Civil Engineering Reference
In-Depth Information
CHAPTER 9
Designing and Proportioning
Normal Concrete Mixtures
The process of determining required and specifiable char-
acteristics of a concrete mixture is called mix design.
Characteristics can include: (1) fresh concrete properties;
(2) required mechanical properties of hardened concrete
such as strength and durability requirements; and (3) the
inclusion, exclusion, or limits on specific ingredients. Mix
design leads to the development of a concrete specification.
Mixture proportioning refers to the process of deter-
mining the quantities of concrete ingredients, using local
materials, to achieve the specified characteristics of the
concrete. A properly proportioned concrete mix should
possess these qualities:
1. Acceptable workability of the freshly mixed concrete
2. Durability, strength, and uniform appearance of the
hardened concrete
3. Economy
Understanding the basic principles of mixture design
is as important as the actual calculations used to establish
mix proportions. Only with proper selection of materials
and mixture characteristics can the above qualities be
obtained in concrete construction (Fig. 9-1) (
Abrams 1918
,
Hover 1998
, and
Shilstone 1990
).
SELECTING MIX CHARACTERISTICS
Before a concrete mixture can be proportioned, mixture
characteristics are selected based on the intended use of
the concrete, the exposure conditions, the size and shape
of building elements, and the physical properties of the
concrete (such as frost resistance and strength) required
for the structure. The characteristics should reflect the
needs of the structure; for example, resistance to chloride
ions should be verifiable and the appropriate test
methods specified.
Once the characteristics are selected, the mixture can
be proportioned from field or laboratory data. Since most
of the desirable properties of hardened concrete depend
primarily upon the quality of the cementitious paste, the
first step in proportioning a concrete mixture is the selec-
tion of the appropriate water-cementing materials ratio
for the durability and strength needed. Concrete mixtures
should be kept as simple as possible, as an excessive
number of ingredients often make a concrete mixture dif-
ficult to control. The concrete technologist should not,
however, overlook the opportunities provided by modern
concrete technology.
Water-Cementing Materials Ratio and
Strength Relationship
Strength (compressive or flexural) is the most universally
used measure for concrete quality. Although it is an impor-
tant characteristic, other properties such as durability, per-
meability, and wear resistance are now recognized as being
equal and in some cases more important, especially when
considering life-cycle design of structures.
Within the normal range of strengths used in concrete
construction, the compressive strength is inversely related
to the water-cement ratio or water-cementing materials
ratio. For fully compacted concrete made with clean,
sound aggregates, the strength and other desirable prop-
Fig. 9-1. Trial batching (in-
set) verifies that a concrete
mixture meets design re-
quirements prior to use in
construction.
(69899, 70008).
