From what I understand, it allows a reduction in water and Portland cement, can get incredible strengths... 3000 psi in 20 hours, for example. The concrete is less permeable and shrinkage is reduced. Also good for reactive aggregate.
I'm thinking of including it in my General Notes... adding a max of 5%. I used it about 20 years back for some high strength concrete columns and it worked well. Just looking for some feedback...
They tell me it is hard to finish well, since it makes the surface of wet concrete "sticky". That is also my experience with it in small "labcrete" batches.
You will have more reliable reduction in your water requirements by using a high range water-reducing admix.
High early strength is also a function of the rest of the mix design, and increasing strength excessively can result in increased cracking/crack width of flexural members (the stronger concrete may crack farther apart, which often results in wider cracks).
I guess with the higher strength, the material is more brittle; I was thinking with the high strengths involved that it would be suitable for columns, etc, to reduce size. I've used it for 10 ksi concrete columns a while back, but understand that it can produce 60 or 80 ksi concrete. I also understand that it is used to minimise shrinkage, but that entails the use of extreme tuning of aggregate gradation to minimise the air voids. Some DOT's in the US (I've heard, but not experienced) are leaning towards it. Most of my info is 'heard', as I've only used it the once (worked well). It seems suited for parkades, etc. where a broomed finish is often used as well as resistance to intrusion of de-icing salts.
Silica fume is advantageous if you want to make your concrete more impermeable. The particle size is much smaller than a concrete particle, so it fills the voids nicely.
The article I saw had a parkage structure and the slab was post-tensioned the next day... They had 3 ksi in 20 hours. Bay spacing was 60'x54'... and the commentator said that they could achieve these 'ultra high' strengths.
"1.1-Historical background
Although high-strength concrete is often considered a
relatively new material, its development has been gradual
over many years. As the development has continued, the
definition of high-strength concrete has changed. In the
1950s, concrete with a compressive strength of 5000 psi
(34 MPa) was considered high strength. In the 1960s,
concrete with 6000 and 7500 psi (41 and 52 MPa) compressive
strengths were used commercially. In the early
1970s, 9000 psi (62 MPa) concrete was being produced.
More recently, compressive strengths approaching 20,000
psi (138 MPa) have been used in cast-in-place buildings.
For many years, concrete with compressive strength in
excess of 6000 psi (41 MPa) was available at only a few
locations. However, in recent years, the applications of
high-strength concrete have increased, and high-strength
concrete has now been used in many parts of the world.
The growth has been possible as a result of recent developments in material technology and a demand for
higher-strength concrete. The construction of Chicago’s
Water Tower Place and 311 South Wacker Drive concrete
buildings would not have been possible without the
development of high-strength concrete. The use of concrete
superstructures in long span cable-stayed bridges
such as East Huntington, W.V., bridge over the Ohio
River would not have taken place without the availability
of high-strength concrete."
They imply that 20 ksi is not uncommon for buildings. I used half that strength in the late 70's and that was very high strength at the time... and I understand that there is research/work being done with ultra high strength concretes.
I deleted from my earlier reply a comment about Colorado DOT using it in bridge decks as a measure against chloride migration into and through the matrix. Yes, it is being used, but caution is required since crack width is a very important predictor of durability.
The US's current tallest structure, Chicago's Trump Tower used silica fume in the 12 ksi and above mixes. The specification was 56 day strength (some elements were reported as 16 ksi / 90 day in magazine articles.)
This is a good example of where properties other than strength (low shrink/creep, ease of consolidation, etc.) were important early and strength development could lag.
In University one of our mix designs for a 3/4" thick concrete toboggan tested 66MPa (almost 20yrs ago now), and that is not high in the world of high strength concrete mixes. 80 - 100MPa is very possible as I understand. Some of the towers in Dubai could not have been built using concrete if it were not for High Performance concrete. The negative of concrete with this strength is the failure mechanism is far more brittle than typical concrete. The other downside is the cost. Large quantities of Silica Fume will drive your concrete costs way up. I priced out some 50MPa concrete for a 1mil gallon tank and it was not a cost effective solution.
