Main Properties of Concrete In Hardened Stage And Their Explanation

Introduction

The hardening of freshly placed concrete is caused by the chemical reaction between water and cement and continues for a long time. Concrete attains 99% strength in 28 days and attains full strength with age. To get a “stone-like” hardened mass, it is desired to maintain humidity and temperature which can be achieved by proper curing↗️. 

The characteristics or qualities which deal with the study of the behavior of concrete is known as properties of concrete. The quality of concrete depends upon the correct proportioning of ingredients to be used. Concrete from its plastic state to the hardened state passes through various states. At every state, concrete has to be strictly supervised and look after. 

The main objective of studying the properties of concrete is to design a proportionate concrete mix for maintaining workability strength, durability, water-cement ratio and to avoid bleeding, segregation, and cracks. The stages of concrete along with their properties.

The properties of concrete are different in plastic and hardened stages. Therefore, the properties of concrete are the following, and the second type given below article post.

Properties of Concrete In Hardened Stage

Hardening is the phenomenon by which the weak set paste develops strength. The hardening of the concrete mix begins after the cement has set. It occurs rapidly during the first few days and continues at a slow rate indefinitely. The properties of concrete in the hardened stage are the following below.

1. Strength

The property of hardened concrete to resist external loading is termed as its strength. The strength of concrete mainly depends upon the W/C ratio, quality, and quantity of cement, degree of compaction, and other concrete operations. Stronger concretes are durable and impermeable. The strength of concrete may be : 

1. Compressive strength
2. Tensile strength
3. Shear strength
4. Bond strength

1. Compressive strength - Out of various strengths of concrete, the determination of compressive strength has received a large amount of attention because the concrete is primarily meant to withstand compressive stresses. Compressive strength is the resistance of concrete to crushing. It is measured in N/mm2. As per IS: 456 - 2000, the minimum grade of concrete to be used for building construction is M20. Similarly, for roads and bridges, M40 grade is used and for lean concretes the grade can be as low as M10. SA Concrete Technology Cubes of 15 cm size are generally used to determine the compressive strength of concrete. The specimens prepared are cast, cured and tested as per the guidelines laid down by Indian Standards The compressive strength of concrete depends upon the type of cement used, size and shape of aggregates, W/C ratio, degree of compaction and curing.

2. Tensile strength (Flexural Strength) - Concrete is very weak in tension. It is only 10% of the compressive strength. That is the reason concrete is reinforced, with steel bars in tension portion, to take up the tensile stresses.

Practically, it is difficult to measure tensile strength. Hence, the tensile strength of concrete is taken as the strength of concrete in bending. The strength of concrete in bending is also termed as flexural strength. The determination of flexural strength is essential to estimate the load at which the concrete member may crack.

3. Shear strength - The shear strength of concrete is 10 to 12% of compressive strength. Shearing of concrete is always accompanied by tensile and compressive stresses, developed due to bending. Generally, the failure of concrete in shear is due to diagonal tension. 

4. Bond strength - The bond strength is 10% of the compressive strength of concrete. Bond strength is the measurement of adhesion between concrete and steel in RCC work, Bond strength can be increased by using deformed bars instead of plain bars in a concrete mix. By decreasing the W/C ratio the bond strength can be enhanced. 

2. Durability

The durability of a concrete mix is the resistance offered to forces of deterioration and disintegration such as a change in temperature, change in moisture content, the action of water containing chemical, and weathering effects. A durable concrete is one that can withstand the conditions for which it has been designed without disintegration and deterioration over a period of years. Factors affecting durability. The factors affecting the durability can be internal or external causes discuss below.

• Environmental exposure conditions.
• Cover to reinforcement.
• Attack by natural or industrial liquids or gases.
• Alkali-aggregate reaction.
• Volume changes due to the difference in the thermal properties of the aggregates and cement paste.
• Permeability of concrete.
• Poor workmanship in compaction and curing.

The durability of concrete can be controlled by using given below 

• Strict supervision in all concrete operations
• Lesser W/C ratio (using optimum water)
• Crushed stone (which are durable)
• Good quality cement (which is sound)
• Proper curing
• Well graded aggregates

3. Impermeability

Concrete is considered to be impermeable if it shows resistance to the flow of water into the pores (voids) in it. Hardened concrete must be impermeable to moisture penetration. Impermeability in concrete is of great interest as it is used in liquid retaining structures. 

Concrete mix prepared with an excess of water generally leaves behind pores after evaporating. Concrete of high density and high compressive strength generally leads to impermeable concrete, Impermeability in concrete can be achieved by given below.

• Selecting well-graded aggregates (having minimum voids)
• Using concretes with a low W/C ratio.
• Proper and uniform compaction.
• Proper curing

4. Dimensional Changes

In addition to the deformation due to loads and any type of other forces, the concrete specimen exhibits other types of deformations also. The following properties of concrete are accompanied by dimensional changes according to time and situation its mainly very little bit changes.

a) Shrinkage

The shrinkage that takes place after the concrete has set and hardened is called drying shrinkage and most of it takes place in the first few months. The extent of shrinkage is dependent on the efficiency of the curing arrangement and the porosity of the formworks. 

Also, the magnitude of shrinkage is dependent upon the mix proportion and water-cement ratio. The extent of shrinkage increases by high cement contents and a high W/C ratio. Shrinkage cannot be eliminated completely but can be reduced by using : 

• Properly designed low W/C ratio
• Using saturated aggregates
• Tight and non-absorbent formwork
• Reducing the height of the fall of concrete during placing.

The only advantage of shrinkage is that concrete grips the reinforcement thus increasing the bond strength. Bond strength reduces the chances of slipping of steel bars from concrete.

b) Creep

When the concrete is subjected to sustained loading then it shows continuous deformation which is termed as a creep. Creep is also known as time yield or plastic flow of concrete. On removal of sustained load, the creep does not disappear completely i.e., creep is permanent. Factors affecting creep are : 

• Age - The rate of creep also decreases with the increase in time. Creep is a long term process and hence takes further time.

• Strength - More is the strength of concrete, lesser will be the effect of creep.

• Aggregates - Creep is more as the size of aggregates becomes finer but creep increases when aggregates are porous in nature.

• Curing time - Due to an increase in curing time, the hydration of cement proceeds then the chances of creep decreases.

• Wetting and drying of concrete - Alternate wetting and drying of concrete lead to increased magnitude of creep.

c) Elasticity

Concrete is not perfectly elastic because stress, in this case, is not proportional to strain. The resistance of concrete to deformation is termed as elasticity. The factors which influence modulus of elasticity are the type of ingredients used, strength, moisture content, age of concrete, etc. Modulus of elasticity can be improved by using low W/C ratios, rich concrete and proper and longer curing periods

d) Thermal Expansion

Concrete expands with a rise in temperature and shrinks as the temperature falls. The change in unit length, of concrete, per degree change in temperature, is known as Coefficient of Thermal Expansion. The value of the coefficient is more for richer mixes than lean mixes. The expansion coefficient is highest for quartz and lowest for limestone. As the temperature rises the expansion in concrete increases due to heat released during exothermic chemical reactions between cement and water. Due to the thermal expansion in concrete also causes considerable deformation and cracks. It can be controlled by providing expansion and contraction joints in big concrete structures. 

Also read - Properties Of Concrete In Plastic Stage