The unit weight of lightweight concrete for structural purposes 
ranges from 85 to 120 pcf. The greatest strengths are attained with 
aggregates derived from shale, clay, or slate and expanded into pellets 
by means of the rotary-kiln process. Lightweight aggregates should 
conform to ASTM Standard Specification C330, and should have a unit 
weight within the 85 to 120 pcf limits, in order to assure a concrete 
compressive strength of at least 2,500 psi. 
The lightweight concrete mixture should be designed in accordance 
with ACI Standard 211.2, and due regard given to the cement content 
rather than the water/cement ratio. As lightweight aggregate absorbs 
mixing water more readily than does natural aggregate, the slump and 
cement content are more important than water/cement ratio in controlling 
the quality of concrete. On the basis of equal workability, the slump 
of lightweight concrete is usually one-half to two-thirds that of con- 
ventional concrete. 
Improved workability for a given slump and a simultaneous improve- 
ment in strength can be attained by using water-reducing and set-retard- 
ing admixtures that conform to ASTM Standard Specification C494. An 
air-entraining admixture also should be used to assure optimum work- 
ability. The amount of entrained air needed in lightweight concrete 
normally ranges between 6 and 8% whereas conventional concrete requires 
from 3 to 54; however, as explained previously, higher percentages are 
needed to resist freezing and thawing in seawater. 
Slump, air content, and unit weight are the principal physical 
characteristics utilized on controlling quality of lightweight concrete. 
Testing samples of the freshly mixed concrete with regard to these three 
characteristics is more important than testing 28-day compressive 
strengths. Maintaining uniformity is facilitated if the tolerance in 
unit-weight variations is restricted to +1 pcf, the slump is the lowest 
compatible with practicable placement and full compaction, and the air 
content is determined by the volumetric method specified in ASTM Standard 
Test Method C173. 
Compacting freshly mixed lightweight concrete by vibration must 
be done carefully to preclude segregation of the aggregate. The fine 
aggregate usually consititues about one-half the volume of the total 
aggregate, but the weight of the fine portion is about double that of 
the coarse portion. 
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