RVR f 2w strength (less than 0.3 w/c ratio) are more easily produced by application of high range of
superplasticizing admixtures. It is generally known that the addition of 0.5 to 1.5% of a
conventional superplasticizer by weight of cement to a concrete mix with 50 to 75 mm slump
will cause a dramatic increase in the consistency (200 to 250 mm); however, this high consistency
concrete tends to revert back to original consistency within 30 to 60 minutes. One way is to
maintain large increases in the slump of superplasticized concrete by repeated dosages of the
admixture. However, segregation may occur, when the second or third dosage of admixture is
added after slump loss. The other approach involves modification of the admixture composition
with a retarding agent so that the original high consistency may be maintained for 3 to 6 hrs.
g
In the early stages of its development, the HSC has a tendency to be sticky and
stiff due to large amounts of fines (high cement content, and pozzolana), a low water-cement
ratio, and a normal water-reducing admixture. However, with the advent of superplasticizers
it is possible to have a desired high workability without causing segregation even at a lower w/
c ratio of 0.3.
The most noteworthy point about HSCs is their capacity to develop strength at a
rapid rate without steam curing. Concrete can develop 20 to 27 MPa on normal curing within
24 hours and the ultra HSC can develop 42 MPa in 12 hours and 64 MPa in 24 hours.
wD2E 2 D2g
2 2p As a result of reduction in the
size and number of micro cracks in HSCs, its stress-strain relation, creep and fracture behaviour
is different from the normal concretes. HSCs, having compressive strengths in the range of 30
to 75 MPa behave more like a homogeneous material as compared to PCC. For HSCs the stress-
strain curves are steeper and more linear to a higher stress-strength ratio than in normal
strength concretes, because of a decrease in the amount and extent of micro-cracking in the
transition zone. Thus it shows a more brittle mode of fracture and less volumetric dilation. Also
the amount of micro-cracking in HSC associated with shrinkage, short term loading, and
sustained loading is significantly less.
h It has been found that primarily due to low permeability, HSCs exhibit excellent
durability to various physical and chemical agents that are normally responsible for concrete
deterioration. Due to high cement content thermal cracking can be durability problem in
structures using HSC. It has also been found that there is an expected temperature rise of 10–
40°C for every 100 kg/m^3 cement content.
e
The use of the highest possible strength concrete and minimum steel offers the
most economical solution for columns of high rise buildings. It has been found that there is an
increase of only 3.1 times in price for an increase of 4.7 times in load carrying capacity. This
clearly demonstrates the economy of using HSC in multistorey buildings With concrete, one
can “fast track”, that is, start construction even when the superstructure is only partially built
and thus get the building completed sooner, as against with the use of a steel frame. So far
industrial application, HSCs are limited to structural members that are not exposed to freeze
thaw cycles. It seems that superplasticized, low w/c ratio HSC containing high cement content
and a good quality puzzolana should have a great potential of use where impermeability or
durability, not strength, is the main consideration. Such applications include floors in the
chemical and food industry, and bridge deck overlays that are subject to severe chemical and