Concrete is the most widely used building material that has been in use since 3000 BC in Egyptian Civilization to build pyramids, especially after the invention of binding agents like Lime and Cement. With the passage of time, more research led towards many refinements in concrete ingredients and production.
Concrete you can mold, you can press it into – after all, you haven’t any straight lines in your body. Why should we have straight lines in our architecture? You’d be surprised when you go into a room that has no straight line – how marvelous it is that you can feel the walls talking back to you, as it were.
Philip Johnson
Concrete has been used globally for decades and will be immensely used near future. Maybe the binding agents change, maybe the coarse aggregate change but concrete will still be there; living and surviving.
1. What is Concrete?
Concrete is a matrix / composite that is composed of mainly the following ingredients;
- Coarse Aggregate
- Fine Aggregate
- Binding Agent
- Water
- Secondary Raw Materials (Optional)
- Plasticizers (Optional)
2. Classification of Concrete According to Strength
Concrete maybe divided Strength wise as follows;
- Normal Strength Concrete
- High Strength Concrete
3. Normal Strength Concrete (NSC)
3.1 Use – Normal Strength Concrete
Normal Strength Concrete (NSC) is used for ordinary construction purposes like residential buildings, apartments, single and multi-dwelling units, etc. It maybe used anywhere where the Unit Load is nearly 60 lb./sft to 100 lb./sft.
3.2 Mix Proportion – Normal Strength Concrete
Normal Strength Concrete is prepared by the use of Coarse Aggregate, Fine Aggregate, Cement and Water. To determine the mix proportion of these aggregates in order to achieve Normal Strength Concrete, proper mix design needs to be conducted. Hit and trail is the only method to achieve it as size of aggregates, properties of aggregates, cement quality, water quality, temperature, etc. differ from case to case.
3.3 Strength Requirements – Normal Strength Concrete
If concrete strength is equal to or lower then 6000 psi (pounds per square inch) or lower then M40 (40 x 145 = 5800 psi), then concrete will be categorized as Normal Strength Concrete.
3.4 Setting Time – Normal Strength Concrete
Normally, the Initial Setting Time of Normal Strength Concrete varies from 30 min. to 90 min. while Final Setting Time of Normal Strength Concrete maybe 10 hours, which may further depend upon fineness of cement and fine aggregates, atmospheric temperature, the moisture content, quantity of cement, etc.
3.4.1 Development of Strength – Normal Strength Concrete
After placing the concrete in formwork which is known as Casting of Concrete, concrete starts gaining its strength passing from setting to Harding phases. Usually, 7-days, 14-days and 28-days strengths are considered in research.
However, development of enough strength starts from 7-days of casting which 10 MPa (10 x 145 = 1450 psi) to 28-days strength of 40 MPa (40 x 145 = 5800 psi) which is approximately equal to 6000 psi.
3.5 Water Cement Ratio – Normal Strength Concrete
Water-Cement Ratio of Normal Strength Concrete should not be more then 0.6. Proper mix design will demarcate the value of w/c ratio.
3.5.1 Why 28-days Strength is Significant?
Usually at 28-days almost 90% of the strength of concrete has been achieved, that is why 28-days strength is considered significant.
Then question arises Why not more then 28-days strength is consider?
Yes, you are right. Why don’t we wait for 100% strength gain? Because it may take 6 months to years to achieve 100% strength of concrete. Material Testing Engineers cannot wait so long. Furthermore, the difference in strength of 10% is not considered significant.
At 90-days, almost 95% strength of Normal Strength Concrete is achieved.
3.6 Physical Properties of Normal Strength Concrete
- For Normal Strength Concrete, Slump Test should have values between 1 – 4 inches.
- Density of Normal Strength Concrete varies from 140 pcf (pounds per cubic feet) to 175 pcf.
- Air Content should vary from 1% to 2%.
Normal Strength Concrete is usually not durable against severe environments like extreme weathers, freeze and thaw cycles, corrosive environments, etc.
Normal Strength Concrete is weak in tension but strong in compression.
4. High Strength Concrete (HSC)
Concrete that is having ultimate compressive strength at 28-days is more then 6000 psi is categorized as High Strength Concrete. Such concrete can be prepared by reducing the water-cement ratio to 0.35 and below.
Secondary raw materials, plasticizers and super-plasticizers maybe added to High Strength Concrete in order to attain some high performance features. Furthermore, high quality aggregate needs to be used.
4.1 Use – High Strength Concrete (HSC)
High Strength Concrete (HSC) is mainly used for the following purposes;
- Saving Transportation Cost
Higher Strength having smaller volume of concrete is added value to reduce the transportation cost of High Strength Concrete as compared to Normal Strength Concrete in which volume of concrete is more as compared to its strength.
2. Smaller Dimensions of Elements – Creating Space
Second benefit of using High Strength Concrete is the creation of space for architectural requirements. Members cast with High Strength Concrete are slender and carrying much larger loads as its strength is more, as compared to Normal Strength Concrete. Structures get roomy and spacious.
3. Taller Buildings
With High Strength Concrete, taller and more elegant structures especially building structures can be constructed. As their load bearing capacity is more and structural supports are slender (thin) thus providing economy with strength.
4. Quick Setting – Rapid Construction
High Strength Concrete sets rapidly as compared to Normal Strength Concrete, thus, creating an added value specially in places where there is high traffic volumes in case of roads, bridges and culverts. Rapid construction can assist in early completion of projects.
5. Saving Cost
Slender structural elements can be constructed with the help of High Strength Concrete as compared to Normal Strength Concrete thus saving concrete volume and construction cost, the transportation costs, the casting costs, the formwork costs, the member’s size costs, etc. Overall cost of High Strength Concrete maybe lower then Normal Strength Concrete.
5. Difference between Normal Strength Concrete and High Strength Concrete
| S.No | Normal Strength Concrete (NSC) | High Strength Concrete |
| 1 | The water cement ratio used in normal strength concrete should not exceed 0.6. | The water cement ratio used in high strength concrete is 0.25 to 0.35. |
| 2 | Moderate quantity of cement is used in normal strength concrete. | High amount or quantity of cement is used in high strength concrete. |
| 3 | In normal strength concrete not high quality of aggregates are used and ordinary Portland cement is mostly used. | In this type of concrete high quality of aggregates are used. Sometimes ordinary Portland cement can be used. |
| 4 | The concrete mix ratio of normal strength concrete varies from M10 to M40. | The concrete mix ratio of high strength concrete varies from M50 to M70. |
| 5 | The compressive strength of normal strength concrete is 10 MPa or 1450 psi to 40 Mpa or 5800 psi. | The compressive strength of high strength concrete is 50 MPa or 7,250 Psi to 70 MPa or 10,150 psi. |
| 6 | Workability of normal strength concrete is more than high strength concrete. | Workability of high strength concrete is low. |
| 7 | Normal strength concrete is used in small projects where high strength is not required. | High strength concrete is used heavy construction works where high strength is required. |
| 8 | Bleeding can develop in normal strength concrete. | High strength concrete does not bleed. |
| 9 | Less cementitious material is used. | High cementitious material is used. |
| 10 | Permeability of normal strength concrete is moderate. | Permeability of high strength concrete is very low. |
| 11 | Fracture surface made in normal strength concrete is very rough | Fracture surface formed in High strength concrete is smooth. |
| 12 | In common structures, the normal strength concrete is used more than high strength concrete. | The high strength concrete is used in specific works such as high strength projects. |
| 13 | Preparation of the normal strength concrete it is not costly. | To prepare the high strength concrete it is slightly costly than normal concrete. |
| 14 | Normal strength concrete columns may not bear more load and therefore be constructed thicker. | Concrete columns can bear more load and therefore be constructed slender than normal strength concrete columns. |
| 15 | The initial setting of normal strength concrete is 30 minutes. | The initial setting of high strength concrete is 140 minutes. |
| 16 | The final setting time of normal strength concrete is 10 hours or 600 minutes. | The final setting time of high strength concrete is 290 minutes. |
| 17 | Durability of normal strength concrete is not high. | Durability of high strength concrete is high. |
| 18 | To preparing the normal strength concrete not high skilled labor is required. | To preparing high strength concrete highly skilled labor is required. |
Conclusion
Selection of Normal Strength Concrete (NSC) or High Strength Concrete is dependent on load requirements and the type of structure. Aesthetic requirements set by the Architect may also play a pivotal role. Whereas, construction also involves high economy which must also be taken into consideration. Both types are having their own benefits and drawbacks. An experienced designed will quantify which type of concrete needs to be poured to keep the structure in ductile performance range.
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