PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 299

7.3 A project specifies a concrete strength of at least 3000 psi. Materials

engineers will design the mix for a strength higher than that. Calcu-
late the required average compressive strength of the mix design if
the standard deviation is Estimate the modulus of elas-
ticity of the concrete at the required average compressive strength
(the calculated strength, not the given strength).

s = 350 psi.

7.4 What is your recommendation for the maximum nominal size of

coarse aggregate for the following situation?
A continuously reinforced concrete pavement cross section contains
a layer of No. 6 reinforcing bars at 6-inch centers, such that the steel
is just above mid-depth of a 10-inch thick slab. Cover over the top of
the steel is therefore about 4 inches.

7.5 A concrete mix with a 3-inch slump, w/c ratio of 0.50, and sand

with a fineness modulus of 2.4 contains 1700 lb of coarse aggregate.
Compute the required weight of coarse aggregate per cubic yard. To
adjust the mix so as to increase the compressive strength, the water-
to-cement ratio is reduced to 0.45. Will the quantity of coarse aggre-
gate increase, decrease, or stay the same? Explain your answer.

7.6 You are working on a concrete mix design that requires each cubic

yard of concrete to have 0.45 water-to-cementitious materials ratio,
of dry gravel, 4% air content, and of cement.
The available gravel has a specific gravity of a mois-
ture content of 1.6%, and absorption of 2.4%. The available sand
has a specific gravity of a moisture content of 4.8%, and
absorption of 1.5%. For each cubic yard of concrete needed on the
job, calculate the weights of cement, moist gravel, moist sand, and
water that should be added to the batch. Summarize and total the
mix design when finished (don't include air entrainer in summary).

1963 lb/yd 3 565 lb/ft 3

G gravel = 2.7,

G sand = 2.5,

7.7 Design the concrete mix according to the following conditions:

Design Environment
Building frame
Required design
Minimum
Minimum space between
Minimum cover over
Statistical data indicate a standard deviation of compressive
strength of 2.1 MPa is expected (more than 30 samples).
Only air entrainer is allowed.
Available Materials
Air entrainer: Manufacture specification 6.3 ml/1% air/100 kg
cement.
Coarse aggregate: 19 mm maximum size, river gravel (rounded)
Bulk oven-dry specific
Oven-dry rodded
Moisture
Fine aggregate: Natural sand

strength = 27.6 MPa

dimension = 150 mm

rebar = 40 mm

rebar = 40 mm

gravity

1761 kg/m 3 Absorption

= 2.55, = 0.3%

density =

content = 2.5%