Introduction of High Strength Concrete (HSC)
High-strength concrete has a compressive strength greater than 600 kg/cm sq. HSC is made by lowering the water-cement (W/C) ratio to 0.35 or lower.
Often silica fume is added to prevent the formation of free calcium hydroxide crystals in the cement matrix, which might reduce the strength of the cement- aggregate bond.
Low W/C ratios and the use of silica fume make concrete mixes significantly less workable, which is particularly likely to be a problem in high-strength concrete applications where dense rebar cages are likely to be used.
To compensate for the reduced workability, superplasticizers are commonly added to high-strength mixtures. Aggregate must be selected carefully for high-strength mixes, as weaker aggregates may not be strong enough to resist the loads imposed on the concrete and cause failure to start in the aggregate rather than in the matrix or at a void, as normally occurs in regular concrete.
In some applications of HSC, the design criterion is the elastic modulus rather than the ultimate compressive strength. It is used especially for the research centers, nuclear stations, foundations of heavy machines, etc.
Types of high strength concrete
- HSC having strength up to 1100 kg/cm”.
- HSC having strength up to 1100-6700 kg/cm.
Method of Production of High Strength Concrete
Following methods are used for making high strength concrete:-
a) High-speed slurry mixing:- In this method, the high-speed slurry is obtained by more efficient hydration of cement resulting from the more intimate contact between cement particles and water.
b) Use of cementitious aggregates:- In this method cementitious aggregate is used in concrete to produce high strength using SLAG (cementitious aggregate) as aggregate, strength up to 1240 kg/cm has been obtained with a w/c ratio of 0.32.
c) Re-vibration:- Concrete undergoes plastic shrinkage. Mixing water creates continuous capillary channels, bleeding and water accumulates at some selected places. All these reduce the strength of concrete. Controlled re-vibration (within initial setting time) removes all these defects and increases the strength of concrete.
d) Admixtures:- Water reducing admixture results in an increase in compressive strength.
e) Seeding:- In this method, a small percentage of finely ground, fully hydrated portland cement is added to the fresh concrete mix.
f) Sulphur impregnation:- Satisfactory HSC has been produced by impregnating low strength porous concrete with Sulphur. This type of concrete is achieved up to a strength of 575 kg/cm2 g)
g) Inhibition of cracks:- Concrete fails by formation & propagation of the crack. If the propagation of crack is inhibited, the strength will be higher. Replacement of 2-3% of fine aggregate by polythene 0.025mm thick & 3 to 4mm in diameter results in higher strength, 1050kg/cm.
Note:- In spite of (HSC) ultra high strength concrete is also produced by the following method:-
1. Compaction by pressure
2. Helical binding
3. Polymer concrete
Some of the basic concepts in preparing HSC
- Aggregates used must be strong and durable. Generally, small size aggregates are used for making HSC.
- High-strength concrete generally uses a low water-cement ratio which ranges from 0.23 to 0.35. Thus, superplasticizers are added to obtain the desired workability.
- The total cement content for this type of concrete is should not be more than 650 kilograms per meter cube as high cement content results in cracking due to hydration.
- Sometimes to obtain the desired strength cement is mixed with supplementary materials like fly ash, ground granulated blast furnace, etc.
- The components of the design mix like mix proportioning the shape of aggregates, supplementary cementitious materials, and superplasticizers should be given special care while preparing the high-strength concrete.
Most common applications of HSC
- To put the concrete into service at a
much earlier age.
- To build high-rise buildings by reducing
column sizes and increasing available
- To build the superstructures of long-span bridges and to enhance the durability of bridge decks.
- To satisfy the specific needs of special applications such as durability modulus of elasticity and flexural strength.
- Some areas of application are dams, grandstand roofs, marine foundations, parking, garages, and heavy-duty industrial floors.
Happy Learning – Civil Concept
Civil Engineer – Ranjeet Sahani