Mix design, workability heat evolution and strength development of high strength concrete.
A literature survey of the properties and uses of high strength concrete, defined for this
study as having a strength in excess of 60 N/tnm2, has shown that of prime need is a
systematic, reproducible procedure for attaining high strength concrete.
The "Maximum Density Theory", i.e. the requirement that the aggregate occupies as large
a relative volume as possible, has been adopted as an approach to optimisation of the mix
proportions. However, this does not consider the effect that the aggregate suIface area has
on the requirement of excess paste for lubrication. To investigate the combined effect of
void content and surface area, mixes with lower sand proportions than that required for
minimum void content were tested for slump. The optimum sand proportion is the one
that produces the highest slump, for a particular cement content. This procedure has been
called: "The Modified Maximum Density Theory".
Having thus optimised the cement and aggregate contents, partial cement replacement by
mineral admixtures, at low water-cement ratios, has been investigated in order to assess:
a) their contribution to long term strengths,
b) their contribution to reducing the heat evolution of concrete mixes,
and c) their effect on the workability of concrete.
Condensed silica fume (at replacement levels of up to 15%) produced higher compressive
strengths than ordinary Portland cement. Ground granulated blast furnace slag (at
replacement levels of up to 30%) can be used without decreasing the 28-day strength.
Replacement by 20% pulverised fuel ash resulted in a 15% decrease in the 28-day
strength and equal strength to ordinary Portland cement concrete at ages beyond 56-days.
Temperature measurements during hydration, under adiabatic conditions, have however
shown that these replacement levels do not lower the temperature rise at a water-binder
ratio of 0.26. The higher levels required for significant temperature reduction will also
cause a significant reduction in the strength. To offset this ground granulated blast furnace
slag (58%) and pulverised fuel ash (36%) in combination with 10% condensed silica fume
were used. These combinations reduced the temperature rise by more than 10°C while the
reduction in the 28-day compressive strength was less than 15%.
Partial cement replacement by pulverised fuel ash and ground granulated blast furnace
slag improved the workability and therefore allowed a reduction in the superplasticiser
dosage required for a given slump. The use of condensed silica fume reduces the
workability at low superplasticiser dosages, but it has a water-reducing effect above a
certain superplasticiser dosage.
Results from these studies have been used to formulate guidelines for the proportioning
of materials for producing high strength concrete.