The current trend in the literature of the last 10 years is to rely on the surface or near-surface properties, but this has not appeared much in practice. Permeability and water absorption methods are generally favored. A number of these claim to be useful for field-testing, but most are not simple. One recurring problem with near-surface test methods applied to field concrete is the effect of concrete's moisture condition on the determination. Most of these methods work best when test specimens are prepared and analyzed in the laboratory because the methods include a drying period to remove this effect. However, some of the field methods give moisture correction procedures. The simplest test found was based on a relative humidity button containing moisture sensitive dye, allowing visual verification that the surface of the concrete was wet (Carrier and Cady, 1970).⁽⁵¹⁾ To our knowledge, this device has not been widely used (Senbetta, 1994)⁽⁵²⁾, but there was no information on whether this was because of technical problems or a general disregard for this kind of verification.

Some of the methods described in ACI 228 potentially could be adapted for measuring changes in near-surface properties as an indicator of curing.⁽⁴⁹⁾ The rebound hammer appears to have promise here. This method has been commonly criticized as a measure of in place strength because of the very large effect of surface features on readings. One of these problems, near-surface aggregate particles, can be averaged out statistically, so that the method should have potential for measuring quality of near-surface curing, particularly if calibrated properly with a section of field concrete known to be well cured.

Accounting for Concrete Materials and Mixture Proportions

Four properties of a concrete mixtures and concrete materials that affect curing practice are addressed in standard guidance: (1) type of cement, (2) presence of pozzolan or slag, (3) water-cement ratio and, (4) rate of strength gain. The variables are not independent. In the main, these variables affect the duration of curing, and have been addressed in that section of this report. The presence of some pozzolans, notably silica fume, and water-cement ratios less than 0.40 requires water-added curing, which was discussed in the section "Selecting Curing Methods." One of the most critical features of a mixture design with respect to the way it affects curing practice may be the amount of bleeding that occurs.

Type of cement. ASTM C 150 Type I, Type II, and Type V cement is used for paving.⁽⁴⁴⁾ Many cements meet both Type I and II requirements and are called Type I/II. This is not an ASTM recognized category. Types II and V are usually cited for their sulfate resisting properties. Type III is sometimes used for small areas when rapid early strength development is required. Type IV cement is largely unavailable and not of interest for paving. Equivalent cements defined under ASTM C 1157 are Type GU (equivalent to Type I), Type MS (equivalent to the moderate sulfate resisting feature of Type II), and Type HS (equivalent to Type V).⁽⁵³⁾ At least one State (Florida) imposed a heat of hydration requirement on Type II cement used in summer paving of 80 calories per gram (cal/g) at 7 days.

As discussed above, ACI 308 makes a significant distinction between Type I and Type II cement with respect the length of curing required (7 versus 14 days, respectively).⁽³¹⁾ There was once a significant basis for this distinction, but strength-gain rates in modern Types I and II cement are very similar. Type V cement typically does gain strength somewhat more slowly than Types I and II, but little distinction is made in curing guidance.

Pozzolans. Pozzolans typically have two effects on portland cement concrete that pertain to curing. One is that time of setting is sometimes retarded as much as several hours. This mostly affects the timing of application of curing procedures. The other property is that strength gain is slower than with pure portland cement concrete. This is particularly acute with Class F fly ash. Some research has recommended 3-7 days extra curing to get desired strength properties when using this type of pozzolan as a replacement for portland cement (See Appendix B). In some practices pozzolan is simply added to the cementitious content, requiring adjustments to the other ingredients of the concrete. This practice effectively results in an increase in the total cementitious materials. Additional curing is probably not required to reach target strengths when this practice is employed.

Fineness of cementitious materials. Very finely divided cements and pozzolans are noted for contributing to reduced bleed and increased susceptibility to plastic shrinkage cracking. Typical values of fineness for cements are 350 to 400 m²/kg (squared meters per kilogram) (ASTM C 204), and for pozzolans are 10-30 percent retained on a 45 micrometer (µm) sieve.⁽⁵⁴⁾ Values much finer than these may contribute significantly to these problems. Type III portland cements typically have a fineness of 500 m²/kg, or more. Silica fume is about an order of magnitude higher still. Slag tends to be a little finer than Types I or II cement, particularly Grade 120, which may reach a fineness of 500 m²/Mg (squared meters per megagram) (ASTM C 204).⁽⁵⁴⁾

Retarders. As with pozzolans, excessive retardation of time of setting may impact susceptibility to plastic shrinkage cracking by extending the initial curing period. This should not be a problem if evaporative losses are managed properly, but any retardation increases the length of time during which this must happen.

Water-Cement Ratio. Concretes with water-cement ratios less than about 0.45 tend to be more susceptible to plastic shrinkage cracking because of the reduced amount of bleed water that is formed. On the positive side, the more rapid strength gain of low water-cement ratio concretes shortens the time of setting and the total curing time required.

Cement Content. Concretes containing high levels of cementitious materials tend to show lower bleeding rates (although water-cement ratio is the major variable here). High cement contents are also associated with higher ultimate drying shrinkage. Long-term drying shrinkage is related to the amount of hydrated cement paste in a concrete. Reducing cement contents, if compatible with development of other properties is sometimes recommended as a way to reduce ultimate shrinkage.

Amount of pozzolan. As noted above, portland-pozzolan mixtures hydrate slower than pure portland cement, and setting times are often extended. These effects appear approximately in proportion to the amount of pozzolan. Pozzolan is commonly limited to about 20 percent by mass of total cementitious materials in paving concrete because of these effects.