Monthly ArchivesFebruary 2020 - Q Green Techon PVT. LTD.

Concrete is used as construction material and it is categorized as normal concrete or high strength concrete based on its compressive strength. The compressive strength of normal concrete is between 20 and 40 MPa. The strength of high strength concrete is above 40 MPa.

The high strength concrete has compressive strength between 40 and 140 MPa which is discussed in this article.

As time goes the difference between normal and high strength concrete also changes. 100 years ago the concrete having a compressive strength of 28 MPa was considered as high strength concrete. Now the concrete can get strength greater than 800 MPa and is called reactive powder concrete.

When we talk about the application the normal strength concrete is widely used compared to high strength concrete. The main benefit of high strength concrete is for reducing weight, creep or the permeability issues, for improving the durability of the structure and for special architectural requirements were elements carry smaller loads.

What are the properties of Normal and High Strength Concrete?

Whether it is normal or high strength concrete, it should be mixed having the nature of plastic or semi-fluid that it can mold by hand or by using mechanically.

It is very important that the mixture does not go through bleeding or segregation in handling or transportation. The uniform distribution of aggregates helps the concrete to control the segregation.

Workability factors of Normal and High strength Concrete

The workability factor is the ease where the concrete is placed, compact and finished in its fresh state.

The normal strength concrete possesses having good workability that all concrete ingredients are in proper and accurate proportions. These aggregates must be of a proper gradation.

The high strength concrete mix is often sticky and also found difficult to be handled and placed. This is even if the plasticizes are used. It is due to high cement content in it.

Bleeding Factors for Normal and High Strength Concrete

The bleeding is the settlement of solid particles of cement and the aggregate in the fresh concrete mix which results in the development of a layer of water on the top of the concrete surface. The smaller bleeding does not make any issue but the large-scale bleeding affects the durability and strength of the concrete.

When we compared the normal and hard strength concrete the high strength concrete does not bleed. Which is because the high strength concrete has smaller water content and a high amount of cementitious materials. The air-entrained concrete also has fewer chances to bleed.

Permeability of Normal and High Strength Concrete

The durability problems like corrosion resistance, chemical and creep have a direct relationship with the permeability of the concrete. If the foreign substance enters inside the concrete the damage occurs. It depends on the permeability property and the paste and aggregates present in the concrete.

The decrease in permeability beneficial in

• Improves sulfate and chemical attack resistance
• Resistance to corrosion
• Resistance to chloride penetration

The below table shows the test results of various permeability tests conducted on the different concrete mixes. The table also provides information on normal strength concrete and high strength concrete in water and cement ratio.

The lower water-cement ratio with an adequate curing period helps in having a concrete of lower permeability. For the normal strength concrete, the permeability is found to be in the range of 1 x 10 ^-10 cm/sec.

The additional cementitious materials added in the concrete mix like silica fume, fly ash and GGBFS helps to reduce the permeability of the concrete.

High strength concrete has a lower value of permeability compared to normal strength concrete. which is because the high-strength concrete is designed with a lower water-cement ratio. The silica fume is commonly used in the mix. The high strength concrete has permeability ranges from 1 x 10 ^-11 to 1 x 10^-13 cm/sec.

The high strength concrete has low permeability and high resistance to chloride attack which makes it suitable for bridge construction, parking decks and structures are exposed to seawater.

Carbonation of Normal Strength Concrete and High Strength Concrete

The carbonation happens on the surface of the concrete. the carbonation is related to the permeability of the concrete. carbon dioxide of the air reacts with the compounds present in hardened cement paste. This reaction is called calcium carbonates.

The effect of carbonation is mentioned in the permeability factor is less in high strength concrete compared with normal strength concrete.

In both types of concrete mix, the essential amount of protective concrete covers the reinforcement steel which reduces the easy reach to the reinforcement.

Difference between Normal Strength Concrete and High Strength Concrete

The normal strength at 40% of compressive strength value the micro-cracks are formed. These interconnects and propagates reaches 80 to 90% of the strength.

In the normal strength concrete, the fracture surface is very rough. This zone is formed along with the transition zone between the paste matrix and aggregates. The fracture surface in high strength concrete is smooth.

Fly Ash is used as an admixture in concrete. The durability of the concrete with fly ash is discussed here.

The use of concrete in aggressive environmental conditions has been increased substantially. Concrete structures are used to provide support for types of machinery, staff, and products of oil and gas exploration and production.

The concrete structures are used in a nuclear reactor to keep the gases and vapors which released at high temperature and pressure in emergency situations. The fly ash plays an important role.

Effects of Fly Ash on Durability of Concrete

It has effects like

• Permeability of Concrete
• Carbonation of Concrete
• The durability of concrete subjected to repeated cycles of freezing and thawing
• Abrasion and erosion of fly ash concrete
• Sulfate resistance of concrete
• Alkali aggregate reactions in concrete
• Corrosion of steel reinforcement in concrete
• Concrete exposed to seawater

Effect of Fly Ash on Concrete permeability

The permeability of concrete is based on the quantity of hydrated cementitious material at any given time. It is said that the permeability of fly ash was lower than the permeability of controlled concrete after 28 days of curing.

On the other side after six months the fly ash concrete is more impermeable and also achieves substantial imperviousness. The above difference is because of the pozzolanic activity of fly ash and the pozzolanic reaction is low at the beginning.

This is how the fly ash could produce better concrete durability.

Fly ash effect on Carbonation of Concrete

The carbonation is where the carbon dioxide in the air which reacts with calcium hydroxide, calcium silicate and aluminates in hydrates cement and make calcium carbonate.

This process took place in mostly moist situations and the rate of the carbonation of concrete is specified by concrete permeability, saturation degree, and quantity of calcium hydroxide which is ready for the reaction.

The carbonation is the main reason for the steel corrosion resistance. The main focus should be on proportions of concrete mixture, concrete cover and the period of moist curing when a high amount of fly ash is used.

The effect of fly ash on the durability of the concrete with repeated cycles of freezing and thawing

If the concrete is made with exact proportions it has become frost resistance. The fly ash may lead to an increase the number of admixtures that are necessary to obtain the particular level of entrained air and it is also influenced by entrained air stability in fresh concrete.

The research is carried on the effect of the fly ash on the durability of concrete for repeated cycles of freezing and thawing and supports the statement by Larson that Fly ash has no apparent ill effects on the air voids in hardened concrete.

When the proper volume of air is entrained the characteristics of the void system meet the criteria.

Abrasion and Erosion of Fly Ash Concrete

There are many situations where concrete is used for scraping, attrition, sliding of cars, ice, and other items. It has been said that the concrete resistance against abrasion is proportional to its compressive strength.

The low abrasion resistant fly ash concrete might expect unless the concrete is adequately and thoroughly curried. It is proved that concrete with ASTM class F fly ash provider for better abrasion resistance than the ASTM class C or with no fly ash content.

The concrete is eroded when the water flows over the surfaces. With the fixed slump value, the concrete resistance against erosion can be increased by increasing the strength and cement content.

Fly Ash effect on Sulfate Resistance of Concrete

It is proven in research carried out by Dikeou that sulfate resistance of concrete can be improved by using fly ash.

The effect of fly ash on alkali-aggregate reactions on concrete

It is proven that fly ash is very effective in decreasing the detrimental effect of alkali-aggregate reactions (AARs). This effect is only decreased where the siliceous aggregates are involved.

The alkali-aggregate carbonation is one type of AAR that is less responding to fly ash inclusion. But the expansion of alkali-aggregate can be decreased with low calcium fly ash replaces the 25-30 percent and under the condition that alkali is less than four percent.

Effect of the fly ash on the corrosion of steel reinforcement in concrete

There is always a concern of corrosion of steel reinforcement in fly ash concrete. This is happened because of chloride ions from the seawater or de-icing. It has happened when the carbonation depth in concrete reaches the steel reinforcement and if the oxygen and moisture are reached to the surface of the reinforcement the steel bar will corrode.

It will be protected from corrosion if the fly ash concrete cover is sufficient.

Fly Ash effect on concrete exposed to seawater

The concrete used in marine has a danger of wetting and dying, waves, abrasion by debris and sand, freezing and thawing cycles and reinforcement corrosion which are occurred in a chemical medium.

But the entirely submerged concrete is less affected by the above factor. If the fly ash concrete has 25 percent replacement by mass and it is under the condition of water to cementitious materials less than 0.50 have a good performance under freezing and thawing and wetting and drying conditions.