MEMORANDUM
To:
From:
Date:
Re:
Guidelines for Concrete Acceptance
Air Content:
1.
Specification requires air content to be in the range of 5-7 percent.
2.
Anytime the measured air content is outside the range of 5-7 percent, the Contractor should be
notified immediately and asked what steps will be taken to correct the air content for the next
truck.
3.
Occasional (isolated truck) with an air content in the range of 7 to <9 percent is acceptable.
However, the Contractor should be notified immediately. If the next truckload is not within the
specified range (5-7 percent), it should be rejected.
4.
Any concrete with air content at 9 percent or above should be rejected.
5.
Air content of below 4 percent in any exterior above-grade placement or placement near the water
surface should be rejected.
Water/Cement Ratio and Slump:
1.
Water/cement ratio is more important than slump as a guide to monitor concrete quality. The
maximum water/cement ratio for mix “B” on this project is ____________. An occasional
(isolated truck) with a water/cement ratio of ____________ could be allowed. Any second truck at
____________ should be rejected. Any truck over ____________ should be rejected.
2.
Water/cement ratio should be checked on every truckload of concrete.
3.
After plasticizer addition, the allowable target slump should be 6 inches. After plasticizer addition,
a load with a slump >8 inches should be rejected.
4.
Retarder and plasticizer should not be added together unless the Contractor prepares and submits
for approval, a new trial mix.
TEMPERATURE:
Hot Weather:
1.
Visit the batch plant to find out how they are going to deal with hot temperatures.
2.
Using water drawn out of a vat filled with ice OR adding shaved ice directly to the mix. (Ice has to
be accounted for in water/cement ratio).
3.
Specification limits concrete as-placed temperature at 85°F.
4.
Concrete may arrive at the job site hotter as the day progresses, so if the first truck is 86°F or
hotter, reject it.
5.
Hot concrete will require more water to be workable. Maintain water/cement ratio.
6.
Don’t give more than a few degrees once a concrete placement has started. Judgment needs to be
used here. If the batch plant is nearby, reject a subsequent load at 87.0°F or higher. If the truck
travels 30 minutes to the job site, and the batch plant is making an effort to cool the concrete,
maybe go to 88.0°F, before the subsequent trucks are rejected.
7.
If the Contractor elects to use “rejected” concrete, immediately notify the Contractor of potentially
defective work, in writing, and make good notes to document where the concrete was placed.
8.
High temperatures will decrease strength, increase potential for cracking, and lead to rapid set and
poor finish.
Cold Weather:
1.
Visit the batch plant to find out how they are going to heat the concrete. The choice is usually
limited to using hot water. Does the plant have adequate hot water capacity? An electric tank is
probably not going to work past one load. A couple of high recovery gas or oil hot water heaters
will be needed.
2.
In fall, start a log to track mean daily temperature. Get data from local weather service.
3.
Concrete, as placed, must be at or above 55°F.
4.
The Contractor could mix on site for an extended period (5-10 minutes) to raise the temperature to
55°F, but he must be able to discharge completely within the 90 minute time which started at the
batch plant mixing.
(continued)
5.
Reject any concrete ready to place at 53°F or lower. If the Contractor elects to place “rejected”
concrete, put him on notice, in writing, that concrete is considered as potentially defective,
document exactly where it is being placed. Field cured test cylinders are required.
6.
Curing is still required. Cover with poly and/or use 2 heavy coats of curing compound. Maintain
moist surface.
7.
In heated enclosures, poly placed directly on the concrete is required to avoid surface drying due to
the applied heat.
8.
Continuous spraying may over-cool the concrete and/or lead to icing conditions.
9.
Curing compounds are allowed only during cold weather concreting; see Specification Section
03370 for specific cold weather requirements.
Unit Weight:
1.
Ref. ACI 211 and ASTM C138.
2.
Monitor fresh concrete properties (slump, air content, and unit weight) to verify consistency of
materials, mixing, and final product.
3.
With a coarse aggregate of 3/4” to 1-1/2” size, typical unit weight will be between 137-145 pcf.
4.
Unit weight is measured after every air content test.
5.
After several measurements for the project are recorded and then a change in unit weight is
observed (other factors remaining constant such as air content and batch weights), the following
can be realized:
Low Unit Weight
Concrete will have a lower than planned
cement content
Probably a batching error
Will most likely produce a lower strength
concrete
Consider rejecting the truck of concrete
when low strength could be a problem *
* Consult with the Structural Group when there is a question.
High Unit Weight
Concrete will have a higher than planned
cement content
Probably of batching error
Will most likely result in higher number
of shrinkage cracks
Consider rejecting the truck of concrete
when aesthetics and/or leakage could be a
problem *
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