Criterios de resolución

4.16.1 General

C4.16 Requirements for Transporting and Placing of Concrete

C4.16.1 General Proven and effective procedures for placing concrete

are described in detail in the following publications of the American Concrete Institute:

In arranging equipment to minimize separation or segregation, it is important to deposit the concrete vertically into the center of transport containers or during

DIVISION 4 CONCRETE

Standard Commentary

Page 4.32 MNL-116 4th Edition

1. Recommended Practice for Measuring, Mixing, Transporting, and Placing Concrete (ACI 304) 2. Recommended Practice for Hot Weather

Concreting (ACI 305)

3. Recommended Practice for Cold Weather Concreting (ACI 306)

These publications shall be available in the precast concrete plant, and supervisory personnel shall be familiar with the contents.

final placement. The importance of this increases with an increase in slump, maximum size and amount of coarse aggregate, or a reduction in cement content. The height of free fall of concrete need not be limited unless a separation of coarse particles occurs (resulting in honeycomb) or uniformity of appearance is affected; in which case, a limit of 3 to 5 ft (0.9 to 1.5 m) may be adequate. However, to protect spacers, embedded features, form surfaces, and to prevent displacement of reinforcement, concrete fall should be limited to a few feet by means of a suitable drop chute or other devices.

Sufficient mixing and placing capacity shall be provided so that concrete can be provided on a continuous basis, allowing the precast products produced to be free from unintentional cold joints. If delays occur that result in the concrete attaining initial set or loss of plasticity, partially completed products shall be washed out of the forms or rejected.

As concrete is placed in layers to produce a monolithic and visually acceptable finished product, it is important that each layer of concrete be shallow enough so that it may be placed while the previous layer is still plastic.

This will allow for proper consolidation between layers.

4.16.2 Transporting and Placing Concrete C4.16.2 Transporting and Placing Concrete Concrete shall be transported from the mixer to the

forms in the shortest possible time and in such a manner as to prevent segregation or loss of mortar.

The effects of transporting the concrete to the placing site should be evaluated and procedures maintained to avoid undesirable changes to the concrete characteristics. The need to agitate the concrete during transportation will depend on the length of time between completion of mixing and placement.

The use of nonagitating trucks to transport concrete containing silica fume or metakaolin is not recommended.

Regardless of the manner of transportation, concrete shall be discharged into the forms while in its original mixed or plastic state. Retempering by adding water and remixing concrete that has started to stiffen shall not be permitted.

Before beginning casting operations, hardened concrete and foreign matter shall be removed from the surfaces of the transportation and placing equipment.

Free water or excess grout shall not be permitted to drip from transport equipment onto previously finished concrete.

As concre te is placed into the forms, the paste coating of strand or mild steel should be of no concern up to the top of the section of the precast element. However, cement paste should be kept from or cleaned from all reinforcing or embedments that will extend out of the concrete section.

4.16.3 Preventing Aggregate Segregation C4.16.3 Preventing Aggregate Segregation Procedures and arrangements of equipment shall be

used that result in placing concrete in a uniform and compacted condition without segregation. Placing methods shall preserve the quality of the concrete in

Obvious clusters and pockets of coarse aggregate are objectionable and shall be scattered prior to covering with subsequent lifts of concrete to ensure against rock pockets and honeycombing in the completed unit.

terms of w/cm, slump, uniformity, air content, and homogeneity.

Scattered individual pieces of separated coarse aggregate are not objectionable if readily enclosed and consolidated into the concrete.

Concrete should not be deposited in the forms and then leveled or moved horizontally into final position. Such practices result in segregation, as the mortar tends to flow ahead of coarser material. This can result in visible flow lines on the exposed surface. Placing concrete as near to its final location as possible minimizes segregation, the potential for increased entrapped air, and other surface-related aesthetic concerns. Once concrete segregates due to improper handling, the segregation cannot be corrected by subsequent placing and consolidation operations.

4.16.4 Preparation of the Forms C4.16.4 Preparation of the Forms Prior to concreting, the forms shall be cleaned. Loose

and other unwanted materials, such as tie wire clippings, shall be removed by compressed air, vacuum cleaning, or other acceptable methods. If required, a form release agent shall be uniformly applied to all form surfaces that will be in contact with the concrete to provide easy release and stripping of the element from the forms. Form release agent shall be applied prior to placement of the steel and shall provide for the required concrete surface finish.

Preparation of the forms also involves the setting of form surfaces, bulkheads, and features to within specified dimensional tolerances.

4.16.5 Placing Concrete Under Severe Weather Conditions

C4.16.5 Placing Concrete Under Severe Weather Conditions

Freshly deposited concrete shall be protected from freezing, excessively high or differential tem-peratures, premature drying, and moisture loss during the curing period.

Using weather data for the production area, temperature extremes and durations can be identified. A plan of procedures for production response to severe weather conditions should be developed for use whenever weather conditions dictate.

Concrete shall be placed at a temperature between 50°F (10°C) and 95°F (35°C) unless placement at a higher temperature is confirmed to be acceptable in accordance with ACI 305, Recommended Practice for Hot Weather Concreting. Measures shall be taken during severe weather conditions to ensure the concrete remains plastic during placement and finishing operations and that favorable curing conditions are provided thereafter.

The optimum temperature for concrete placement is recommended as 70°F (21°C).

DIVISION 4 CONCRETE

Standard Commentary

Page 4.34 MNL-116 4th Edition

4.16.6 Placing Concrete in Wet and Rainy Conditions

The producer shall have adequate weather protection provisions on hand at all times for outside production activities. It shall be possible to deploy the weather protection provisions without compromising the quality of the product.

4.16.6 Placing Concrete in Wet and Rainy Conditions

4.16.7 Placing Concrete in Hot or Windy Conditions

C4.16.7 Placing Concrete in Hot or Windy Conditions

For concrete placed in hot weather conditions, the temperature shall not be above 95°F (35°C) unless placement at a higher temperature is confirmed to be acceptable in accordance with ACI 305, Recommended Practice for Hot Weather Concreting.

Adequate moisture shall be retained in the concrete during the curing period to prevent surface drying. The temperature of the concrete at placement shall not be so high as to cause difficulty from loss of slump, flash set, or cold joints.

The upper limit of 95°F (35°C) for concrete placement temperature is a guideline that may need to be lowered to satisfy specific conditions or requirements. If problems are encountered using 95°F (35°C) as an upper limit, the maximum temperature should be reduced until problems are eliminated.

If hot and/or drying weather conditions occur, any of the following procedures or combination of procedures may be used to prevent plastic shrinkage cracking or loss of strength of the concrete:

1. Shaded storage for aggregates.

2. Sprinkling, fog spraying, or chilling aggregates with liquid nitrogen.

3. Burying, insulating, and/or shading the water-supply facilities.

4. Use of cold water in batching.

5. Use of shaved or crushed ice for a portion of the mixing water. Only as much ice should be used as will be entirely melted at the completion of the mixing period.

6. Maintaining concrete surfaces in a cool and moist condition by use of wet coverings such as burlap, sprinkling, or ponding as soon as the exposed concrete surfaces are finished or as soon as the water sheen disappears. This is especially important for exposed locations in hot and windy conditions.

7. Shading of the product surface during and after casting to avoid heat buildup in direct sunlight.

8. Use of cement with temperatures under 170°F (77°C), unless special measures to control concrete temperature as outlined in ACI 305, Recommended Practice for Hot Weather Concreting, are used and verified to produce concrete of acceptable performance.

9. White-pigmented membranes may be used but are not recommended in very hot weather until after the first 24 hours, as such membranes do not cool the

concrete as well as wet curing methods.

10. Set-retarding admixtures, which can delay the concrete setting time and provide a longer period for placing and finishing.

11. When the temperature of steel is greater than 120°F (49°C), steel forms and reinforcement should be sprayed with water just prior to placing the concrete.

The surface of the forms should be free of visible water droplets prior to placing the concrete to avoid potential surface finish problems.

The requirements of ACI 305, Recommended Practice for Hot Weather Concreting, shall be observed when these conditions are present.

Wind or direct sunlight can have significant and rapid drying and shrinkage effects on fresh concrete. These along with low humidity are conditions that should be considered and accounted for during placement of concrete. When the ambient temperature is above 80°F (27°C), steps should be taken to protect the concrete from the effects of hot and/or drying weather conditions.

4.16.8 Placing Concrete in Cold Weather Conditions

C4.16.8 Placing Concrete in Cold Weather Conditions

Special precautions shall be taken to protect concrete placements in cold weather that is below 40°F (5°C) to ensure that the concrete gains strength under favorable curing conditions. The requirements of ACI 306, Recommended Practice for Cold Weather Concreting, shall be observed.

Concrete can be placed successfully when ambient temperature falls below 40°F (5°C). This is done by heating the forms to maintain a minimum concrete temperature of 50°F (10°C). The existence of windy and cold conditions may dictate added protection to maintain the temperature of the forms and the concrete after placement.

The combined temperature of the concrete and forms should ensure that the concrete has a minimum temperature of 50°F (10°C) after placement. Concrete curing temperatures must be maintained above 50°F (10°C) for the cement hydration process to properly progress.

4.16.9 Placing Concrete C4.16.9 Placing Concrete

Concrete shall be carefully placed, as nearly as possible, in final position and worked into all details of the forms. The whole mass shall be consolidated by proper vibration. Once placed, concrete shall only be spread with solid-faced tools to prevent concrete segregation. It shall be placed in successive layers and in a manner that the flow is away from corners and ends.

When mixtures of dry or stiff consistencies are required, the placement rate should be slow enough to permit proper vibration. This will help to prevent bug holes and honeycombing. However, placing concrete too slowly can produce layer lines or cold joints due to improper consolidation. The rate of placement and vibration (intensity and spacing) should be selected to minimize entrapped air in the concrete. Thin, even layers of 6 in.

(150 mm) or less can generally be consolidated with minimum occurrence of air voids, while thicker layers will increase the possibility of trapping more and larger air voids.

DIVISION 4 CONCRETE

Standard Commentary

Page 4.36 MNL-116 4th Edition

Concrete shall not be subjected to any procedure that will cause segregation or flow lines. Concrete placing on a slope shall begin at the lower end of the slope and progress upward thereby increasing consolidation of the concrete and avoiding segregation.

4.17 Consolidation of Concrete

4.17.1 General

Concrete shall be uniformly consolidated by internal or external vibration, spading, impact, or a combination of these methods that ensures adequate consolidation of the concrete comprising the finished product.