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|Publication Number: FHWA-HRT-05-038 Date: August 2006|
Publication Number: FHWA-HRT-05-038
Date: August 2006
The American Concrete Institute (ACI) Manual of Concrete Practice(91) contains the major general purpose concrete standards and state-of-the-art committee reports on concrete in the United States. Several of these standards and reports address curing. The highest level standard, in a regulatory context, is the Building Code Requirements for Structural Concrete (ACI 318‑95(3)). Curing is one topic in this general standard for concrete. The Standard Specification for Structural Concrete (ACI 301-96(43)) is also a general concrete standard, but it does not have the regulatory authority of the Building Code. This standard is used to develop contract agreements on execution of concrete construction. Recommendations in ACI 301(43) should not exceed the guidance in ACI 318-95(3).
ACI standards specific to curing are ACI 308-92, Standard Practice for Curing Concrete(31), and ACI 308.1-98(45), Standard Specification for Curing Concrete. As with ACI 301(43), ACI 308.1(45) is used to develop contract agreements, the requirements of which should also fall within the limits of the ACI 318-95.(3) A new standard, Guide for Curing Concrete, has been written by ACI Committee 308.(92) The eventual result is probably that ACI 308R(6) and ACI 308.1(45) together will replace ACI 308.(31) Other official documents that contain guidance on curing are ACI 305R(12) on Hot Weather Concreting, ACI 306R(14) on Cold Weather Concreting, ACI 325.9R(21) on Construction of Concrete Pavements and Concrete Bases, ACI 330R(45) on Design and Construction of Concrete Parking Lots, ACI 330(93) on Specification for Concrete Parking Lots, and ACI 228.1R(49) on In-Place Methods to Estimate Concrete Strength.
Generally, ACI standards and committee reports focus primarily on prescribed curing times or on some fraction of specified strength as quantitative criteria. Durability criteria allowances are discussed but have not been developed. There is little consideration for the specific materials or proportions of materials used in the concrete mixture, although some ACI standards consider portland cement type and rate of strength gain. There is some mention of accounting for water-cement ratio, but this is not well developed. Considerable attention is given to details of different curing methods. Temperature is mostly considered in the context of maximum and minimum allowable concrete temperatures, protection from freezing, and thermal shocks. Time-temperature considerations are dealt with in the context of cold weather and in use of the maturity method when it is allowed for predicting the time to end curing. The following paragraphs discuss each standard in some detail.
Meeks and Carino (1999) review the history of the Building Code guidance on curing.(4) Guidance in the 1995 version is relatively simple. It directs that concrete, other than high-early strength concrete, be maintained above 10 °C and kept moist for at least 7 days. Curing for high-early strength concrete should be maintained for 3 days with temperatures also above 10 °C. Temperature-accelerated curing is allowed, but the details must be developed by the user and durability must not be worse than when the time-based prescriptive requirements are used. For details, reference is made to ACI 308(31), ACI 306R(14) (cold weather concreting) and ACI 305R (hot weather concreting).(12)
Curing is addressed in section 5.3.6 of the body of the specification. There is no mention of curing in the Mandatory Requirements Checklist. The general requirement is to cure for 7 days after placement (3 days for high early-strength concrete), but moisture retention measures may be terminated when: (1) field cured cylinders reach 70 percent of required strength (fc¢); or (2) the temperature is 10 °C or higher for the time required to achieve 85 percent of fc¢ in laboratory cured cylinders; or (3) the concrete's strength reaches fc¢ as determined by accepted nondestructive methods. A reference is given for the accepted nondestructive methods (paragraph 188.8.131.52). These methods include cast-in-place cylinders (ASTM C 873(94)), penetration resistance (ASTM C 803(95)), pullout strength (ASTM C 900(96)), and maturity (ASTM C 1074(39)).
Methods allowed for moisture preservation include:
A proposed revision of ACI 301(43) was published in the July 1999 issue of Concrete International (v. 21 n. 7). The new guidance allows that curing compounds be specified by ASTM C 1315(24), as an alternative to C 309(23) and that concretes containing silica fume be cured with added-water methods.
Additional information on the use of curing compounds is given in ACI 301‑96.(43) Curing compound should be applied according to the manufacturer's instructions after the water sheen has disappeared from the concrete surface and finishing operations are complete. The rate of application should not exceed 5 m2/L. For rough surfaces, two coats are required, applied at right angles, at an application rate not to exceed 5 m2/L. ACI 301‑96(43) also cautions that curing compounds act as bond breakers.
Section 184.108.40.206 sets minimum concrete temperatures, varying from 10 °C to 13 °C, depending on the cross-sectional area. Maximum allowable concrete temperature is 32 °C.
Chapter 1 of ACI 308(31) discusses general curing needs and presents the well known nomograph relating evaporation from a free-water surface to temperature, wind velocity, and relative humidity. A value of evaporation of 1.0 kg/m2/h or more requires measures to prevent excessive moisture loss from the surface of the concrete. An evaporation rate greater than 0.5 kg/m2/h may also require some measures to control evaporation. Chapter 1 also defines the temperature limits for placing concrete. The practical lower limit is 10 °C, although hydration has been shown to continue down to -10 °C. The recommended upper limit is approximately the maximum temperature anticipated during service, although temperatures over 100 °C are sometimes used for accelerated-curing purposes.
Chapter 2 describes various curing methods, materials, and evaluation procedures. Materials that allow extra water to be applied to the concrete (aside from the mixing water) via ponding, spraying, and continuously wetting include mats, earth, sand, sawdust, straw, and hay. Materials that simply retain mixing water include plastic sheets, treated paper, and liquid membrane-forming curing compound. ASTM C 156(18) is cited (section 2.8) as the general method to use to evaluate curing materials, although the most recent revision of this test method includes only liquid membrane-forming curing compounds. This method is discussed in detail below.
Section 2.3.3 describes curing compound practices. Application rates of 0.20 to 0.25 L/m2, usually expressed as 4.0-5.0 m2/L are recommended. There are recommendations on application practices and a caution about the damage to concrete surfaces that can result if curing compound is applied too early, while bleeding is still occurring, even though the bleeding is imperceptible due to the high evaporation rates.
Section 2.9 gives criteria for effectiveness of curing. The basic criterion is prescriptive. The document states that, if surface moisture and temperature are maintained within "desired" levels, then adequate curing will result. Procedures are described for measuring strength development and for using maturity to assess the degree of curing. Neither of these is sensitive to the degree of curing of surface or near-surface concrete, which can be essential in determining some durability properties.
Section 2.10 recommends the length of curing to be 3 days for concretes containing Type III cement and 7 days for concrete containing Type I cement. Length of curing for concretes containing Type II cement is recommended to be 14 days. There is no recommendation for concretes containing Type IV cements (which are either nonexistent or extremely rare) or Type V cement or for concretes containing pozzolans or slag.
Chapter 3 deals with specific types of construction. Section 3.1 is specific to pavements and slabs. These types of structures are particularly sensitive to curing because of the high surface-volume ratios. They are also sensitive to thermal stresses caused by environmentally related temperature cycling. Under extreme evaporation conditions (unspecified), curing may be required even before the concrete has set. At temperatures greater than 5 °C, curing should be maintained for 7 days or until concrete reaches 70 percent of its specified compressive or flexural strength.
This standard defines two new terms: initial curing and a final curing. Initial curing is defined as "deliberate action taken between placement and final finishing of concrete to reduce the loss of moisture from the surface of concrete." Final curing is defined as "deliberate action taken between the final finishing and termination of curing."
The initial curing period is normally neglected unless hot weather or water-cement ratios lower than 0.40 exist. The latter condition is only mentioned in a "Note to the Architect/Engineer" in the "Mandatory Specification Checklist." There is no other mention of mixture proportions. Fogging (section 5) and entrapment of bleed water under a uniform film of evaporation retardant are the only methods allowed for curing during the initial curing period.
Length of curing is either a predetermined period of time or determined by a level of strength or durability (paragraphs 1.1.6 and 1.6.4). The default curing time is 7 days if the temperature is greater 10 °C. Curing may be terminated early if strength is at least 70 percent of fc¢ or if desired levels of durability are reached. There is no additional information on the latter criterion, except in the "Notes to the Architect/Engineer," which references ACI 308(31), ACI 201.2R, and ASTM STP 169C (Senbetta (1994)).(52) Durability criteria must be determined by the designer. Additionally, any of the following moisture-retention techniques are allowed: sheet materials, pigmented curing compounds, addition of water, water-absorbent materials.
Section 2 discusses moisture retention using sheet materials (ASTM C 171).(17) Inspection directions require that the concrete surface be verified visually as being continuously wet. If dry spots are found, then additional water must be added. It is noted in the "Optional Specification Checklist" that the Architect/Engineer may specify the frequency of inspections. It also directs that dark sheets be used when temperatures are less than 15 °C and that white sheets be used when daily high temperatures are greater than 30 °C. The optional checklist also advises that when the temperature during the first day after placing exceeds 20 °C, then white or similarly reflective sheeting is preferable.
Section 3 on liquid membrane-forming curing compounds requires that materials comply with ASTM C 309 and be white or gray pigmented to reflect light.(23) It also requires use of power spray equipment unless the surface to be covered is less than 200 m2 or if overspray of curing compound to adjacent areas must be avoided, in which case hand pump spraying is allowed. Materials must comply with regulations on VOCs, although no information is given on these regulations. The optional checklist allows non-VOC compliant materials if they are allowed by regulations of the using agency.
For floors with high wear resistance requirements, optimum top-surface strength, and minimal crack widths, the optional checklist allows that a curing compound may be specified with lower moisture loss requirements than in ASTM C 309.(23) A maximum moisture loss of 0.3 kg/m2 at a coverage rate of 7.4 m2/L is recommended. Heavier application rates or multiple application rates may also be specified as optional requirements.
Section 4 on ponding requires that the temperature of curing water be no more than 10 °C cooler than the surface of the concrete. Water must meet the requirements of ASTM C 94.(14)
Section 5 on fog spraying requires that the water meet the same requirements as for ponding. During initial curing, water must be directed so that the fog drifts down onto the concrete to prevent damage to the surface. No water is allowed to accumulate until after the time of setting. During final curing, the requirement is to keep the concrete continuously wet.
Section 6 on sprinkling requires that the water meets the same requirements as for ponding. Sprinkling is allowed only for final curing to avoid erosion of the concrete surface, unless the concrete surface is protected by a form.
Section 7 on water-absorbent materials requires that water meet the same requirements as for ponding. There is a caution on staining of materials, and burlap must meet AASHTO M 182.(20) This technique must only be used for final curing and materials must be kept continuously wet.
This standard is essentially a major revision of ACI 308(31) and contains quite a bit of pertinent information. The guidance on recommended requirements is not radically different than provided in other ACI standards. The basic requirements are already published in ACI 308.1(45) described above, but the guide contains considerable supplementary information that is not currently in any of the other ACI guidance.
The guide presents a good discussion of important considerations in the first minutes and hours after placing concrete, before final finishing and application of final curing. Duration of curing and properties of the surface-affected zone are also considered in ways different from most other guidance. The guide contains a section specific to pavements and slabs and a chapter on monitoring curing and curing effectiveness.
This standard is an advisory document intended as supplemental material to the contract specification. Much of it deals with practices useful in avoiding freezing concrete at early ages. Section 6 deals with the issue of slow strength development at low temperatures. An example of the use of the maturity method is given. A tabulation (table 18, below) is given for minimum length of curing at two temperatures (10 and 21 °C) needed to obtain various percentages of 28-day strength. Allowance is made for cement type (I, II, and III).
Section 7 is about insulation. Extensive tabular data are presented by which the user can select the insulating factor (R) of insulation needed to maintain concrete temperature at 10 °C for either 3 or 7 days. The tables account for thickness of the structural member, number of sides exposed to air, and ambient temperature. This procedure is based on retention of the heat of hydration of the cementitious materials in the concrete. There is no adjustment for the variable heat of hydration exhibited by different cementitious materials.
Section 8 addresses curing requirements and methods. Heated enclosures tend to create drying conditions, so attention must be paid to moisture retention. Water curing is not recommended because of the practical problems associated with freezing of leaking water.
|Percentage of standard-cured 28-d strength||At 10 °C, d||At 21 °C, d|
|Type of Cement||Type of Cement|
*This number is in error. It was 4 d in the previous edition.
This standard is also an advisory document and is intended as supplemental material to the contract specification. It defines hot weather concreting as any combination of temperature, wind velocity, relative humidity, and solar radiation that impairs the quality of fresh or hardened concrete because of effects on moisture loss or rate of cement hydration. Quantitative guidance is given on methods to calculate expected temperature of concrete from the temperature of ingredients and on techniques to reduce the temperature of fresh concrete.
Section 4 addresses the details of curing in hot weather. It references ACI 308(31) for general guidance, but it also cautions in section 4.4 that concrete should not be allowed to get too hot since ultimate strength is negatively impacted by high initial curing temperatures. It also cautions against too rapid cooling in young concrete, which may result in formation of thermal shrinkage cracks. Curing water should be near the concrete temperature, or at least the cooling rate should be no more than 3 °C /h or no more than 28 °C in 24 h.
White pigmented curing compound is recommended to reflect light and reduce solar heating. It is recommended that liquid membrane-forming curing compounds exceed the ASTM C 309(23) requirements, which indicate that some agencies require a limit on water loss, as determined by ASTM C 156(18), of 0.39 kg/m2. The requirement in ASTM C 309(23) is 0.55 kg/m2.
If the concrete water-cement ratio is less than 0.40, then the concrete should be water cured rather than using a water-retentive membrane. The standard cautions that pavements 30.5- to 45.7-cm thick can experience significant thermal heating due to the cement's heat of hydration. Temperature rises of 6.7 °C per 60 kg/m3 of cement can be expected in 18 to 72 h if no heat is lost.
Chapter 2, "Materials," Section 2.5 addresses curing materials. Guidance is offered on burlap, waterproof paper, and liquid membrane-forming curing compounds. Burlap is required to comply with AASHTO M 182(20), with further provisions that nonabsorptive materials and unit weights of less than 0.24 kg/m2 not be used. M 182(20) allows a minimum unit weight of 0.23 kg/m2. ASTM C 171(17) is referenced for waterproof paper and ASTM C 309(23) is referenced for liquid membrane-forming curing compounds, with Type 2, white-pigmented, being preferred.
Chapter 10 on curing and protection refers to practices in ACI 308, with additional guidance that surface water must be gone, plus the surface of the concrete must not be sensitive to marring.(31) Curing duration is 7 days if temperatures are above 4 °C or strength is greater than or equal to 70 percent of fc¢. Section 10.1.1 on curing compound practice directs application rates follow manufacturer's recommendation or be no less than 3.7 m2/L in the absence of manufacturer's recommendation. Section 10.1.2 cautions not to use mono-molecular coatings (evaporation retardants) for curing. Details for use of cotton mats, waterproof paper, and white- pigmented polyethylene sheets are given. Section 10.1.6 provides guidance on addressing curing problems associated with saw cuts in pavements, including the use of ropes or twisted fabric inserted into the cut or placing strips of adhesive polyethylene over the cut.
Chapter 4 on construction recommends ASTM C 309(23) for curing compounds with a minimum application rate of 5 m2/L or manufacturer's recommendations. Opening to traffic is recommended to be no less than 3 days for automobile traffic and no less than 7 days for all other traffic if temperatures are above 16 °C. More time (unspecified) is required if temperatures are cooler. A compressive strength of 21 MPa or greater can substitute for the time requirement.
This standard presents the guidance in ACI 330R(46) in a format suitable for writing project specifications.
This standard describes a number of in-place test methods for measuring strength that might be of practical value in evaluating curing. Methods include rebound number (ASTM C 805(80)), penetration resistance (ASTM C 803(95)), pullout test (ASTM C 900(96), break-off test (ASTM C 1150),(97) ultrasonic pulse velocity (ASTM C 597(98), maturity method (ASTM C 1074(39)), and cast-in-place cylinders (ASTM C 873(94)). The standard also describes principles of the test methods and some practical details on limitations.
ASTM standards cover material specifications and test methods. These are the standards primarily referenced in the ACI standards. ASTM's curing standards are described below. Other standards are useful for measuring or estimating development of properties that depend on curing. These are summarized in ACI 228.1R-95, as described above.(49)
This is the original and principal standard referenced for liquid membrane-forming curing compound. The standard classifies curing compounds into Types 1 and 2, and Classes A and B. Type 1 is clear or translucent (Type 1-D may contain a fugitive dye). Type 2 is white pigmented. Older versions of the specification allowed a gray-pigmented curing compound. The class defines the type of solids in the curing compound. Class A is unrestricted. Class B must be a resin.
The standard includes qualitative requirements, called "General Requirement," on sprayability, surface adhesion to concrete, shelf life, toxicity, and deleterious reactions with concrete. There are quantitative requirements on water-retention properties, reflectance properties (for Type 2), and drying times. The water-retention requirement is the principal performance requirement pertaining to the curing function. The limit is 0.55 kg/m2 at 72 h when tested according to ASTM C 156 at an application rate of 5 m2/L or according to the manufacturer's recommendation.(18)
This standard covers some of the same scope as ASTM C 309(23), but requires higher performance in moisture retention properties at a lighter application rate and some additional properties that are independent of moisture retention that are not required in C 309(23). Like C 309(23), materials are classified as Types and Classes. Type I is clear or translucent. Type II is white pigmented. There is no provision for fugitive dyes. Class A is nonyellowing. Class B is moderately yellowing, and Class C may undergo severe darkening.
As in C 309, there are qualitative requirements on applicability, adhesion, deleterious reactions with concrete, toxicity, and shelf life.(23) Minimum solids content is 25 percent. This requirement is intended to ensure a minimum membrane thickness of 0.025 mm. Maximum moisture loss, tested according to C 156, is 0.40 kg/m2 in 72 h.(18) Unless otherwise specified, the application rate for acceptance testing is 7.5 m2/L for Type I materials and 5 m2/L for Type II materials. There are also quantitative requirements for reflectance, drying time, yellowing in ultraviolet light, acid and alkali resistance, and adhesion of tile cement placed over the curing compound.
This test method measures the moisture retention properties of curing compounds and sheet materials by measuring mass loss at 72 h of a standard mortar specimen coated with the material under test. Test conditions are 37.8 °C, 32 percent RH, and airflow sufficient to cause a water evaporation rate of 2.0 to 3.4 g/h from a free-water surface. The coverage rate for curing compounds is 5 m2/L or according to manufacturer's requirements. Application method is according to manufacturer's guidance.
This test method, or slight variants of it, is the most commonly used method for evaluating curing compounds for water retention. The test method is often criticized for lack of precision. The published within-laboratory standard deviation is 0.13 kg/m2. A laboratory would be expected to duplicate test results to within a range of 0.36 kg/m2. The between-laboratory standard deviation is 0.30 kg/m2. Two laboratories would be expected to duplicate test results to within a range of 0.84 kg/m2. Work is underway to improve the precision of the test method.
This test method, like C 156, is intended to test curing compounds and sheet materials.(18) This test method is based on measurement of water absorption by a cured sample of mortar, comparing the absorption of the exterior surface with an interior surface exposed by cutting a specimen. The concept behind the method is that a poor quality curing compound will allow the surface to dry, resulting in a less compact microstructure, and hence more water absorption relative to the interior of the specimen, which is not sensitive to the quality of curing compound. Senbetta (1994) reviews the development of this test method. There is little data on its precision.(52) A within-laboratory standard deviation is reported, but this was the result of a single laboratory's effort. When expressed as a coefficient of variation, the precision is 18.6 percent, using the recommended limit of 3.7 by 10-6 cm2/s as the basis for the calculation. There is no information on the between-laboratory precision. This method is not apparently widely used for acceptance testing of curing materials, although water absorption test methods are widely recommended in the literature as a test to measure the quality of curing in the near-surface zone of concrete.
The scope of this specification is for sheet materials for use in moisture retention and in minimizing temperature rise due to solar radiation. For moisture retention, it cites the 0.55 kg/m2 water-loss requirement at 72 h used for curing compounds, determined according to C 156.(18) There are also requirements on physical properties that address durability consideration and a reflectance requirement for white sheet materials.
AASHTO has its own set of materials specifications and test methods. These include specifications M 182(20) for burlap, M 171-97(17) for sheet materials, and M 148-97(24) for curing compounds. The only test method pertinent to curing is T 155-91.(55)
M 171(17) is identical to ASTM C 171-95(99) and M 148(24) is identical to ASTM C 309-94(23), except that SI units are the preferred standard units of measure. Test method T 155(55) is identical to ASTM C 156-95(18). Specification M 182 for burlap is a uniquely AASHTO standard.(20)
AASHTO also publishes a "Guide Specification for Highway Construction,"(33) a "Quality Assurance Guide Specification,"(100) and a "Construction Manual for Highway Construction."(101) All of these contain guidance on curing.
The scope of this specification covers burlap for use in curing concrete. Four classes are defined according to the density (expressed in the non-SI units of ounces per square yards) of the material. There are quantitative requirements on the physical properties of the material (density, thread counts and on minimum widths and lengths of pieces) and qualitative requirements on defects. There are no performance requirements on burlap's suitability as a curing material.
Division 500 covers rigid pavements. Curing information is in Section 501.03 on Construction, paragraph M. The basic requirement is to cure the concrete for at least 3 days, starting immediately after the finishing operation. It requires avoiding exposing concrete for more than 30 min at any time during the curing operation. Fogging is required for curing until the final curing method is in place.
Placing concrete is limited to ambient temperatures between 10 and 30 °C. The concrete must be protected (presumably from freezing) for at least 10 days or until a flexural strength of 15 MPa is achieved (AASHTO T 97).(102) Protection with blankets is required if the temperature drops below 2 °C.
Four curing methods are allowed, as described below. There is no guidance for choosing among them:
Division 800, Section 808 is on concrete structures. Paragraph I deals with curing including guidance on bridge decks.
General guidance is to start curing immediately after free water has left the surface of the concrete and finishing is complete. Curing is to continue for 7 uninterrupted days, 10 days when more than 10 percent (by mass) pozzolan is used. Curing time can be reduced if field-cured test cylinders indicate that 70 percent of specified strength has been achieved. In hot weather, water is applied to surfaces that have been previously treated with curing compound or that are covered with forms until cooling is no longer needed.
Five curing methods are allowed, but there is no guidance on choosing among them:
There is special guidance on curing bridge decks. Bridge decks are to be cured with a combination of curing compound and water. Type 2 (white pigmented) curing compound is required, with application starting immediately after finishing is complete. Water curing is applied not more than 4 h after finishing. There is no additional guidance on length of cure other than the 7 to 10 days given in the general guidance, above.
This standard was published 1996, but has not been balloted by AASHTO, so it is not an official
AASHTO guide or standard. Section QA-501 addresses portland cement concrete pavement. Curing is not specifically addressed in the body of the document. Compressive strength, thickness, and ride are the three major items addressed in this document.
Appendix A is a "Guide for Quality Control and Acceptance Requirements for Portland Cement Concrete." It states that the contractor must have a quality control plan that describes tests and test frequencies. It also states that the plan must address all elements that affect the quality of structural concrete, listing "Finishing and Curing" as one of them. Otherwise, there is no guidance on curing issues.
This book is intended to be a guide to State highway departments in writing their own construction manuals. Section 501.03 deals with construction requirements. Paragraph O addresses curing, but only directs the reader to project specifications for guidance. Section 501.06 deals with inspection and instructs the inspector to check curing application and to keep a diary, but there is no specific guidance. Section 707 deals with sampling and testing frequency for materials. There is no mention of curing.
USACE guidance is contained in the Corps of Engineers Guide Specifications (CEGS), technical manuals (TM), engineer manuals (EM), and test methods and specifications. Test method and specifications are normally ASTM standards, however, there are some standards that are unique to the USACE. These have an alphanumeric prefix of "CRD-C." In the relatively recent past, guide specifications were distinct for military and civil construction, having designations of CEGS and Civil Works Guide Specification (CWGS), respectively. There is currently an effort to consolidate these into a single set of standards (all designated CSGS), but this is not yet complete. The two guide specifications described below have yet been consolidated and their titles suggest duplication. Guidance is largely the same, with the noted exceptions.
Section 2.4 on curing materials, gives specifications for curing compounds and for burlap. Curing compound must meet ASTM C 309, Type 1-D (fugitive dye) or Type 2 (white pigmented).(23) For surfaces that are to be painted, or are to receive bituminous roofing, or waterproofing, or floors that are to receive adhesive applications of resilient flooring, only styrene acrylate or chlorinated rubber compounds meeting Class B requirements can be used. Type 1‑D curing compound shall have the reflective requirements of C 309 waived.(23) AASHTO M 182 for burlap and cotton mat is cited.(20) Curing water shall be fresh, clean, potable, and free of injurious amounts of oil, acid, salt, or alkali, except that nonpotable water may be used if it meets the requirements of USACE specification CRD-C 400.(16) This specification calls for tests on strength development and staining.
Paragraph 3.15 describes required practice. A general caution is given that concrete shall be protected from premature drying, extremes in temperature, rapid temperature change, mechanical damage or effects of flowing water throughout the curing period. The curing period is 3 days for concrete containing Type III cement (ASTM C 150) and 7 days for other concrete.(44) Temperature of air and forms in contact with concrete must be above 10 °C for the first 3 days and above 0 °C for the remainder of the curing period. If the temperature is less than 0 °C, then measures must be taken to ensure that the concrete is kept at 5 °C or above for the 7-day period. On removal of protection, temperature at 50 mm inside concrete shall not differ from ambient by more than 13 °C.
Apart from the exceptions listed in section 2.4 on compatibility with coatings (described above) moist curing shall be used on areas that are to receive hardeners, paint, or other applied coating. Concrete containing silica fume shall be cured by fog misting during finishing, followed immediately by continuous moist curing.
For moist curing, wooden forms are to be kept continuously wet. In hot weather, nonsupporting steel forms shall be loosened and the formed surface kept wet. Burlap and mats shall be completely saturated before application to the concrete surface. When ponding is used, temperature of the water shall no be more than 10 °C cooler than the concrete.
On slabs, curing compound shall be applied as soon as bleed water disappears, with tops of joints sealed to prevent curing compound from seeping in and moisture being lost. Curing compound shall be applied in a two-coat continuous operation at a minimum pressure of 500 kPa with an application rate of not more than 10 m2/L for each coat, with the second coat applied perpendicular to the first. If it rains within 3 h, the entire application must be redone. Surfaces on which nonpigmented curing compound are used shall be shaded from direct sun for the first 3 days.
Except for plastic coated burlap, impervious sheeting alone shall not be used for curing. Such impervious sheeting shall only be used on horizontal or nearly horizontal surfaces. Additional guidance is given on details of use.
Paragraph 3.17.9 gives guidance on curing inspection. Moist curing inspection shall occur once per shift or not less than two times per day, including nonworkdays. Moisture condition is recorded. If an area is found to be dry, corrective action is taken and curing extended 1 day. When curing compound is used, the contractor verifies that it is properly mixed. At end of each operation, the contractor shall estimate the quantity of curing compound used and the surface area covered. When the calculated rate of application is below that specified (no tolerance given) or is not uniform, the entire surface must be sprayed again. Sheets are inspected once per shift and once per day on nonworkdays, noting conditions of sheets, laps, and joints. If deficiencies are found, they are repaired and curing extended by 1 day. Reports are completed in writing daily, and weekly summaries are prepared.
The major difference between CEGS 03300(19) and 03301(103) is in the guidance on duration of curing. CEGS 03301 gives considerably more detail with respect to differences among cementitious materials. The following is excerpted from that standard:
Type III portland cement......................................................................................3 days
Portland cement when accelerator is used to achieve high early strength, except when fly ash or ground granulated blast furnace slag (GGBFS) is used.............................................................................3 days
Type I portland cement........................................................................................7 days
Type IS or Type IP cement..................................................................................7 days
Portland cement blended with silica fume.............................................................14 days
Type II portland cement.......................................................................................14 days
Portland cement blended with 25 percent or less fly ash GGBFS............................14 days
Portland cement blended with more than 25 percent fly ash or GGBFS....................21 days
This guide specification covers airfield pavements as well as other heavy-duty pavements. It directs that pavement be cured for at least 7 days. Three methods are allowed: membrane curing (curing compounds), moist curing, and impervious sheet curing.
Guidance on membrane curing directs that curing compound be applied as soon as free water has disappeared from the surface after finishing. If evaporation is high and bleeding has not stopped, then fog spray shall be used to keep the surface wet until the concrete reaches time of setting and bleed has stopped. No instructions on how to determine these conditions and events are included.
Curing compound is required to meet the USACE specification CRD-C 300,(84) which is a more stringent specification that ASTM C 309(23) (see description below). Some manufacturers advertise a line of curing compounds that meet this requirement.
The concrete surface shall not be dry on application of curing compound and additional moistening may be necessary to avoid this condition. Curing compound must be applied with power equipment that spans the entire width of the pavement. Equipment must be fitted with mechanical agitation to prevent settling of solids in the storage tank. Equipment shall be well maintained. An approved quality assurance system is required to insure a continuously wet condition.
Application rate is 5.0 m2/L (±5 percent) in either one or two coats. The dried membrane shall be free of cracks or pin holes (noncompliance requires reapplication). If heavy rainfall occurs within 3 h of application, then the curing compound must be applied again. Instructions are included for dealing with hand spraying of irregular areas, for repairing damaged areas, and for spraying after joint cutting. Contingency curing capability is required to be on site in the event of failure of curing compound application equipment.
Moist curing must start immediately after finishing. Methods include ponding, sprinkling, wet mats or burlap, and wet plastic coated burlap.
Guidance on sheet curing focuses almost exclusively on sizes of sheets and overlap of sheets, and details of covering exposed edges.
As part of general guidance on placing, ACI 306R(14) is referenced for cold weather concreting (temperatures below 4 °C) and ACI 305R(12) is referenced for hot weather concreting (temperatures above 32 °C). Calcium chloride is allowed as an accelerator in cold weather. In hot weather, it is recommended to avoid placing when evaporation rates are in excess of 1.0 kg/m2/h as determined from the evaporation rate nomograph in ACI 308(31). It is further suggested that an evaporation rate of 0.75 kg/m2/h would be a safer limit to avoid plastic shrinkage cracking. Concrete should not be placed when concrete temperature exceeds 32 °C or when the air temperature is less than 4 °C.
In the section on curing, it is cautioned that all equipment and supplies must be on hand before concreting begins to insure no delays in application of moisture retention and temperature control measures. In general, pigmented curing compound is required. Equipment for applying curing compound must be power driven and constructed so as to straddle the newly paved lane and to give uniform and complete coverage. Nozzles must be surrounded by a hood to prevent wind from blowing the spray. Hand-operated pressure sprayers are allowed on odd widths and shapes, and on edges exposed by form removal. Curing compound must form a continuous void-free membrane and be maintained throughout the curing period. No guidance is given on how to determine this condition. No guidance is given on the length of the curing period.
Where experience has shown that curing compound alone does not prevent shrinkage cracking, then a combination of curing compound and moist curing may be specified.
Low melting point wax-based curing compounds should be avoided if the concrete surface is to be painted.
Curing is covered in chapter 8 on "Concrete Construction." As part of the general guidance, it is recommended that the contractor submit a plan for curing before construction begins. It emphasizes that positive curing procedures must be used. Form curing-where no additional water is added-is not acceptable. Forms left in place must be kept continuously wet. Water cannot have staining capability and hand sprinkling is not satisfactory, except as an emergency procedure.
Membrane-forming curing compound is particularly recommended because of ease of inspection. Uneven application is easily detected visually. When nonpigmented curing compound is used because of aesthetics, an inspector must be on hand to check uniformity of application, although there is no guidance on how to do this. When using nonpigmented curing compound and temperatures are above 32 °C, concrete must be shaded for 3 days. Compressed air lines must have traps to prevent moisture or oil from contaminating the curing compound. Ordinary, hand-held garden type sprayers are not satisfactory.
In cold weather concreting, as defined in ACI 306R(14), concrete temperatures should be monitored at several locations, particularly at corners and should be protected from cycles of freezing and thawing until it reaches a strength of 24 MPa. Drying is not likely to be a problem in cold weather unless protection systems create a particularly dry or warm condition. Precautions should be taken if concrete is warmer than 15 °C and exposed to air warmer than 10 °C.
During hot weather concreting, as defined in ACI 305R(12), precautions against plastic shrinkage cracking should be taken if evaporation rates exceed 1.0 kg/m2/h, as determined from the evaporation rate nomograph or as determined by direct measurement of evaporation from an open dish. Moist curing is recommended as the best way to prevent plastic shrinkage cracks in hot weather. There are no maximum limits on concrete or ambient temperatures, but general guidance on hot weather concreting is provided.
For curing high strength concrete, water curing is recommended, particularly at early ages and when water-cement ratios are below 0.4.
As part of the concrete report required after project completion, a section on curing methods, inspections, and nonconformances is recommended.
This specification closely resembles ASTM C 309(23) except for the water-retention requirement (actually a water-loss requirement). The specification requires no more than 0.31 kg/m2 loss at 7 days. Curing compounds meeting this requirement are normally specified for airfield pavements. Most other USACE specifications require curing compounds that meet ASTM C 309.(23) Several manufacturers make a curing compound complying with CRD-C 300, and this is indicated in the product literature.
This test method closely resembles ASTM C 156(18) except that the conditions in the curing cabinet require an airflow of 10 m/s (C 156(18) requires sufficient air flow to produce a target evaporation), and the test period is 7 days.
Guidance on curing water is that it "... must be free of materials that significantly affect the hydration of reactions of portland cement or that otherwise interfere with the phenomena that are intended to occur during the curing of concrete," (p. 1). There is no test method or quantitative requirement on this interference. However, when the same statement is used in the context of specifying mixing water, the requirement is a comparison of strength development of 50.8 mm mortar cubes made with the water in question versus strength of cubes made with deionized water.
British Standard, BS 8110 (1997), "Structural Use of Concrete: Part 1. Code of practice for design and construction" addresses curing in Section 6 titled, "Concrete, materials, specification, and construction."(40)
Curing and protection should start immediately after the compaction of the concrete to protect it from:
Minimum length of curing required depends on type of cement, ambient conditions, and temperature of concrete. The guidance is reproduced below (table 19) with annotations in italics relating cement types to the nearest ASTM equivalent. Surface temperature should not be allowed to fall below 5 °C during this time.
|Type of cement||Ambient conditions after casting||Minimum periods of curing and protection|
|Average surface temperature of concrete|
|5 to 10 °C||t °C (any temperature between 10 °C and 25 °C)|
|PC 42.5 or PC 52.5 to BS 12
SRPC 42.5 to BS 4027
|All cements in table 1 of BS 5328, Part 1 (1997), except those listed above. Blended cements, ASTM C 595(60), C 1157(53)||Average||6||80
|All||Good||No special requirements|
|Note 1. Abbreviations for the type of cement used are as follows:
Note 2. Ambient conditions after casting are as follows:
Five curing methods are allowed. There is no guidance on choosing among them:
BS 7542 is the test method used to test curing compounds.(28) This method is very much like ASTM C 156, in that a standard mortar specimen is coated with curing compound, then exposed to a hot, dry conditions, and mass loss measured.(18) However, instead of reporting simple mass loss, as in C 156, the method reports mass loss relative to the mass loss of an uncoated control specimen, in units of percent.(18) The property is called the "curing efficiency index." A value of 0 percent indicates that the curing compound-treated specimen lost as much water as the control. An index result of 100 percent means that the curing compound-treated specimen lost no water. A value of 90 percent is considered acceptable. Test conditions are: temperature 38 °C, 35 percent RH, and wind velocity of 0.5 m/s. The test age is 3 days. Precision is not given in the test method, but within-laboratory standard deviations of about 4.5 percent for three materials with curing efficiencies higher than 80 percent, and 11 percent for one material with a curing efficiency of 56 percent were reported by Cabrera, Gowiripalan, and Wainwright (1989).(107)
A summary of the guidance on curing in the Euro-International Committee for Concrete (CEB)- International Federation for Prestressing (FIP) Model Code (1990)(41) is taken from Meeks and Carino (1999)(4). The title of the section in the code is "Curing and Protection." This code only considers the moisture balance aspect of curing. Time-temperature considerations are not addressed. Protection involves avoiding effects of rain or flowing water, early freezing, thermal stresses, vibration, and impact.
It is recognized that curing has only a minor effect on the strength development of concrete, with the exception of very thin sections, because the core of thick sections maintains enough water for hydration to occur even without any deliberate curing. Lack of curing has a major effect on the properties of surface or near-surface concrete.
The recognized methods of curing are:
The code notes that not all methods work equally well; therefore, it is important to have frequent inspections, although no specific guidance is given. It is also noted that methods that involve addition of water give a concrete with a denser pore structure than methods that only retain water. For low water-cement ratio concretes, it is recommended that a type of curing that involves adding additional water be used.
Duration of curing for concrete in normal service environments (humid, wet, seawater, frost) is prescribed according to ambient conditions during curing and rate of development of impermeability in the concrete. Ambient conditions are categorized into three levels. Condition I is defined as exposure to no direct sun and RH greater than 80 percent. Condition II is defined as exposure to medium sun, medium wind velocity, or RH between 50 and 80 percent. Condition III is defined as exposure to strong sun, high wind velocity, or RH of less than 50 percent. No quantitative values of illumination or wind velocity are given.
Rate of development of impermeability is prescribed according to type of cement and the water cement ratio of the concrete. Curing times range from 1 to 8 days depending on the combination of these factors. The tabulation of this information is reproduced in table 20. This guidance is for ambient temperatures of approximately 20 °C. The maturity method is recommended to estimate required curing time for temperatures higher or lower than this. Such calculations result in an approximate doubling of the curing time at 10 °C. For temperatures around 30 °C, maturity calculations result in an approximate halving of the curing time. For pozzolan-containing concretes, curing time is to be increased by 1 or 2 days. For severe chemical exposures, it is recommended that curing time be extended by 3 to 5 days.
Information on European standards for curing was taken from Litzner and Becker (1999).(108) A table on curing requirements was presented in this reference, cited as being taken from EN 206.(42) Table 21 is a reproduction of this table. Exposure classes XO and XC1 are for dry concrete inside buildings.
|Rate of development of impermeability of concrete||Very Rapid||Rapid||Medium||Slow|
|Exposed ambient conditions during and after curing||I No direct sunshine, relative humidity of surrounding air >80 percent||1||2||3||4|
|II Exposed to medium sunshine or medium wind velocity or relative humidity 50-80 percent||2||3||4||5|
|III Exposed to strong sunshine or high wind velocity or relative humidity less than 50 percent||3||4||6||8|
|Rate of development of impermeability of concrete|
|Rate of development of impermeability||Water-cement ratio||Class of cement*|
|Slow||All other cases|
|Surface concrete temperature (T), ° C||Minimum curing period, days*, + for a concrete strength development r = fcm,2/fcm,28|
|r ≥ 0.50 Rapid||r ≥ 0.30 Medium||r ≥ 0.15 Slow||r < 0.15 Very Slow|
|T ≥ 25||1.0||1.5||2.0||3.0|
|25 > T ≤ 15||1.0||2.0||3.0||5|
|15 > T ≤ 10||2.0||4.0||7||10|
|10 > T ≤ 5‡||3.0||6||10||15|
* Plus any period of setting exceeding 5 h.
+ Linear interpolation between values in the rows is acceptable.
‡ For temperatures below 5 °C, the duration should be extended for a period equal to the time below 5 °C.
The Canadian guidance on concrete curing is in CSA23.1-94,(109) as summarized by Meeks and Carino (1999).(4) The basic curing requirement is for 3 days moist curing, with temperatures greater than 10 °C, or for the time required to reach 35 percent of the specified 28-day strength. Field cured specimens or nondestructive methods may be used. For aggressive chemical or abrasive service conditions, curing time should be increased by 4 days over the basic requirement or to the time necessary to reach 75 percent of specified 28-day strength. There is no quantitative guidance for curing of concretes that develop strength slowly, but there is a general caution about the need to account for this in project specifications. Reference is made to ACI 308 for curing methods and details of practice.(31)
Norwegian Standard 3420, as reported in Meeks and Carino (1999)(4) addresses concrete curing requirements, as summarized in Meeks and Carino (1999).(4) Concrete shall be kept moist for 3 days after setting, either with water or by prevention of evaporation. If the water-cement ratio is less than 0.40, then water curing must be used. For special requirements on impermeability, absence of cracks, and resistance to chemical attack, 14 days of curing is required. No artificial drying is allowed until 70 percent of specified 28‑day strength is reached. Special care is advised with use of silica fume or large amounts of fly ash to prevent early drying of the concrete surface and resultant plastic shrinkage cracks. In very aggressive environments, maximum curing temperature is 65 °C.
Australian Standard 3600 (AS 3600, 1994(48)) addresses curing, as summarized by Meeks and Carino (1999).(4) Guidance is based mostly on severity of exposure condition in service. Exposure is classified according to climate zones (temperate, tropical, and arid) and environmental aggressiveness within each of these. Curing guidance is then based on minimum curing time or, alternatively by minimum compressive strength. For relatively mild exposures, at least 3 days moist curing is required, or if accelerated curing is used, curing to a compressive strength of 15 MPa is required. Design strengths of 20 to 25 MPa, depending on details of exposure, are required. For more aggressive exposures, at least 7 days curing under ambient conditions is required, or if accelerated curing is used, curing is required to a strength of 20 MPa for a 32 MPa design strength, to 25 MPa for a 40 MPa design strength, or to 32 MPa for a 50 MPa design strength. The design strengths vary according to exposure class.
The Roads and Traffic Authority of New South Wales (RTA)(110) has established field performance criteria for curing. Water sorptivity is used to determine effectiveness of curing on site. See Chirgwin and Ho (1995)(111) for the test method and Ho and Chirgwin (1996)(112) for the performance specification. Table 22 is reproduced from the latter reference. Concretes containing large amounts of slag must be cured for an additional 7 days in moderate to severe exposures.
Silica fume concrete is required to be cured with water for 3 to 6 days. There is also a limit on crack width on bridge decks.
|Exposure||Minimum bindera content, kg/m3||Maximum water-binder ratio||Minimum period of standard curing, d||Maximum sorptivity depth, mm|
|A||Dry climate, no industry, nonaggressive||0.56||7 (7)b||45|
|B1||Industrial areas, inland||320||0.56||7 (7)||35|
|B2||Close to coast or permanently in salt water||390||0.46||7 (15)||17|
|C||Tidal/splash zone||450||0.40||14 (22)||11|
|aBinder = cementitious materials
bNumber of days in parentheses are for high slag cement
A limited survey of State DOT standards showed that States differ in many of the details of curing practice. A more thorough review of these standards was done; the results are summarized in appendix B.