Aci 318 Manual Of Concrete Practice !FREE!
The Concrete Manual, now updated to the 2021 IBC and ACI 318-19, provides the guidance and information that inspectors and other construction professionals need to become more proficient in concrete field practices and inspection.
Aci 318 Manual Of Concrete Practice
A Resource Reference section includes a list of the concrete industry and technical organizations to contact for additional information. Your purchase of the Concrete Manual includes exclusive online access to the Concrete Manual Workbook to help you master concrete inspection and field practices. The workbook contains learning objectives, lesson notes, key points for studying, and quizzes for each chapter. The answer key includes references to the applicable sections in the Concrete Manual.
The Concrete Manual, now updated to the 2021 IBC and ACI 318-19, provides the guidance and information that inspectors and other construction professionals need to become more proficient in concrete field practices and inspection. The Concrete Manual will:
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 126.96.36.199). 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)).
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.
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."
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 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.
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).