Aggregates for concrete - Concrete Mix Design, Quality Control and Specification - page 61

of workability. The problem tends to occur on low-story work where an
attempt may be made to pump concrete with aggregate that is not fully
saturated. This may be successful for a limited time but as soon as any
difficulty is experienced the concrete comes under greater pressure and
the problem is exacerbated. Once the aggregate is fully saturated, such
concrete can be pumped just as well as dense aggregate concrete. Indeed,
being lighter, it may well be easier to pump to heights of 50 stories or
more by reducing the hydraulic head.
It is interesting to note that at least one of the Scandinavian floating
oil platforms used lightweight aggregate concrete. What was particularly
interesting was that the aggregate is deliberately used dry. The Norwegians
admit that this causes the problems outlined earlier but state that it was nec-
essary to achieve the desired low density. On a dry land project, this would
be ridiculous because the concrete would eventually have the same mois-
ture content and the same density whether the aggregate was initially wet
or dry. The Norwegians said that this was not the case when the concrete
is to be permanently immersed in water from a relatively early age. In con-
crete with extremely low water to cementitious materials ratios (w/cm) the
amount of water removed from saturated aggregate by hydration can help
overcome this concern.
The use of saturated aggregate has benefits other than improved slump
stability. The weight differential between the mortar and the aggregate is
reduced, and therefore less trouble is experienced with floating aggregates.
This differential is also reduced by the use of air entrainment, and the air
also impedes the movement of water through the mix, so reducing slump
loss. The entrapped water in lightweight concrete acts as internal curing by
providing built-in reservoirs of water. This greatly improves development
of hardened properties and can virtually eliminate autogenous shrinkage.
Internal curing is more effective than surface curing in massive or low w/cm
concrete but does not eliminate the need for surface protection. The density
of the concrete is substantially affected by the moisture content and the
weight loss on drying can be as much as 200 kg/m 3 with some concretes.
It is also important to note that the crushing strength of the concrete may
be substantially reduced by its being fully saturated at the time of test.
Unlike dense aggregate concrete, lightweight concrete should not be tested
fully saturated unless it will be fully saturated in use.
Lightweight concrete should not be thought of as necessarily permeable,
nondurable, or less capable of protecting steel. Such material has been used
to produce concrete ships and found to protect the steel very well over
many years. It has been shown to give improved resistance to rain penetra-
tion in precast housing. This should not be surprising as the penetrability
properties of concrete are generally determined by the properties of the
matrix between the aggregate particles and the interfacial zone not the
aggregate itself.