Comisiones de Reconocimientos de Estudios

C4.15.1 General

Concrete of satisfactory quality requires the materials to be thoroughly mixed until there is a uniform distribution of the materials and the mix is uniform in appearance.

The necessary mixing time will depend on many factors including batch size, workability of the batch, size and grading of the aggregate, type of mixer, condition of blades, and ability of the mixer to produce uniform concrete throughout the batch and from batch to batch.

Some mixes require longer mixing times to improve workability and help eliminate the potential for surface voids and other consistency related problems.

4.15.2 Methods of Concrete Mixing C4.15.2 Methods of Concrete Mixing Concrete shall be mixed by one of the following

methods:

Definitions:

1. Central mixed concrete 1. Concrete mixed in a central stationary mixer and delivered to the casting area by buckets, truck mixer, truck agitator or nonagitating trucks.

2. Shrink mixed concrete 2. Concrete that is partially mixed in a stationary mixer then mixed completely and delivered to the casting site in a truck mixer.

3. Truck mixed concrete 3. Concrete that is completely mixed in a truck mixer as it is delivered to the casting site.

All requirements for mixing of concrete, as given in this Article, are valid for both normal weight aggregates as specified in ASTM C33 and for lightweight aggregates for structural concrete, as specified in ASTM C330.

The time from the start of concrete mixing to placement shall not exceed 1 hour. Retempering (with water) of concrete which has started to stiffen shall not be allowed.

The practice of adding superplasticizer to counteract slump loss must be carefully monitored to avoid potential uniformity problems.

DIVISION 4 CONCRETE

Standard Commentary

Page 4.28 MNL-116 4th Edition

4.15.3 Mixing Time and Concrete Uniformity Mixing time shall be measured from the time all cement and aggregates are charged into the mixer drum. All water shall be in the drum by the end of the first one-fourth of the established mixing time.

C4.15.3 Mixing Time and Concrete Uniformity Both overmixing and undermixing are to be avoided.

Undermixing will result in concrete of variable consistency and low strength. Overmixing may result in a reduction of air in air-entrained mixes, grinding of aggregates, and loss of workability. Concrete mixing procedures should be established for each type of mixer.

Variations in mix designs such as those for lightweight concrete and the use of superplasticizers may require adjustments to standard mixing procedures.

Mixing time for air-entrained concrete shall be verified and controlled to ensure the specified air content.

For a given dosage of air-entraining admixture, the amount of entrained air will vary with the type and condition of the mixer, the amount of concrete being mixed, mixing speed, and the mixing duration. The entrained air content will generally increase as mixing conditions are optimized. Poor mixing conditions will not facilitate the full potential of air entrainment, and an excessive rate of mixing may reduce the entrained air content. The amount of entrained air generally increases with mixing time up to about 5 minutes, beyond which it slowly decreases. However, the air void system, as characterized by specific surface and spacing factors, generally is not harmed by prolonged agitation.

The mixing time required for each batch shall be based on the ability of the mixer to produce uniform concrete throughout a given batch and between batches. For each type of mixer, the optimum mixing time shall be established by the manufacturer’s recommendations. The mixer performance shall be determined from tests using two samples taken no more than 15 minutes apart. The first sample shall be obtained after discharge of approximately 15% of the batch and the second after 85% of the batch.

Scheduling of operations should be controlled to prevent delays in the period between charging the mixer and depositing the concrete in the forms. Such delays may result in nonuniform concrete.

When uniformity sampling is performed, slump tests, taken at the time of sampling, shall be made to check the relative degree of uniformity. If the slumps differ more than the specified values in Table 4.15.3, the mixer shall not be used unless corrective measures are taken.

Mixers should not be loaded above the rated capacities and should be operated at the speeds recommended by the mixer manufacturer. It may prove beneficial to reduce the batch size below the rated capacity to ensure more efficient mixing. Increased output should be obtained by using a larger mixer or additional mixers, rather than by speeding up or overloading the mixer. If the blades of the mixer become worn or coated with hardened concrete, the mixing action will be less efficient. Badly worn blades should be replaced, and hardened concrete should be removed periodically, preferably after each production day.

Table 4.15.3 Requirements for Uniformity of Concrete (from ASTM C94)

Test

Maximum Allowable Deviation Between Samples Weight per cubic foot calculated to

an air-free basis, lb/ft³ 1.0

Air content, volume % of concrete 1.0 Slump:

If average slump is 4 in. or less, in.

If average slump is 4 to 6 in.

If superplasticizers used in mix, in.

1.0 1.5 2.5

Coarse aggregate content, portion by weight of each sample retained on No. 4 sieve, %

6.0

Unit weight of air-free mortar based on average for all comparative samples tested, %

1.6

4.15.4 Mixing Time − Stationary Mixers C4.15.4 Mixing Time − Stationary Mixers Unless otherwise recommended by the mixer

manufacturer, the minimum mixing time for stationary mixers shall be one minute for batches of 1 cubic yard (0.76 cubic meter) or less. This mixing time shall be increased by at least 15 seconds for each cubic yard, or fraction thereof, in excess of 1 cubic yard (0.76 cubic meter). The mixing time shall be increased when necessary to ensure the required uniformity and consistency of the concrete. Both undermixing and overmixing shall be avoided.

Mixing time less than that specified by the manufacturer may be permitted provided performance tests indicate that the time is sufficient to produce uniform concrete.

For stationary mixers, mixing time should not exceed the manufacturer’s recommended optimum mixing time by more than three times. In the event a batch has to be held in the mixer for a longer period, the speed of the drum blades should be reduced to agitating speed, or the mixer should be stopped in intervals to prevent overmixing.

4.15.5 Mixing Time − Shrink Mixing 4.15.5 Mixing Time − Shrink Mi xing When a stationary mixer is used for the partial mixing

of the concrete during shrink mixing operations, the time may be reduced to a minimum of 30 seconds, followed by not less than 50 revolutions or more than 100 revolutions at mixing speed in a truck mixer. Any additional revolutions in the truck mixer shall be at agitating speed in accordance with mixer manufacturer’s recommendations.

4.15.6 Mixing Time − Truck Mixing C4.15.6 Mixing Time − Truck Mixing

For mixing in a truck mixer loaded to the maximum When truck mixing is used, depending on the

DIVISION 4 CONCRETE

Standard Commentary

Page 4.30 MNL-116 4th Edition

rated mixing capacity, the number of revolutions of the drum or blades at mixing speed shall not be less than 70 nor more than 100, unless special conditions necessitate additional mixing time. If the batch is at least 1/2 cubic yard (0.4 cubic meter) less than the maximum mixer capacity, the number of revolutions at mixing speed may be reduced to 50. All revolutions in excess of 100 shall be at agitating speed per the mixer manufacturer’s recommen-dations.

circumstances involved, it may be necessary to develop a focused quality control effort to ensure that the truck drivers perform only authorized actions in terms of mixing speed, mixing time, and the addition of water.

A minimum of 100 revolutions at a speed of at least 15 revolutions per minute shall be used for mixing concrete containing silica fume or metakaolin. When using dry, densified silica fume, these requirements shall be increased to 120 revolutions at a minimum speed of 15 revolutions per minute. Additionally, the load size shall be limited to not more than 75% of the rated mixing capacity.

4.15.7 Special Batching and Mixing Requirements for Lightweight Aggregates

C4.15.7 Special Batching and Mixing Requirements for Lightweight Aggregates

Lightweight aggregate concrete shall be batched and mixed as recommended by the producer of the aggregate. Batch weights of lightweight aggregates shall be based on oven-dry weights corrected for absorbed moisture and surface moisture. In some instances, the accurate control of concrete with lightweight aggregate is more feasible measuring by bulk volume rather than by weight. In such cases, batching shall be in accordance with ASTM C685.

ACI 211.2, Standard Practice for Selecting Proportions for Structural Lightweight Concrete, provides additional information on the use of lightweight aggregates.

When a lightweight concrete mix is used for the first time, the mixing and handling procedures should be checked to ensure they are adequate to produce uniform concrete of the quality specified. In such a case, the aggregate producer’s recommendations may be particularly helpful.

Lightweight aggregate may require predampening prior to batching. Aggregate shall be tested for water absorption at the minimum moisture content likely to occur during production. For aggregates having less than 10% total absorption by weight or shown to absorb less than 2% water by weight during the first hour after immersion in water, predampening prior to mixing is not required. If the lightweight aggregate absorbs more than that shown above, it shall be predampened and a mixing procedure developed that is shown to produce concrete of uniform quality.

Knowledge of the aggregate water absorption characteristics simplifies the process of batching lightweight mixtures. This also helps to optimize mixing time. If the lightweight aggregate unit weight is variable, then batching by volume rather than by weight is necessary.

Dampening of the aggregates may be carried out by submerging the lightweight aggregate or by use of a sprinkling system.

4.15.8 Cold Weather Mixing C4.15.8 Cold Weather Mixing Concrete temperatures at the mixer shall be

maintained above a minimum of 50°F (10°C). Materials shall be free from ice, snow, and frozen lumps before entering the mixer.

When concrete temperatures are less than 50°F (10°C), the time required for the concrete to gain strength is greatly extended or strength gain may actually stop.

When exposure to cold weather is severe, due either to low air temperature or because the concrete sections are thin, the temperature of the fresh concrete shall be increased by heating to ensure the concrete temperature does not fall below 50°F (10°C).

When it is necessary to heat the concrete component materials, the aggregates shall not be heated above 180°F (82°C).

The benefits and associated cost of heating the components of the fresh concrete should be balanced against the benefits and costs of heating the concrete after placement and during curing.

To avoid the possibility of premature stiffening or flash set of the cement when either the water or aggregates are heated to a temperature over 100°F (38°C), the water and aggregate shall come together first in the mixer. This is to ensure that the temperature of the combination is reduced to a temperature that is below 100°F (38°C) before the cement is added.

Where hot water is used for maintaining a minimum concrete temperature, provisions should be made for the operator to read the temperature of the water before it enters the mixer and after possible blending with cold water.

Liquids or slurries shall be protected from freezing.

Powdered materials shall be protected from moisture that may freeze. Materials that have been damaged by frost, deteriorated, or contaminated shall not be used in the production of concrete.

4.15.9 Hot Weather Mixing C4.15.9 Hot Weather Mixing

The concrete temperature at the mixer shall be

maintained below a maximum of 95°F (35°C). Care should be taken in hot climates to protect batch- material storage bins and waterlines from direct sun. The temperature of the mixing water and aggregate play a more important role in determining the concrete temperature than does the temperature of the cement. To avoid mixing and uniformity problems, it is not advisable to substitute ice for all of the mixing water when admixtures are used in the concrete mix.

If high temperatures are encountered, the mix ingredients shall be cooled before or during mixing.

Flake ice, or well-crushed ice of a size that will completely melt during mixing may be substituted for a portion of the mixing water. Liquid nitrogen may also be used to cool the aggregates.

As the concrete temperature increases, the setting rate also increases. Because higher temperatures generally reduce the time period for optimal workability, proper scheduling of the concrete placement is critical.

4.16 Requirements for Transporting