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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-02-099
Date: January 2005
The final curing period is defined as the time interval between application of curing procedures and the end of deliberate curing. Final curing methods can be classified into three types: curing-compound methods, water-added methods, and water-retentive methods. For most paving applications, selection among these methods is largely a matter of economics of materials and labor. Technical issues enter into the decision process for some special-purpose concretes. Figure 14 summarizes major considerations.
Figure 14. Chart. Major items requiring attention during construction-final curing period.
Curing compounds are normally the most economical method for curing large areas of paving because of the relatively low labor costs. Once the application is satisfactorily completed, little or no additional attention is required. The negative side to using curing compound methods is the relatively complicated selection and specification-compliance issues that are frequently encountered, and the skill required to apply the material correctly. Figure 15 summarizes the major issues associated with use of curing compounds.
Figure 15. Chart. Major features in curing compound practice.
Specifications for curing compounds are covered by AASHTO M 148,(20) which is equivalent to ASTM C 309,(21) and by ASTM C 1315.(22) The guidance in ASTM C 1315(22) contains more stringent requirements on water retention than ASTM C 309(21) and a minimum requirement on solids content. Both of these documents are pertinent to paving applications, but the specification also contains several requirements that usually do not pertain to paving applications. These include compatibility with adhesives (flooring) and surface sealing properties.
Selection criteria for curing compounds for paving applications include:
Tendency to run (viscosity) is another important criteria for tined (textured) pavements, but not part of most standard criteria. Each of these criteria is discussed below.
Water retention is the major performance property of curing compounds. It is tested according to AASHTO T 155(23) or ASTM C 156(24) (or some variant of this test), which measures water loss after a fixed period (usually 72 hours) of exposure to a standard drying condition. Most of the moisture loss occurs in the first 24 hours, and at least one State Department of Transportation (DOT)-Caltrans-uses that test age with a modified specification limit. The environmental conditions used in ASTM C 156 represent an evaporation rate, as determined by direct measurement, of between 0.65 and 1.1 kg/m2/h.(24) The between-laboratory precision of this method is poor, and causes considerable contention between buyer and seller.
The standard water-retention limit (AASHTO M 148,(20) ASTM C 309(21)) is a moisture loss of 0.55 kg/m2 (max). Some State DOTs require lower values, down as low as about 0.25 kg/m2. ACI 305 R (on hot weather) recommends reducing the limit to 0.39 kg/m2 for hot weather concreting.(5) Hot weather concreting is vaguely defined in this document, but can reasonably be interpreted to mean conditions in which evaporation rates exceed 0.50 kg/m2/h, as determined by the evaporation rate nomograph in ACI 308 and 305 R.(4,5)
The research behind the 0.55 kg/m2 limit dates to the 1930s and 1940s. It was determined that this level of moisture loss led to strength development in concrete (of stripped and coated cylinders) at least as high as with moist curing. Recent research is limited, but suggests this limit to be adequate for most applications.
For a material to conform to standards for volatile organic compounds (VOCs), it can contain no more than 350 grams per liter (g/L) of volatile solvents. Many products list the VOC content on the package or on the materials safety data sheet. New federal regulations require use of low VOC materials in certain applications, particularly in enclosed areas. This regulation is also commonly applied to pavement construction even though the enclosed-area concept does not usually apply. Most manufacturers market a wide variety of low VOC products. The major problem with low VOC materials is that they contain a relatively large amount of water, which can make the product very slow to dry under conditions of low evaporation rates (see below).
Drying time is an important property because a curing compound is susceptible to washing off if it is rained on before it has dried. Wet curing compound also limits walking on the pavement. The standard drying time in AASHTO M 148(20) and ASTM C 150(7) is 4 hours under prescribed laboratory drying conditions. The laboratory conditions represent an evaporation rate of approximately 0.43 kg/m2/hr. While this rate is not exactly specified, it is the value from the conditions described in the test method. The test method is rather approximate and could be the cause of some compliance disputes; some State DOTs require shorter drying times. As mentioned in the previous paragraph, low VOC curing compounds may be slow to dry. The following empirical equation has been found to be useful in anticipating the amount of drying time needed under poor drying conditions, such as evaporation rates of less than 0.10 kg/m2/h. The equation given in figure 16, below, does not really apply well to estimating drying times less than about 4 hours.
Figure 16. Equation. Drying time for curing compound-temperature correction.
In figure 16, ER is the evaporation rate (units of kg/m2/h) as estimated using the ACI 308 nomograph.(4) According to this equation, evaporation rates less than about 0.1 kg/m2/h may result in prolonged drying times.
White-pigmented curing compound is most often used in paving. The white pigment reflects sunlight and helps with temperature control in hot weather. In addition to these functional properties, the pigment is a very strong indicator of the amount and uniformity of the application. Other pigments, typically pink or yellow-green, are used as fugitive pigments (they bleach out within a few days) and are designed principally for architectural applications where the more persistent white pigments is objectionable. These colored pigments are not as good for estimating application properties as white pigment. Methods exist for estimating application rates from reflectance of white pigment.
The solid fraction of the curing compound contains ingredients that form the curing membrane. The white pigment is also part of the solids, but is not active in membrane formation. It is termed "vehicle solids." Active solids are classified as either wax or resin. Both are competent materials. Waxes are sometimes not favored for paving for practical reasons; waxes are less resistant to marring than resins, and some are believed to clog nozzles easily. Resins come in a wide variety of chemical types, and often are not specifically identified by the manufacturer.
Total solids content is an approximate indicator of water retention properties, within a given type of chemistry. Total solids of approximately 12-15 percent are usually required to meet the 0.55 kg/m2/h limit stated in AASHTO M 148(20) and ASTM C 309.(21) Total solids content of about 25 percent may be required to meet the more demanding requirement of ASTM C 1315.(22)
Some curing compounds act as bond breakers. If paint or adhesives are intended to be applied to the finished surface, then either the curing compound must be removed or a compound compatible with coatings selected. The manufacturer's literature clearly indicates this special property of a curing compounds. Absence of information on compatibility with coatings is an indication that this property does not exist with a particular product.
On grooved or textured pavements, some curing compounds are of such low viscosity that they tend to run down into the bottom of the grooves, leaving a deficiency on the vertical surfaces. A limited laboratory study found wide variation among products in the maximum application rate that could be tolerated without running. Some products tended to run at applications as light as 10 m2/L, while others could be applied at 5.0 m2/L without running, which is the application rate typically recommended by most manufacturers. At least one State DOT has a test and specification requirement for this property.(25) Application of curing compound in two light coats will help avoid this problem, if it exists. There are curing compounds manufactured specifically for application to vertical surfaces, but the viscosity is so high that they may not work with the type of application equipment normally used in highway paving.
Curing compounds perform best if applied after time of initial setting. Typical guidance on paving is to apply the curing compound when the surface sheen has disappeared. Taken literally, this practice can lead to poor performance. Paving concretes tend to be made with a relatively low w/c ratio, so under even relatively mild drying conditions, the surface sheen may disappear soon after placing even though bleeding is continuing. Application of the curing compound then slows or stops the evaporation of bleed water, which then either accumulates under the membrane or dilutes the curing compound. In either case, the membrane is likely to be damaged and suffer reduced performance during the final curing period. This damage is sometimes visible as cracks or tears in the membrane. In other cases the damage can only be seen with moderate magnification. If drying conditions are mild (e.g., <0.5 kg/m2/h), this result may have no detrimental effect.
There have been occasions in which curing compound applied under these conditions will apparently bond to the surface layer of mortar, which will then delaminate as the bleed water works its way to the surface.(4) The damage develops as thin spalls (a few millimeters thick) of surface mortar early in the pavement history, but not necessarily immediately. This is apparently not a common phenomenon.
It is relatively common practice to apply curing compound very soon after placing. When this happens, the curing compound may act as a relatively good evaporation reducer. The potential difficulty with this practice is that the curing compound will not retain water during the final curing period to the level of performance expected from the job specification on the material for the reasons cited above. However, it may be reasonable practice to apply part of the curing compound early, for purposes of controlling evaporative losses during the initial curing period, then applying the remainder after time of initial setting to restore the integrity of the membrane from any damage suffered from the early application. If this practice is anticipated, it should be verified with laboratory testing. This verification can reasonably be done at the same time curing compound compliance testing is being done. Rather than waiting for the sheen to disappear from the test specimen, curing compound can be applied within a few minutes after forming, the water loss measured, and the physical integrity of the membrane and mortar surface verified.
AASHTO guidance recommends a coverage rate of no more than 5 m2/L. This rate is also common in the manufacturers' guidance. As discussed above, many curing compounds cannot be applied at this rate in a single pass without serious running into low areas. Grooving patterns increase the effective surface area. This effect must be accounted for to maintain the target application rate. The equation given in figure 17, below, can be used to calculate the application rate necessary to compensate for the increased area.
Figure 17. Equation. Application rate for curing compound-correction for texturing.
AR = the adjusted application rate (m2/L)
ARungrooved = the specified application rate for an ungrooved pavement
S = the space between grooves
W = the width of the grooves
D = the depth of the grooves
Curing compound is best applied to textured pavements in two perpendicularly applied coats.
After-the-fact verification of application rates is sometimes accomplished by documenting the volume of curing compound used and the area of pavement covered. Another way is to directly verify the flow rate through nozzles by measuring the volume delivered over a known time interval; by monitoring the movement rate of the application equipment, an application rate can be determined. Both of these methods are competent to determine average application rates, but neither is useful for verifying uniformity of application.
No method exists for verifying uniformity of nonpigmented compounds, but two approaches are potentially useful for white-pigmented compounds. The visual check is the easiest to use. The inspector looks for areas of less-than-white appearance. Whitepigmented curing compound meeting AASHTO M 148(20) and ASTM C 309,(21) and applied at a rate of 5 m2/L, has a very white appearance. Any hint of gray is an indication of serious under-application. Variations on the order of 2 m2/L are detectable by this method. This method is most effective soon after the curing compound has been applied. Concrete tends to lighten in shade considerably as it dries, so an underapplication may be difficult to perceive after about 24 hours.
Portable reflectometers are manufactured for assessing coverage of paint. These devices are effective in measuring reflectance of field applications of white-pigmented curing compounds.(26) This instrument is more sensitive than the human eye to variations in whiteness (hence application rate). A limited evaluation indicates that this equipment can detect variations on the order of 1 m2/L if a calibration is prepared using job concrete with a range of known application rates to form a standard curve.
Water-added methods include ponding, fogging, and wet burlap or other wetted absorbent materials. Major criteria are summarized in figure 18.
Figure 18. Chart. Major features of curing with added water.
Water-added methods are not practical for curing pavements, but may be practical for patching and slab replacements. Water-added methods are commonly believed to be the only effective curing method for avoiding cracking due to internal desiccation in very low w/c ratio concrete (which becomes critical at w/c ratio levels starting at about 0.40 and less). Consequently this method is often used for bridge decks. However, it is not clear whether water effectively penetrates very deeply into low w/c ratio concretes. Concrete made with type K expansive cement, which has been uncommonly available in recent years, performs better with water-added curing.
Requirements on curing water are relatively simple. ASTM C 94 contains requirements on mixing and curing water.(27) Major limits are on chlorides and sulfates. Where staining is a concern, there are often limits on iron. Requirements on burlap are more complicated. Burlap is covered by AASHTO M 182.(28) Burlap is made in a variety of weights and thread counts, which are typically the basis for job specifications. Some job specifications also limit use of previously used burlap. The basis for this is unclear.
Fogging can be applied at any time after placing as long as it is not so heavy that runoff develops. Flowing water cannot be tolerated before time of initial setting because of the danger of washing out cement fines. Absorbent materials/ should usually not be applied before time of initial setting because of the danger of physically damaging the surface.
Sprinklers and soaker hoses are effective methods for keeping a horizontal surface wet. Water-absorbent materials can be covered with plastic to eliminate evaporation and reduce the amount of water needed for effective curing. Burlap can be purchased with an impervious layer on one side.
Extreme drying conditions may cause a problem when using some water-added curing methods. High rates of evaporation of the curing water create a strong cooling effect and can result in a sufficiently strong temperature gradient that cracking occurs. The U.S. Army Corps of Engineers limits the temperature gradient in the outer 50 mm of a concrete structure to no more than 13 °C. Laboratory work has shown that a wet concrete exposed top evaporation rates greater than 1.4 kg/m2/h can develop a cooling gradient over a 50 mm depth larger than 13 °C. Covering wet absorbent materials with a layer of plastic sheeting will prevent significant evaporative cooling. The HIPERPAV software program contains features that can help with an analysis of this condition.
Verification of curing is a matter of visual inspections. Typical guidance is that all concrete be inspected at least once per day or more often if conditions warrant, and if dry concrete is found, the situation is corrected and an additional day added to the curing requirement.
Methods involving impervious sheeting are simple and relatively free of specification compliance issues. Major criteria are summarized in figure 19. This method is probably impractical for large areas of paving and/or windy conditions, but may be very practical when used for smaller areas. Some agencies (e.g., U.S. Army Corps of Engineers) do not allow use of plastic sheeting directly against the surface of the concrete because of the mottled pattern that sometimes develops.
Figure 19. Chart. Major features of curing with water-retention methods.
Specification compliance issues are simplest with this type of curing. AASHTO M 171(30) and ASTM C 171(29) both describe the specification covering plastic sheeting, and contain relatively simple requirements. A major feature of specifications on application has to do with overlap of sheets. Typically a 50-mm overlap is required. The sheet material must cover exposed edges of the concrete, be sealed so that moisture does not escape, and be weighted down so that wind will not lift the sheet off of the pavement, either by getting under the edges or by aerodynamic lifting.
Simple polyethylene sheets meeting AASHTO M 171 specifications work well for water retention, but use of this type of material may result in a mottled pattern on the concrete surface.(30) For this reason, some agencies do not allow curing with plastic sheets. Plastic sheet-burlap laminates are also manufactured that would avoid this problem.
White-pigmented sheeting is available, which helps with temperature control by reflecting sunlight.
Sheet materials should be applied after time of initial setting. Applying sheeting prior to initial time of setting is likely to cause marring of the surface.
As in water-added methods, daily visual inspection is the normal verification method. If drying areas are found, the problem is fixed and an additional day added to the curing time.
The effects of temperature variations in concrete on events that occur during or at the end of the initial curing period have been described above. This section addresses problems associated with the volume changes in hardened concrete that occur within the first few days of placing.
Figure 20. Chart. Thermal effects.
Temperature changes cause volume changes in concrete. A typical coefficient of thermal expansion for concrete is 10-5/°C (i.e. in/in/°C, cm/cm/°C etc) ± 2x10-5/°C. The coefficient of thermal expansion is dominated by the type of aggregate. Carbonate aggregates have a lower coefficient of expansion than do silicate aggregates. The volume changes caused by temperature changes are not a problem per se, unless the concrete is not free to expand or contract due to some physical limitation. For considerations of curing practice, it is the early heating, then cooling, along with environmentally induced warming and cooling cycles and accompanying volume change of the concrete that is the immediate problem.
Concrete generates heat internally starting soon after placement due to the hydration of the cementitious materials. The most intense heating from this source occurs in the first 24 hours, reaching a peak approximately 6-8 hours after placing, depending on the environment and the chemistry of the cement. In thin pavements, this heat is usually dissipated to the environment about as fast as it develops, and does not contribute significantly to the overall heating of the pavement. In thick highway pavements, some of the heat can accumulate.
Concrete can also warm if the air temperature is higher than the placement temperature and if there is significant solar radiation. Cool atmospheric conditions and evaporation of water from the surface of the concrete act against the warming. The typical pattern in warm-weather placements is for the concrete to warm up at least a little. However, if the heat of hydration of the cement, peak air temperature, and peak solar radiation occur at the same time, then temperatures as high as about 60 °C (140 °F) can be reached if measures are not taken to prevent this.
Problems do not usually occur during this heating phase, but as the concrete develops strength and becomes somewhat brittle, the shrinkage associated with cooling can cause cracking. An exception to this occurs if warming and cooling cycles of sufficient intensity occur early in the history of the pavement. Under such conditions, the rapid warming of the surface of the concrete can cause a warping reaction that can result in cracking. A temperature change of about 15 °C (27 °F) in concrete that is restrained from shrinking is sufficient to cause cracking.
The restraint that prevents free shrinkage of the pavement may be the friction with the road base, or be deep-lying concrete may experience slower cooling because it is not in contact with cool air.
The purpose of joint cutting is basically to anticipate these cracks and to cause them to form in a controlled location. The art in joint cutting is determining when the concrete is mature enough to withstand the sawing operation without damage, but not to wait too long so that significant cooling shrinkage occurs, causing cracks to form in uncontrolled locations.
Because of the number of variables involved in the final curing process, anticipating problems is a very complicated process. FHWA has developed a calculation procedure that captures all of these effects and anticipates these kinds of problems. This procedure is captured in a software product called HIPERPAV. This software accepts inputs on the concrete properties, pavement properties, and environmental conditions, then calculates temperature changes and gradients. The output is a time-dependent graph that displays development of compression and tension in the pavement, and anticipates points at which cracking is likely to develop.
In the absence of this computational tool, the pavement engineer's best option is to try to minimize the maximum temperature difference between the average ambient temperature and the peak temperature of the pavement. This method will also limit the size of the temperature gradients during cooling. In searching standard guidance, no limits were found that address this feature, although a maximum temperature difference of about 20 °C is commonly applied to large-section bridge elements.
Another approach commonly used in construction of mass concrete structures is to cool the concrete to a temperature below the average ambient temperature, so that the temperature increase caused by the heat of hydration and environmental heating will result in a smaller ambient-peak value (i.e., the concrete will have less cooling to do, and hence the gradients should be smaller).
Fresh concrete is cooled most effectively by keeping the aggregates cool by sprinkling the stockpile. Aggregates normally comprise 75-80 percent of the mass of concrete, so their temperature dominates the temperature of the fresh concrete. However, cooling stockpiles may be impractical in some paving operations. The fallback method is to use ice as partial replacement for mixing water, or to use liquid nitrogen injections.