Building Design and Construction Handbook - Merritt - Ventech!

4.22 SECTION FOUR
4.17.1 Normal-Weight Concrete
The nominal weight of normal concrete is 144 lb/ft 3 for non-air-entrained concrete,
but is less the air-entrained concrete. (The weight of concrete plus steel reinforcement
is often assumed as 150 lb/ft 3 .)
Strength for normal-weight concrete ranges from 2000 to 20,000 psi. It is generally
measured using a standard test cylinder 6 in in diameter by 12 in high. The
strength of a concrete is defined as the average strength of two cylinders taken from
the same load and tested at the same age. Flexural beams 6 � 6 � 20 in may be
used for concrete paving mixes. The strength gains of air-entrained and non-airentrained
concretes are graphically shown in Fig. 9.2.
As illustrated in Fig. 9.2, the strength of a given mix is determined by the watercement
ratio (W/C), and whether or not air entraining is used. Other factors are
the maximum-size aggregate and the desired fluidity (slump) of the concrete at the
point of placement. When no historical record is available for the aggregates and
cements to be used, the water-cement ratios in Table 9.2 can provide guidance for
the initial designs.
Each combination of coarse and fine aggregates has a specific water demand for
a given mix fluidity, or slump. Two general guidelines are:
1. For a constant slump, the water demand increases with increase in maximumsize
aggregate.
2. For a constant maximum-size aggregate, as the slump increases, the water demand
increases.
There are many different methods for designing a normal-weight concrete mix.
A standard method is given in ACI 211, ‘‘Standard Practice for Selecting Proportions
for Normal, Heavyweight, and Mass Concrete.’’ See also Art. 9.10.
Workability of a concrete is the property most important to contractors who
must place the concrete into forms and finish it. Workability includes the properties
of cohesiveness, plasticity, and nonsegregation. It is greatly influenced by aggregate
shape and gradation. Mixes that are hard to pump, place, and finish include those
deficient in fines, those with flat and elongated aggregates, and those with an excessive
amount of fines (sand and cement). If the sand is deficient in fines, workability
can be increased by addition of 30 to 50 lb/yd3 of fly ash. The most effective
method of producing workable concrete is to employ a well graded, combined
aggregate gradation.
Modulus of elasticity of normal-weight concrete is between 2,000,000 and
6,000,000 psi. An estimate of the modulus of elasticity for normal-weight concrete
with compressive strengths between 3000 and 5000 psi can be obtained by
ƒ�c ƒ�c multiplying the square root of by 57,000. Above 5000 psi, the modulus should
be determined using the procedure of ASTM C469. [See also Eq. (4.1) in Art.
4.17.2.]
Volume changes occur as either drying shrinkage, creep, or expansion due to
external thermal sources. Drying shrinkage causes the most problems, because it
produces cracks in the concrete surface. The primary cause of drying shrinkage
cracks is an excessive amount of water in the mix. The water has two effects. First,
it increases the water-cement ratio (W/C), weakening the concrete. Second, additional
water beyond that needed for hydration of the cement creates an excessive
number of bleed channels to exposed surfaces. When the cement paste undergoes
its normal drying shrinkage, these channels cannot provide any resistance to penetration
of water or aggressive chemicals.