Page 179 - Civil Engineering Formulas

116 CHAPTER FIVE

non-air-entrained concrete and from 0.54 to 0.35 for air-entrained concrete.
2
These values are for a specified 28-day compressive strength f c in lb/in or
2
2
MPa, of 2500 lb/in (17 MPa) to 5000 lb/in (34 MPa). Again, refer to the ACI
Code before making any design or construction decisions.
Maximum w/c ratios for a variety of construction conditions are also listed in
the ACI Code. Construction conditions include concrete protected from exposure
to freezing and thawing; concrete intended to be watertight; and concrete exposed
to deicing salts, brackish water, seawater, etc. Application formulas for w/c ratios
are given later in this chapter.

JOB MIX CONCRETE VOLUME

A trial batch of concrete can be tested to determine how much concrete is to be
delivered by the job mix. To determine the volume obtained for the job, add the
absolute volume V of the four components—cements, gravel, sand, and water.
a
Find the V for each component from
a
V a W L (5.2)
(SG)W u
3
3
where V absolute volume, ft (m )
a
W weight of material, lb (kg)
L
SG specific gravity of the material
3
3
w density of water at atmospheric conditions (62.4 lb/ft ; 1000 kg/m )
u
Then, job yield equals the sum of V for cement, gravel, sand, and water.
a
MODULUS OF ELASTICITY OF CONCRETE
The modulus of elasticity of concrete E —adopted in modified form by the ACI
c
Code—is given by
2
1.5
f
E c 33w c 2 c lb/in in USCS units (5.3)
1.5
f
0.043w c 2 c MPa in SI units
With normal-weight, normal-density concrete these two relations can be simpli-
fied to
2
E c 57,000 2 c lb/in in USCS units (5.4)
f
f
4700 2 c MPa in SI units
2
where E modulus of elasticity of concrete, lb/in (MPa); and f c specified
c
2
28-day compressive strength of concrete, lb/in (MPa).