| MASTERSPEC - EVALUATIONS | 08/02 (R 05/05) |
The basic premise of this Section is that its scope is limited to small, simple, structurally noncritical concrete projects. The Section covers the basic requirements of cast-in-place concrete without embellishment. It might be used for residential, light-commercial, and light-retail building projects where the use of cast-in-place concrete is largely nonstructural, or for miscellaneous site concrete. Typical cast-in-place concrete uses might include footings, grade beams, slabs-on-grade, and basement or foundation walls without special structural requirements or finish treatments. The Section Text does not specify many of the requirements of buildings with more critical concrete requirements such as multistory frames or industrial floors.
A comprehensive discussion of concrete materials, mixes, and properties appears in the Evaluations in Division 03 Section "Cast-in-Place Concrete."
ACI 301, Specification for Structural Concrete, which is published by the American Concrete Institute/ACI International (ACI), is the foundation of this Section. Provisions of ACI 301 have been incorporated by reference because they set conservative default requirements for cast-in-place concrete.
Another important ACI document is ACI 318, Building Code Requirements for Structural Concrete, along with its metric counterpart, ACI 318M. These documents are intended for adoption by the model codes and the International Building Code (IBC), and they establish minimum design requirements for the design professional and a basis for acceptance of design and construction of structural concrete by the building official. They are not intended by ACI to become a part of the contract document. Fortunately, many of their provisions for concrete materials and mixes that affect a construction specification may be found in ACI 301.
The design professional may be unfamiliar with specifying concrete according to ACI 301. Much of the detail customarily specified in a project specification is not repeated in the Section Text; however, the design professional and the Contractor should have copies of ACI 301 available for reference.
For example, if a project specification does not cover form-facing materials and does not spell out the finishes required on formed concrete, what are the Contractor's obligations? ACI 301 sets these out in detail, emphasizing design, performance, and construction requirements. ACI 301 also prescribes default formed finishes, depending on whether the concrete is exposed to public view.
Concrete construction and surface tolerances in ACI 117, Specifications for Tolerances for Concrete Construction and Materials, are referenced in ACI 301. Offsets between adjacent form-facing panels are limited by ACI 301 to Class C, allowing gradual or abrupt irregularities of 1/2 inch (13 mm).
For concrete surfaces not exposed to public view, a rough-formed finish is allowed. Concrete exposed to public view requires a smooth-formed finish. ACI 301 defines the term exposed to public view as "situated so that it can be seen from a public location after completion of the building." The Section Text expands on options for formed finishes and provides examples of their locations.
Finishing methods depend on whether the concrete is formed or unformed. Formed finishes are those imparted by the formwork itself or applied manually to the newly hardened concrete after form removal. Related unformed surfaces, such as tops of beams, need manual finishing. Unformed concrete includes concrete surfaces to be manually finished even if boundary or support elements may be formed. A suspended cast-in-place slab with an integrated beam system is an example.
Formed Finishes: Two categories of formed finishes, as-cast and rubbed, are recognized by ACI 301. Further finishing and architectural treatment of formed surfaces are considered architectural concrete by ACI and are specified in Division 03 Section "Architectural Concrete." As-cast finishes include rough-formed, smooth-formed, and architectural finishes. Rubbed finishes include smooth-rubbed, grout-cleaned, and cork-floated finishes.
Architectural finishes are not included in the Section Text.
Unformed Finishes: Floor and slab finishes vary from coarse to fine textured. The finishing process is preceded by screeding or leveling the fresh concrete and by initial floating. Depending on the tools used, initial floating is called bull floating, darbying, or highway-type straightedging and takes place immediately after screeding and before bleedwater appears. Subsequent finishes require some stiffening of the concrete.
Hand-tool jointing and, if required, edging are applied at this stage. Jointing precedes final finishing when the concrete is soft enough to groove but hard enough to support foot pressure. By contrast, saw cutting takes place after the final finishing and sufficient concrete hardening to prevent spalling or raveling of edges.
Curing is the process of maintaining concrete at a satisfactory moisture content and temperature while it hydrates.
Newly placed concrete changes from a fluid state to a hardened state in a process called hydration, a chemical reaction between cementitious materials and water. As concrete hardens, it strengthens and develops other properties such as durability, density, watertightness, freeze-thaw resistance, chemical resistance, and abrasion resistance. It also loses moisture by evaporation. If concrete dries too fast or is too cold, hydration stops and problems arise. ACI has issued detailed recommendations for cold- and hot-weather concreting that extend beyond this discussion.
Without surface protection to maintain water in the concrete, strength suffers, differential shrinkage causes cracks over exposed surfaces, and other desired concrete properties may not materialize. Paradoxically, too much mixing water, rather than solving the problem, can make things worse. Most good-quality concrete has 50 to 75% more water than is needed for hydration; poorer quality concrete may easily have twice the amount of needed water.
Curing keeps the hydration process active as long as the temperature remains favorable. Curing measures should be maintained for at least seven days, the minimum period considered critical for developing initial strength and other properties in concrete. Hydration slows when the temperature drops below 50 deg F (10 deg C), retarding strength development. Some concrete items benefit more from curing than others. Floors, slabs, and pavement, with large exposed surface areas, may be more critical than bulky concrete items with comparatively small surface areas.
It is one thing to specify curing; it is another to get it. Because some of the damage to concrete may not be detected during construction, curing is sometimes ignored or halfheartedly performed. Insist on curing unless reliable weather conditions occur during concreting. Conditions of high humidity, rain, mist, and comfortably low temperatures sustained during the curing period can combine to naturally cure the concrete. Because these favorable conditions usually cannot be relied on, curing is necessary. ACI 301 and ACI 318 (ACI 318M) mandate a seven-day curing period for concrete.
Curing must be started as soon as concrete finishing is complete. In severe drying conditions, a combination of high air and concrete temperatures, low humidity, and windy days will require intermediate steps to prevent premature concrete from drying during finishing and before curing can begin. Preventive measures include evaporative retarders applied as a film to the concrete and a misting spray cooling the air around the concrete by evaporation.
Three curing methods predominate: wet curing, impervious sheet covers, and membrane-forming curing compounds. The choice of a single method or a combination of methods can be specified, depending on the requirements. Curing compounds, for example, may interfere with adhesives that bond floor coverings to the concrete. Sometimes curing methods may be combined; e.g., initial wet curing may be followed by a curing compound.
Temperature-control methods differ. In hot weather, wet-curing methods can help lower the temperature. Wet curing is seldom used in cold weather. Insulating blankets or covers, which trap heat and moisture generated by the concrete with or without other dry-curing methods, can be used.
The Section Text can be edited to delete testing requirements. Concrete covered in the Section Text would usually not be subject to the "Special Inspections" provisions of the three national model codes or the IBC. If special inspections will be required, consider using Division 03 Section "Cast-in-Place Concrete" or extract the "Field Quality Control" Article from that Section Text and place it in this Section. However, nothing prevents the design professional from specifying concrete testing, even for limited applications.
The Section Text applies the default material and testing requirements of ACI 301. The Owner may directly engage the independent testing and inspection agency or have the Contractor engage the agency.
Publication dates represent the editions on which the current Section Text is based. Standards are revised periodically, which may occur before this Section is updated again.
The following publications are useful in specifying cast-in-place concrete for limited applications. Other references may be needed for design purposes.