Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.17
Other ACI 318 Building Code requirements for mix design are:
1. Concrete exposed to freezing and thawing or to deicing chemicals while wet
should have air entrained within the limits in Table 9.3, and the water-cementitious
materials ratio by weight should not exceed 0.45. If lightweight aggregate is used,
ƒ�c should be at least 4500 psi.
2. For watertight, normal-weight concrete, maximum water-cementitious materials
ratios by weight are 0.50 for exposure to fresh water and 0.40 for seawater or
deicing chemicals. With lightweight aggregate, minimum is 4000 psi for concrete
exposed to fresh water and is 5000 psi for seawater or deicing chemicals.
ƒ� c
Although the Code does not distinguish between a ‘‘concrete production facility’’
with in-house control and an independent concrete laboratory control service, the
distinction is important. Very large suppliers have in-house professional quality
control. Most smaller suppliers do not. Where the records of one of the latter might
indicate a large standard deviation, but an independent quality-control service is
utilized, the standard deviation used to select should be based on the proven
record of the control agency. Ideally, the overdesign should be based, in these cases,
on the record of the control agency operating in the concrete plant used.
9.11 CHECK TESTS OF MATERIALS
Without follow-up field control, all the statistical theory involved in mixed proportioning
becomes an academic exercise.
The complete description of initial proportions should include: cement analysis
and source; specific gravity, absorption, proportions of each standard sieve size;
fineness modulus; and organic tests for fine and coarse aggregates used, as well as
their weights and maximum nominal sizes.
If the source of any aggregate is changed, new trial batches should be made. A
cement analysis should be obtained for each new shipment of cement.
The aggregate gradings and organic content should be checked at least daily, or
for each 150 yd 3 . The moisture content (or slump) should be checked continuously
for all aggregates, and suitable adjustments should be made in batch weights. When
the limits of ASTM C33 or C330 for grading or organic content are exceeded,
proper materials should be secured and new mix proportions developed, or until
these measurements can be effected, concrete production may continue on an emergency
basis but with a penalty of additional cement.
9.12 AT THE MIXING PLANT—
YIELD ADJUSTMENTS
Well-equipped concrete producers have continuous measuring devices to record
changes in moisture carried in the aggregates or changes in total free water in the
contents of the mixer. The same measurements, however, may be easily made manually
by quality-control personnel.
To illustrate: for the example in Art. 9.5, the surface-dry basic mix is cement,
564 lb; water, 300 lb; sand, 1170 lb; and gravel, 2000 lb. Absorption is 1% for the
ƒ� cr
ƒ� c