ACI 309.2R:1998 (R2005) download.Identification and Control of Visible Effects of Consolidation on Formed Concrete Surfaces.
This report pmvides guidelines for identifying and controlling visible effects of consolidation on precast or cast-in-place formed concrete cur- faces. it includes a .cummarv of direct and indirect causes of such imperfec. lions. An outline to assist in the reporting on surfaces and photographs to illustrate typical concrete surface blemishes are cilso included.
Surface hkmjsl,es in concrete may be minimized by proper planning clur— ing the design and specification stcues. Of equal importance is the employmen! of properly trained and ,noliva ted supervisory and non-supervisory construction personnel to achieve the i,iiended concrete finishes and surface textures.
The report einphasize.s .cignif leant consolidation factors that minimize undesirable surface effects. The reader is cautioned that other potential causes qf such effects may exist beyond those lisu’d in this report. There are documents in which a feature designated as a defect is one that must he prevented, avoided, corrected, remmediated or otherwise dealt with. The term “defect,” asfonnerli’ used in this report, covers blemishes and deparlures from perfection that are noifeatures. and must be avoided or repaired
ACI Committee Reports. Guides. Standard Practices, and Commentaries are intended for guidance in planning. designing. executing, and inspecting construction. ACI 309.2R is intended for the use of individuals who arc competent to evaluate the significance and limitations of its content and recommendations and who will accept responsibility for the application of the material it contains. The American Concrete Institute disclaims any and all responsibility for the stated principles. The Institute shall not be liable br any loss or damage arising therefrom.
Reference to ACI 309.2R shall nOt be made in contract documents. If items found in this document are desired by the ArchitectlEngineer to be a part of the contract documents, they shall he restated in mandatory language for in- corporal ion by the Arch i tect/Engineer.
A formed concrete surface, uniformly smooth or deeply textured and essentially free of blemishes and color variation, is difficult to attain. Since repairs to a defective surface are costly and seldom fully satisfactory, the need for repairs should be minimized by establishing and maintaining the quality of the concrete operation and by adhering to acceptable consolidation procedures. Standards for surface finishes are beyond the scope of this report. Guidance for establishing appropriate standards is offered by the International Council for Building Research (CIB) (1975). which classities formed surface finishes as follows:
• Special—High standards of appearance required (ACI
303R);
• Elaborate—Definite requirements for visual appearance:
• Ordinary—Appearance is of some importance: and
• Rough—No special requirements for finish.
Concrete construction procedures do not always provide the control necessary to consistently obtain blemish free concrete indicated by a special category.
To achieve any concrete finish, the designer and the contractor must use materials as well as design and construction practices that will keep surface effects within acceptable limits. There is a definite need for understanding the causes of unacceptable blemishes and effects encountered in concrete construction and a need for applying more effective measures to minimize or eliminate them. This report addresses those needs and its major emphasis is on consolidation- related effects.
The most serious effects resulting from ineffective consolidation procedures are: honeycomb, subsidence cracks, cold joints, and excessive surface voids. A detailed description of the blemishes and their causes are provided in Table 1. Some imperfections may not conform to contract documents and may be considered as defective work.
CHAPTER 2—FACTORS CAUSING EFFECTS
Causes of consolidation-related effects on formed concrete surfaces (Table I) include:
A. Design and construction-related causes
• Difficult placement due to design of a member
• Improper selection of horizontal construction joint location
• Improper design, manufacture, installation, shipping. preparation and maintenance of forms
• Improper selection of concrete mixture proportions
• Failure to adjust concrete mixture proportions to suit placement condition
• Improper placement practices
• Improper consolidation practices
• improper steel detailing
B. Equipment-related causes
• improper equipment
• Improper equipment maintenance
• Equipment failure (crane. pump. concrete plant)
• Interruption of utility service
C. Material-related causes
• Improper selection of release agent
• Cement characteristics
• Variation in mixture constituents
• Inappropriate use of admixtures
D. Environmental causes
• Extreme weather conditions
Examples of some of the more common blemishes are illustrated in Fig. I through 10.
2.1—Design of structural members
The common problems requiring consideration during design and planning are congested reinforcement (particularly splices), narrow sections. or complex form configurations. Conditions that require closed top forming. embedments, and battered forms also require consideration during design and planning.
To produce properly consolidated concrete with the desired appearance. the placement and consolidation of the concrete must be understood. The designer must have a working knowledge of the concrete placement process. The designer and the constructor should communicate during the early phases of the concreting process. Early recognition of problem areas is important to provide time to take remedial measures, such as staggering splices, grouping reinforcing steel, modifying stirrup spacing. increasing the section size, and selecting locations of horizontal construction joints. When unfavorable conditions exist that could contribute to substandard surfaces, one or more of the following actions should be taken:
I. Redesign the member.
2. Redesign the reinforcing steel;
3. Provide adequate access for consolidation at horizontal
construction joints;
4. Modify mixture proportions;
5. Use mock-up tests w develop a procedure: and/or
6. Alert the constructor to critical conditions.
A review by the designer is essential to ensure thai strength levels, nominal maximum aggregate size, and slump requirements for different structural elements are met. Concrete ingredients should be evaluated and proportions should be selected well in advance of the concreting operation to achieve the desired properties for the fresh concrete. Sticky mixtures may occur if the fine aggregate grading in the 1.18 mm to 300 im (No. 16 to 50) size range approaches the upper limits specified by ASTM C 33. or if high cement contents are used. Some pozzolans also may cause mixtures to be more cohesive. Thus, the passage of entrapped air may be restricted and air voids may be trapped at the Interface between the concrete and the form. If fine aggregate contains the proper amount of materials in the 600 to 300 tm (No. 30 to 50) size range. little bleeding will occur in the resulting concrete. As a result, placement and consolidation of the concrete will be facilitated, thereby minimizing surface effects.
Soft aggregates may degrade and produce additional fines. In some instances, the fines may make the mixture more cohesive and increase the difficulty of removing entrapped air. This is particularly true at high cementilious materials contents. In other instances, the additional fines can significantly increase the water demand, resulting in lower strength, increased shrinkage, and crazing of smooth formed surfaces. Experience indicates that a concrete at a given consistency will generally flow more easily at lower temperatures than at higher temperatures.
When chemical and especially mineral admixtures are used, their effect on placement and consolidation should be evaluated when mixture proportions are being established. All of the factors discussed above need to be considered to obtain a concrete mixture with the desired composition. consistency, and workability to facilitate its placement and consolidation.
2.5—Placement
Concrete should be placed as quickly as possible with a minimum amount of segregation and spattering on the forms. Once the coarse aggregate is separated from the mortar by poor handling and placement practice, it is virtually impossible to work the mortar back into the voids and restore a dense mass by vibration. Segregation and separation cause honeycomb. Spattered mortar on the form produces color variations and poor surface texture. Placing concrete too slowly may allow workability to be lost and can produce layer lines or cold joints due to improper consolidation. The rate of placement and vibration factors (intensity and spacing) should be selected to minimize entrapped air in the concrete.
If concrete is deposited in thick layers of more than 300mm (I 2 in.), more air may be trapped than if it is placed in a thinner, even layer since the air has to travel farther to escape. Where mixtures of dry or stiff consistencies are required, the placement rate should be slower to permit adequate consolidation so as to avoid bug holes and honeycombing. However, in the case of a sanitary treatment structure with steel forms, an increase in lift thickness from 0.6 to 1.2 m (2 104 ft) reduced bug holes by 50 percent when an air content of 5 percent was specified.
2.6—Consolidation
Concrete consists of coarse aggregate particles in a matrix of mortar, and irregularly distributed pockets of entrapped air, lithe concrete is air entrained, an additional evenly distributed system of entrained air bubbles is present. The volume of entrapped air in unconsolidated concrete may vary from about 5 to 20 percent depending on thc workability of the mixture, size and shape of the form, amount of reinforcing steel, and method of depositing the concrete. The purpose of consolidation is to remove as niuch of this entrapped air as practical.
Vibration is the most common method of consolidation. It causes very rapid movement of the concrete mixture particles and briefly liquefies the mixture, thus reducing the internal friction. When vibrated, concrete becomes fluid and through the action of gravity seeks a lower level and denser condition as entrapped air rises to the surface and is expelled. It compacts laterally against the form and around the reinforcing steel. In practice, vibration is normally continued until the entire placement acquires a uniform appearance and its surface just starts to glisten or large bubbles cease to appear. A film of cement paste should be discernible between the concrete and the forms. These visual indicators are not necessarily an accurate indication of good consolidation. ACI 309R provides guidance on judging the adequacy of vibration.
Undervibration is far more common than overvibration. and may he caused by the following:
I. Use of an undersized, underpowered, or poorly maintained vibrator;
2. Excessive or haphazard spacing of vibrator insertions:
3. Inadequate vibration during each insertion:
4. Failure of the vibrator to penetrate into the preceding layer: and/or
5. Vibrator in the wrong position relative to the form.
Common imperfections resulting from under-vibration are honeycomb, excessive entrapped air voids, and layer lines.
Overvibration can occur if vibration is continued for a prolonged time (several times the recomnended time period). Overvibration is generally the result of using oversized equipment. improper procedures, high slump. or improperly proportioned mixtures. It may result in segregation, excessive form deflection, sand streaking, and form damage. Backsirom et al. (1958) found that air content of concrete is decreased by increasing periods of vibration, but little effect is noted on spacing factor of air-entrained concrete. In concrete of nominal 6.5 percent air the air content dropped from 6.7 to 1.2 after 2, 6, 12, 20, 30. and 60 sec of vibration, but the spacing factor was unchanged as was the number of cycles to 25 percent loss in mass.
The consequences of overvibration will be minimized if a well-proportioned mixture with a proper slump is used.
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