ACI 309R-96

Chapter 7—Recommended vibration practices for general construction, p. 309R-16

7.1—Procedure for internal vibration

7.2—Judging the adequacy of internal vibration

7.3—Vibration of reinforcement

7.4—Revibration

7.5—Form vibration

7.6—Consequences of improper vibration

Chapter 8—Structural concrete, p. 309R-21

8.1—Design and detailing prerequisites

8.2—Mixture requirements

8.3—Internal vibration

8.4—Form vibration

8.5—Tunnel

Chapter 9—Mass concrete, p. 309R-22

9.1—Mixture requirements

9.2—Vibration equipment

9.3—Forms

9.4—Vibration practices

9.5—Roller-compacted concrete

Chapter 10—Normal weight concrete floor slabs, p. 309R-25

10.1—Mixture requirements

10.2—Equipment

10.3—Structural slabs

10.4—Slabs on grade

10.5—Heavy-duty industrial floors

10.6—Vacuum dewatering

Chapter 11—Pavements, p. 309R-27

11.1—Mixture requirements

11.2—Equipment

11.3—Vibration procedures

11.4—Special precautions

Chapter 12—Precast products, p. 309R-30

12.1—Mixture requirements

12.2—Forming material

12.3—Production technique

12.4—Other factors affecting choice of consolidation method

12.5—Placing methods

Chapter 13—Lightweight concrete, p. 309R-31

13.1—Mixture requirements

13.2—Behavior of lightweight concrete during vibration

13.3—Consolidation equipment and procedures

13.4—Floors

Chapter 14—High density concrete, p. 309R-32

14.1—Mixture requirements

14.2—Placing techniques

Chapter 15—Quality control and inspection, p. 309R-33

15.1—General

15.2—Adequacy of equipment and procedures

15.3—Checking equipment performance

Chapter 16—Consolidation of test specimens, p. 309R-35

16.1—Strength tests

16.2—Unit weight tests

16.3—Air content tests

16.4—Consolidating very stiff concrete in laboratory specimens

Chapter 17—Consolidation in congested areas, p. 309R-36

17.1—Common placing problems

17.2—Consolidation techniques

Chapter 18—Information sources, p. 309R-37

18.1—Specified and/or recommended references

18.2—Cited references

Appendix A—Fundamentals of vibration, p. 309R-38

A.1—Principles of simple harmonic motion

A.2—Action of a rotary vibrator

A.3—Vibratory motion in the concrete

CHAPTER 1—GENERAL

A mass of freshly placed concrete is usually honey-combed with entrapped air. If allowed to harden in this condition, the concrete will be nonuniform, weak, porous, and poorly bonded to the reinforcement. It will also have a poor appearance. The mixture must be consolidated if it is to have the properties normally desired and expected of concrete.

Consolidation is the process of inducing a closer arrangement of the solid particles in freshly mixed concrete or mortar during placement by the reduction of voids, usually by vibration, centrifugation, rodding, tamping, or some combination of these actions; it is also applicable to similar manipulation of other cementitious mixtures, soils, aggregates, or the like.

Drier and stiffer mixtures require greater effort to achieve proper consolidation. By using certain chemical admixtures, consistencies requiring reduced consolidation effort can be achieved at a lower water content. As the water content of the concrete is reduced, concrete quality (strength, durability, and other properties) improves, provided it is properly consolidated. Alternatively, the cement content can be lowered, reducing the cost while maintaining the same quality. If adequate consolidation is not provided for these drier or stiffer mixtures, the quality of the inplace concrete drops off rapidly.

Equipment and methods are now available for fast and efficient consolidation of concrete over a wide range of placing conditions. Concrete with a relatively low water content can be readily molded into an unlimited variety of shapes, making it a highly versatile and economical construction material. When good consolidation practices are combined with good formwork, concrete surfaces have a highly pleasing appearance [see Fig. 1(a) through 1(c)].

Fig. 1(a)—Pleasing appearance of concrete in church construction

Fig. 1(b)—Pleasing appearance of concrete in utility building construction

Fig. 1(c)—Close-ups of surfaces resulting from good consolidation

2.1—Mixture proportions

Concrete mixtures are proportioned to provide the workability needed during construction and the required properties in the hardened concrete. Mixture proportioning is described in detail in documents prepared by ACI Committee 211, as listed in Chapter 18.1.

2.2—Workability and consistency

Workability of freshly mixed concrete is that property that determines the ease and homogeneity with which it can be mixed, placed, consolidated, and finished. Workability is a function of the rheological properties of the concrete.

As shown in Fig. 2.2, workability may be divided into three main aspects:

1. Stability (resistance to bleeding and segregation).
2. Ease of consolidation.
3. Consistency, affected by the viscosity and cohesion of the concrete and angle of internal friction.

Workability is affected by grading, particle shape, proportions of aggregate and cement, use of chemical and mineral admixtures, air content, and water content of the mixture.

Consistency is the relative mobility or ability of freshly mixed concrete to flow. It also largely determines the ease with which concrete can be consolidated. Once the materials and proportions are selected, the primary control over work-

ability is through changes in the consistency brought about by minor variations in the water content.

The slump test (ASTM C 143) is widely used to indicate consistency of mixtures used in normal construction. The Vebe test is generally recommended for stiffer mixtures.

Values of slump, compacting factor, drop table, and Vebe time for the entire range of consistencies used in construction are given in Table 2.1.

Other measures of consistency such as the Powers remolding test and Kelly ball are available. These are not used as frequently as slump. Information on various consistency tests has been discussed by Neville (1981), Vollick (1966), and Popovics (1982).

2.3—Workability requirements

The concrete should be sufficiently workable so that consolidation equipment, properly used, will give adequate consolidation. A high degree of flowability may be undesirable because it may increase the cost of the mixture and may reduce the quality of the hardened concrete. Where such a high degree of flowability is the result of too much water in the mixture, the mixture will generally be unstable and will probably segregate during the consolidation process.

Mixtures having moderately high slump, small maximum-size aggregate, and excessive fine aggregate are frequently used because the high degree of flowability means less work in placing.

At the other extreme, it is inadvisable to use mixtures that are too stiff for conditions of consolidation. They will require

Fig. 2.2—Parameters of the rheology of fresh concrete

great consolidation effort and even then may not be adequately consolidated. Direction and guidance are often required to achieve the use of mixtures of lower slump or fine aggregate content, or a larger maximum size aggregate, so as to give a more efficient use of the cement.

Concrete containing certain chemical admixtures may be placed in forms with less consolidation effort. Refer to reports of ACI Committee for additional information on chemical admixtures. The use of fly ash, slag, or silica fume may also affect the consolidation of concrete by permitting placement with less consolidation effort. Refer to reports of ACI Committee 226 for more information regarding these materials. The amount of consolidation effort required with or without the use of admixtures can best be determined by trial mixtures under field conditions.

It is the workability of the mixture in the form that determines the consolidation requirements. Workability may be considerably less than at the mixer because of slump loss due to high temperature, false set, delays, or other cause.

CHAPTER 3—METHODS OF CONSOLIDATION

The consolidation method should be compatible with the concrete mixture, placing conditions, form intricacy, amount of reinforcement, etc. Many manual and mechanical methods are available.

3.1—Manual methods

Some consolidation is caused by gravity as the concrete is deposited in the form. This is particularly true for well proportioned flowing mixtures where less additional consolidation effort is required.

Plastic or more flowable mixtures may be consolidated by rodding. Spading is sometimes used at formed surfaces—a flat tool is repeatedly inserted and withdrawn adjacent to the form. Coarse particles are shoved away from the form and movement of air voids and water pockets toward the top surface is facilitated, thereby reducing the number and size of bugholes in the formed concrete surface.

Hand tamping may be used to consolidate stiff mixtures. The concrete is placed in thin layers, and each layer is carefully rammed or tamped. This is an effective consolidation method, but laborious and costly.

The manual consolidation methods are generally only used on smaller nonstructural concrete placement.