Building Design and Construction Handbook - Merritt - Ventech!

9.16 SECTION NINE
record of field test results is not available, the ACI 318 Building Code, with several
restrictions, permits the use of trial batches as a basis for selecting initial proportions.
This condition is likely to occur when new sources of cement or aggregate
are supplied to an established plant, to a new facility, such as a portable plant on
the site, or for the first attempt at a specified strength more than 1000 psi above
previous specified strengths.
The ACI 318 Building Code includes provisions for proportioning concrete
mixes based on other experience or information, if approved by the Engineer. This
alternative procedure is restricted to proportioning concrete with a specified ƒ�c �
4000 psi. The required average compressive strength must be at least 1200 psi
greater than ƒ�. Concrete proportioned by this procedure must also conform to the
c
Code’s durability requirements. These provisions are intended to allow the construction
work to continue when there is an unexpected interruption in concrete supply
and time does not permit tests and evaluation. These provisions are also aimed at
small projects where the cost of trial batches is not justified.
The initially established proportions can be used during progress of a project
only as long as the strength-test results justify them. The process of quality control
of concrete for a project requires maintenance of a running average of strength-test
results and changes in the proportions whenever the actual degree of control (standard
deviation s) varies from that assumed for the initial proportioning. Equations
(9.3) and (9.4) are applied for this analysis. With project specifications based on
the ACI 318 Building Code, no minimum cementitious-materials content is required;
so good control during a long-time project is rewarded by permission to
use a lower cementitious-materials content than would be permitted with inferior
control.
Regardless of the method used for proportioning the basic initial proportions
should be based on mixes with both air content and slump at the maximum permitted
by the project specifications.
TABLE 9.3 Required Air Entrainment in
Concrete Exposed to Freezing and Thawing
Nominal maximum
size of coarse
aggregate, in.
Total air content,
% by volume
Severe
exposure
Moderate
exposure
3
⁄8
1 7 ⁄2 6
1
⁄2 7 1 5 ⁄2
3
⁄4 6 5
1 6 41 ⁄2
11 ⁄2 51 ⁄2 41 ⁄2
2 5 4
3 41 ⁄2 31 ⁄2
* From ACI 318-99, Table 4.2.1. for ƒ�c � 5000 psi,
air content may be reduced 1%. ‘‘Severe exposure’’ is
where concrete in a cold climate may be in almost
continuous contact with moisture prior to freezing, or
where deicing salts are used. ‘‘Moderate exposure’’ is
where concrete in a cold climate will only be exposed
to moisture prior to freezing and where no deicing salts
are used.
ƒ� c
ƒ� cr