Concrete is the oldest and most used man-made material on earth. Its most common construction material is extensively used for buildings, bridges, roads, and dams. Its uses range from structural applications to sidewalk ways, curbs, pipes, and drains.
Concrete is not clearly classified category-wise. But generally, the concrete is classified as;
- Normal strength concrete (NSC)
- High strength concrete (HSC)
- Ultra strength concrete (UHSC)
Indian Standard Recommended is a method of mix design that denotes the boundary at 35 Mpa between HSC and NSC. Indian Standard did not talk about UHSC. But high strength concrete has strength above 40 Mpa.
Last 15 years, the use of high-strength concrete is entered in the construction field such as long-span bridges, high–rise buildings, Dam, Retaining walls, tunnels, etc. high grade concrete is used up to 90 to 120 Mpa.
Generally in Prestressed Concrete widely used high strength concrete for making components of the bridge abutment, girder, wall, etc. The first pre–stressed bridge was built in India for Assam rail and more than 35 mpa strength of concrete was used in the Konkan railway.
In India Ready–mixed Concrete is widely used in everywhere large construction projects.
The batching plants produced high-strength concrete in a mechanical manner. Making high strength concrete must take care of the proportion of concrete, superplasticizer, Silica fume, the shape of aggregate, cementation materials, etc.
The following methods are used to make high strength concrete using a special method.
- Re- vibration
- High speed slurry
- Use of admixtures
- Inhibition o cracks
- Sulphur impregnation
- Use of cementations aggregates
Seeding is involved a small percentage of fully hydrated Portland cement to the fresh concrete. The seeding method is generally developing strength in concrete.
In concrete mixing water creates capillary channels, accumulation of water, bleeding at some places. These entire factors reduce the strength of concrete.
3. High Speed Slurry Mixing
The High-speed slurry mixing process is the advanced preparation of water-cement mixture then this slurry is blended with aggregate and made concrete.
4. Use of Admixtures
5. Inhibition of Cracks
Concrete is failed by the propagation and formation of cracks. So that 2 to 3 % of fine aggregate is replaced by polythene or polystyrene.
6. Sulphur Impregnation
In this process fresh concrete specimen is cured for 24 hours and then drying the specimen at 120 C for 24 hours, immersing the specimen in sulphur under vacuum pressure for 2 hours and then releasing pressure, soaking them for ½ hours for infiltration of sulphur.
This process gives strength up to 58 Mpa. So that low strength porous concrete impregnating by sulphur.
7. Use of Cementations aggregate
It has been found that the use of cementations aggregate has yielded high strength. The cement found is a kind of clinker. This glassy clinker when fine; ground result in a kind of cement.
When coarsely crushed cementations aggregate make algae. Which use as an aggregate they give strength up to 125 MPa has obtained with water/cement ration 0.32.
High Performance Concrete
The New term High strength concrete is used for the concrete mixture which possesses high workability, high dimensional stability, low permeability, high modulus of elasticity, high strength, and resistance to chemical attack.
High performance concrete is also high-strength concrete but it has few attributes specially designed as mentioned above, it may be recalled that in normal concrete, which is low in strength and elastic modulus.
By strengthening and densification the transition zone many desirable properties can be improved. Reduction of the quantity of water is the necessary step for making High performance concrete because the reduction of water-cement ratio is increased in concrete strength.
For increasing strength adding silica fumes become a necessary ingredient for strength above 80 MPa. Also the use of best quality fly ash and GGBS for other nominal benefits. When adding pozzolanic material in concrete is the benefits will outweigh the disadvantages.
In high-performance concrete use of an appropriate superplasticizer is the most important thing because a high slump is possible only with the use of a superplasticizer.
The main problem is the type of cement and selection of superplasticizer, which get a sufficient time till concrete is placed and compact.
Advantages for High Performance Concrete
In normal strength concrete, the strengths of aggregate by itself play a minor role. Any aggregated available at the site could be used with little modification of their grading.
Where the bond between hydrated cement paste and aggregate is strong so that it results in transfer stress across the transition zone.
At the same time, the strength of the cement paste phase on account of the very low w/c ratio is so high that sometimes it is higher than the strength of aggregate particles.
Whenever you observed the fracture surface in high strength concrete that means they pass through the coarse aggregate as often as if not more often than the cement paste itself.
On the basis of practical experience, it is seen that for concrete strength up to 100 MPa maximum sizes of 20 mm aggregate could be used. However for concrete in excess of 100 MPa the maximum size of coarse aggregate should be limited to 10 to 12mm.
Typical High Performance concrete mixture used in some important buildings in USA and other countries.
high strength concrete mix
|Fly ash (Kg/m3)||60||–||–||–||–|
|Silica fume (Kg/m3)||–||–||30||36||43|
|Coarse aggregate (Kg/m3)||1030||1030||1100||1130||1080|
|Fine aggregate (Kg/m3)||630||745||700||745||685|
|Water Reducer (Kg/m3)||975||–||–||900||–|
|W/( c +m)||0.35||0.37||0.27||0.31||0.25|
|Strength at 28 days MPa||65||80||93||83||119|
|Strength at 91 days MPa||79||87||107||93||145|
- Water Tower Place, Chicago 1975
- Joigny Bridge , France 1989
- La Laurentienne Building Montreal 1984
- Scotia Plaza, Toronto 1987
- Two Union , Seattle 1988
Regarding the shape of the aggregate, crushed aggregate can be used, but utmost care should be taken to see that aggregate is cubic in shape, with a minimum amount of flaky or elongated particles.
They affect not only strength but also adversely affect workability. The cubic-shaped coarse aggregate is the more workable. For HPC shape and size of the HPC that are used in some important buildings in the USA and other countries.
In India, it is reported that HPC of the strength 60 MPa was used for the first time for the construction of containment dome at Kaiga and Rajasthan Atomic Power Projects.
You May Also Like