Building Design and Construction Handbook - Merritt - Ventech!

9.2 SECTION NINE
cement, blended hydraulic cements, or expansive cement, or in combination with
fly ash (ASTM specification C618), raw or calcined natural pozzolans (ASTM
C618), ground granulated blast-furnace slag (ASTM C989), or silica fume (ASTM
C1240). Addition to a concrete mix of fly ash, silica fume, or slag decreases permeability,
protects reinforcement, and increases strength. Concrete made with polymers,
plastics with long-chain molecules, can have many qualities much superior
to those of ordinary concrete. See also Sec. 4.
9.2 CEMENTS
The ACI 318 Building Code requires cement to conform to ASTM C150, ‘‘Standard
Specification for Portland Cement;’’ or ASTM C595, ‘‘Standard Specification for
Blended Hydraulic Cements;’’ or ASTM C845, ‘‘Standard Specification for Expansive
Hydraulic Cement.’’ Portland cements meeting the requirements of ASTM
C150 are available in Types I to V and air-entraining Types IA to IIIA for use
under different service conditions. The ACI 318 Building Code prohibits the use
of slag cement, Types A and SA (ASTM C595), because these types are not intended
as principal cementing constituents of structural concrete.
Although all the preceding cements can be used for concrete, they are not interchangeable.
Note that both tensile and compressive strengths vary considerably,
at early ages in particular, even for the five types of basic portland cement. Con-
sequently, although project specifications for concrete strength are usually based
on a standard 28-day age for the concrete, the proportions of ingredients required
differ for each type. For concrete strengths up to 19,000 psi for columns in highrise
buildings, specified compressive strengths are usually required at 56 days after
initial set of the concrete. For the usual building project, where the load-strength
relationship is likely to be critical at a point in strength gain equivalent to 7-day
standard curing (Fig. 9.1), substitution of a different type (sometimes brand) of
cement without reproportioning the mix may be dangerous.
The accepted specifications (ASTM) for cements do not regulate cement temperature
nor color. Nevertheless, in hot-weather concreting, the temperature of the
fresh concrete and therefore of its constituents must be controlled. Cement temperatures
above 170�F are not recommended (‘‘Hot Weathering Concreting,’’ ACI
305R).
For exposed architectural concrete, not intended to be painted, control of color
is desirable. For uniform color, the water-cement ratio and cement content must be
kept constant, because they have significant effects on concrete color. Bear in mind
that because of variations in the proportions of natural materials used, cements from
different sources differ markedly in color. A change in brand of cement therefore
can cause a change in color. Color differences also provoke a convenient check for
substitution of types (or brands) of cement different from those used in trial batches
made to establish proportions to be employed for a building.
9.3 AGGREGATES
Only material conforming to specifications for normal-weight aggregate (ASTM
C33) or lightweight aggregate for structural concrete (ASTM C330) is accepted
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