Building Design and Construction Handbook - Merritt - Ventech!

4.24 SECTION FOUR
weights range from 90 lb/ft 3 (all aggregates lightweight) to 115 lb/ft 3 (sand lightweight).
Typically, compressive strengths range from 2500 to 4000 psi. Highstrength
lightweight concretes, however, have been produced with maximum unit
weights of 125 lb/ft 3 and strengths from 6000 to 9000 psi. Structural lightweight
concretes are defined by the ACI as concretes with a 28-day compressive strength
more than 2500 psi and air-dry unit weight of 115 lb/ft 3 or less.
The variable amount of water absorbed in the voids of lightweight aggregates
makes use of W/C difficult in design of a lightweight-aggregate mix (Table 4.5).
Air entrainment of 4 to 6% is desirable to prevent segregation. Maximum size of
the coarse aggregate should not exceed half the depth of cover over the reinforcing
steel.
Lightweight-aggregate concrete exposed to sulfates should have a compressive
strength ranging from 3750 to 4750 psi (see ACI 318). For marine structures, the
W/C should not exceed 0.40 and at least seven bags of cement should be used per
cubic yard of concrete.
The modulus of elasticity E c of lightweight concrete generally ranges from
1,500,000 to 3,000,000 psi. It may be estimated from
where w � unit weight of concrete, lb/ft3 � 28-day compressive strength of concrete, psi
ƒ� c
1.5 Ec � w �ƒ� c
(4.1)
Volume changes occur in lightweight concrete as in normal-weight concrete,
but lightweight concrete is stabler when exposed to heat. Drying shrinkage causes
the most undesirable volume changes, because it produces cracks in the surfaces
of the concrete. The primary cause of drying-shrinkage cracks is excessive water
in the mix. The water has two effects. First, it increases the W/C and weakens the
concrete. Second, the additional water beyond that needed for hydration of the
cement creates an excessive number of bleed channels to the exposed surfaces.
When the cement paste undergoes normal drying shrinkage, these channels cannot
provide any resistance to ingress of aggressive chemicals.
Creep is an important concern for lightweight concrete, as it is for normal-weight
concrete, especially for prestressed concrete.
(‘‘Standard Practice for Selecting Proportions for Structural Lightweight Concrete,’’
ACI 211.2, and ‘‘Guide for Structural Lightweight Aggregate Concrete,’’
ACI 213.)
TABLE 4.5 Approximate Relationship between
Cement Content and Compressive Strength
Compressive Aggregates all
strength ƒ�c ,
psi
lightweight,
lb/yd3 Sand aggregate
lightweight,
lb/yd3 2500 400–510 400–510
3000 440–560 420–560
4000 530–660 490–660