CAPÍTULO I Disposiciones Generales
Artículo 9. Constancia en el expediente académico
C4.17.1 General
Vibration should be distributed so the concrete is thoroughly consolidated, producing a dense and uniform mass. It shall also ensure surfaces that are as free of imperfections or blemishes as possible. The optimum time of vibration depends on the type of vibrator, the mix characteristics, and the configuration of the forms and reinforcement. Reducing the vibration time on the last lift in returns will result in increased bug holes.
Consolidation of concrete mixes shall accomplish full coating of the coarse aggregate and reinforcement with cement paste. Air pockets around reinforcement, that can create potentially corrosive environments, shall be avoided by selecting proper consolidation methods.
After the proper vibrating equipment has been selected, it shall be operated by trained personnel.
Concrete vibration shall be performed with proper timing and spacing to ensure adequate consolidation.
The selection of the most appropriate vibrator or vibration method involves factors such as:
1. Size, shape, type and stiffness of the forms . 2. Concrete mix and consistency.
3. Prior experience.
The effectiveness of vibration shall be judged by the surface condition of the finished concrete, unless circumstances indicate that a more in-depth evaluation is needed. If unacceptable surface defects such as honeycombing, aggregate or mortar pockets, and excessive air bubbles are noted, the placement and vibration procedures shall be revised to ensure proper consolidation.
It is difficult to damage properly proportioned and mixed concrete by excessive vibration, provided the forms are properly designed. Problems stemming from inadequate vibration are far more common than problems associated with excessive vibration.
Vibration procedures shall be established at the beginning of each project that will ensure adequate concrete consolidation in the various types of products to be cast.
Cold joints shall be avoided unless specifically accounted for in the design.
Cold joints may be required for certain types of members with complex dimensions that cause forming and production difficulties.
4.17.2 Consolidation of Lightweight Concrete In consolidation of lightweight concrete, precautions shall be taken to avoid aggregate floatation caused by overvibration.
C4.17.2 Consolidation of Lightweight Concrete In consolidation of lightweight aggregate concrete, care should be taken not to overvibrate. Since the coarse aggregate particles are the lightest solid ingredients in the mix, overvibration can cause the particles to rise. This may lead to finishing problems and the strength to be nonuniform through the depth of the member. The use of mixes with a slump of less than 4 in. (100 mm) helps to prevent segregation during handling, consolidation, and finishing operations.
4.17.3 Consolidation of Concrete in Complex Precast Concrete Products
C4.17.3 Consolidation of Concrete in Complex Precast Concrete Products
The layers of concrete shall be placed level to minimize vibrator-induced flow of the concrete. High spots shall be leveled by hand or by mechanical screeding.
When large production runs of elements involving complex shapes are planned, it will be cost effective to evaluate alternative consolidation methods in order to identify those methods and procedures that give the most consistent and s atisfactory results.
4.17.4 Use of Internal Vibrators C4.17.4 Use of Internal Vibrators Workers shall be trained in the proper use of internal
vibrators. Vibrators shall not be allowed to contact forms for exposed concrete surfaces. Internal vibrators shall not be inserted closer to the form than 2 to 3 in.
(50 to 75 mm). Care shall be taken to avoid vibrator contact with the reinforcement and to avoid displacing cast-in hardware. When internal vibrators are used to consolidate concrete around epoxy-coated steel reinforcing bars, the vibrators shall be equipped with rubber or nonmetallic vibrator heads to avoid damaging the epoxy coating
Experienced and competent vibrator operators working with regularly maintained vibrators and sufficient standby units are essential to a satisfactory consolidation program. There is a tendency for inexperienced vibrator operators to merely flatten the batch.
The distance between vibrator insertions should generally be about 1 to 1-1/2 times the radius of action, or such that the area visibly affected by the vibrator overlaps the adjacent, vibrated area by a few inches. The vibrator shall not be inserted within 2 ft (0.6 m) of any leading (unconfined) edge.
The vibration frequencies that affect the aggregates and fines of a concrete mix range from 1,200 to 14,000 vibrations per minute (VPM). The lower frequencies activate the large diameter particles, and the higher frequencies affect the fines and cement in the mix. High frequency vibration better ensures complete coverage of larger particles with paste. The movement of the aggregate particles caused by vibration allows trapped water and air bubbles to move upward. This aspect of consolidation is greatly facilitated by a sufficient quantity of mortar.
Vibrators that contact the forms of exposed concrete
DIVISION 4 CONCRETE
Standard Commentary
Page 4.38 MNL-116 4th Edition
surfaces may mar and disfigure the concrete surface.
Internal vibrators inserted close to the form may cause the coarse aggregate to be driven away from the form face producing local pockets of fine material (stinger marks) on the surface. Vibration of reinforcement may cause reinforcing steel reflection features visible in the surface of the finished product.
To consolidate “flowing” or high slump concrete, use a large amplitude (i.e., large diameter) internal vibrator inserted at a close spacing and withdrawn slowly. While consolidating in this manner, the surface should be examined for evidence of excess water or paste. When this occurs, the consolidation effort should be reduced.
The high-slump, self-leveling characteristics of flowing concrete may appear to not need consolidation.
However, as a minimum, nominal vibration should be provided to eliminate large air voids.
4.17.5 Use of External Form Vibrators C4.17.5 Use of External Form Vibrators The size and spacing of form vibrators shall be such
that the proper intensity of vibration is distributed evenly over the desired area of the form.
Proper spacing of external form vibrators is a function of the type, stiffness and shape of the form, depth and thickness of the concrete, force output and frequency of vibrator, workability of the mix, and vibrating time.
Determining the optimum placement and spacing of external vibrators may require trial production runs.
Forms should be placed on elastomeric isolation pads or other resilient-based material to prevent transmission and loss of vibration energy to the supporting foundation.
4.17.6 Use of Surface Vibrators C4.17.6 Use of Surface Vibrators Surface vibrators shall be designed and used in a
manner that prevents separation or displacement of the mix as a result of suction between the concrete and the vibrator surface. Surface vibrators shall be moved at a rate sufficient to embed the coarse aggregate and bring a sufficient quantity of paste to the surface for finishing. The vibration and rate of movement shall be sufficient to compact the full depth of the concrete layer.
Surface vibrators are used to consolidate thin layers of precast concrete mixes. Typical types of surface vibrators include vibrating screeds, vibratory tampers, hand jitterbugs, and bull floats with mounted vibrators.
Vibrators mounted on bull floats should be mounted so that the shaft of each vibrator rotates in opposite directions.
If grate tampers are used, the concrete slump shall not be over 2 in. (50 mm). Vibrating grate tampers shall not be used for structural lightweight aggregate concrete.
4.17.7 Use of Vibrating Tables C4.17.7 Use of Vibrating Tables
Care shall be taken to ensure the proper distribution of vibration when vibrating tables are used. The number and location of external vibrators to be used on a vibrating table shall be determined on the basis of adequate amplitudes of vibration and uniform distribution over the entire concrete surface. The frequency and amplitude shall be checked at several points on the table, using a vibrograph, vibrating reed tachometer, or other suitable methods. The vibrators shall be positioned to ensure dead spots are eliminated and the most uniform vibration is attained.
Vibrating tables or casting decks are best used for flat or low-profile units, and provide an easy and effective method for application of external vibration.
The frequency and amplitude of vibrating forms and vibrating tables equipped with external vibrators should be determined at sufficient points to establish the level of uniformity.