Specification for Concrete Batching and Mixing in Construction Project

Equipment

The Contractor shall provide a modern and dependable wet batch?type mixing plant having sufficient capacity to complete the work within the established progress schedule.  Dry batch-type mixing plant shall not be used. Separate bins shall be provided in the plant for the fine aggregate, the different sizes of coarse aggregate and for bulk cement when used.

Aggregates shall be weighed in separate weight batchers with individual scales, or cumulatively in one weight batcher with a single scale.  Bulk cement shall be weighed on a separate weight batcher.  Water may be measured by weight or by volume.

The concrete mixing equipment shall be capable of combining the materials into a uniform mixture and of discharging this mixture without segregation.

Adequate facilities shall be provided for accurate measurement and control of each of the materials entering each batch of concrete.

Prior to the installation of the batching and mixing plant, aggregate handling facilities and concrete conveying and placing equipment, the Contractor shall submit drawings showing the general arrangement and details of plant and equipment for the approval of the owner.

Cost for the above plant and equipment shall be included in permanent work cost.

Materials delivered to the batching plant shall be within the following limits of accuracy:

Material

% By Weight
Cement

1.0
Water

1.0
Aggregate

2.0
Air-entraining admixture

3.0

Mixing

The first batch of concrete materials placed in the mixer shall contain sufficient excess of cement, sand and water to coat the inside of the drum without reducing the required mortar content of the mix.

The concrete mix shall be air entrained by an approved air entraining agent which shall be separately batched in solution by an accurate mechanical dispenser and distributed uniformly throughout the mix.

The mixing time for each batch, after all ingredients are in the mixer drum, shall be as follows:

                     Capacity of Mixer                      Mixing Time

                        0.38 m3 or less                          1.25 mins

                        0.57 to 1.15 m3                         1.5 mins

                        1.53 and 2.3 m3                        2.0 mins

                        3.06 m3                                     2.5 mins

The mixing periods specified are based on the control of the speed of rotation of the mixer and on the introduction of all of the mixing water before 1/4 of the mixing time has elapsed.  The mixing time shall be increased if, and when, in the opinion of the owner, the charging and mixing operation fails to produce the required uniformity of composition and consistency.

The mixer shall rotate at a uniform speed for a minimum of 12 revolutions after all materials have been placed inside and shall not be charged or operated in excess of the capacity and speed recommended by the manufacturer.  Excessive over-mixing requiring the addition of water to preserve the required concrete consistency shall not be permitted.