What are the Properties of Green Concrete and Hardened Concrete?
Properties of Green Concrete and Hardened Concrete
Green Concrete has completely different properties when it is in the plastic stage and when hardened. Concrete is a plastic stage and is also known as a hunk of green concrete. The properties of green concrete include:
1. Properties in Green Concrete
Workability: This is defined as the ease with which concrete can be compacted fully without segregation and bleeding. It can also be defined (explained) as the same amount of internal concrete works are required to fully compact the concrete into the required optimum density. The concrete workability depends upon the quantity of the water and grading and shape, and the percentage (%) of the aggregates present in the concrete (Green Concrete ).
Workability is measured by
(a) The method of slump test is observed when the frustum of the standard testing cone filled with concrete is lifted after removal.
(b) The compaction (compact) factor is determined after allowing the concrete to fall through in the compaction testing machine.
(c) The time taken in the second period for the shape of the concrete to change from cone to cylinder when tested in Vee-Bee sensitometer (Green Concrete ).
|Slump||Compaction Factor||Time in Vee-Bee|
|Concrete of shallow sections with vibrations|
0.75 – 0.80
10 – 20
|Concrete light reinforced sections with vibrators|
0.80 – 0.85
5 – 10
|Concrete of lightly reinforced sections (mild reinforcement) without vibrations and heavily reinforced sections with vibrations|
25 to 75 mm
0.85 to 0.92
2 to 5
|Concrete of heavily reinforced sections without vibration||75 to 125 mm||More than < 0.92|
Separation (sequestration) of coarse aggregate particles from the green concrete (plain concrete) is called segregation. This may happen due to a lack of sufficient quantity of finer particles in mixed concrete or due to the throwing of the mixed concrete from greater heights at the time of placing the concrete. Because of the segregation (separation), the cohesiveness (selected) of the concrete is lost, and honeycombing results. Ultimately it results in the loss of strength of hardened concrete. Therefore most care is to be taken to avoid segregation (Green Concrete ).
Bleeding: This procedure refers to the appearance of the water along with cement particles on the surface of the freshly laid concrete. This happens when there is an excessive quantity of water in the mix or due to excessive compaction. The Bleeding causes the formation of pores (unmixing concrete) and renders the concrete weak. The process of Bleeding (mixing) can be avoided by suitably controlling the quantity of water (calculation) in the concrete and by using finer grading of aggregates.
Harshness: Harshness (roughness) is the resistance offered by the concrete to its surface finish. Harshness is due to the presence of a lesser quantity of fine aggregates, lesser cement mortar, and the use of poorly graded aggregates. It may result due to an insufficient quantity of water. With harsh (hardness) concrete, it’s difficult to get a smooth concrete surface finish and concrete becomes porous (Green Concrete ).
2. Properties of Hardened Concrete
The properties of hardened concrete are:
- Resistance to wear
- Dimensional changes
- My permeability
Strength: The characteristic strength of concrete is defined as the compressive strength of 150 mm size cubes after 28 days of curing below which not more than 5 percent of the test results are expected to fail. The unit of stress used is N/mm2. An IS-456 grades the methods of concrete based on its characteristic strength as shown in Table below.
|concrete Characteristic strength in M N/mm2||10N/mm2||15 N/mm2||20 N/mm2||25 N/mm2||30 N/mm2||35 N/mm2|
Until 2000, M15 concrete was permitted to be used for reinforced concrete works. But, An IS – 456–2000 specifies a minimum grade of M20 grade should be used for reinforced concrete works in civil engineering.
The strength of concrete depends upon the amount of cement content, quality and grading of aggregates, water-cement ratio, compaction, and curing. The strength of concrete is gained in the initial stages. In 7 days, the strength gained was as much as 60 to 65 percent of 28 days strength. It’s customary to assume the 28 days (curing test) strength as the full strength of the concrete. However, concrete gains strength after 28 days also. The characteristic strength may be increased by the factor given in Table.
The Minimum< age of the testing member, when design the load is expected
|1 month||3month||6 months||12 months|
The tensile strength may be estimated from the formula f t = 0.7 feck N/mm2, where fck is the characteristic compressive stress. The modulus of elasticity may be estimated from the formula is an E = 50 (fifty) fck N/mm2.
Dimensional Change: Concrete shrinks with age. The total calculated shrinkage depends upon the constituents of the concrete and size of the member and the environmental (admas) conditions. Total shrinkage is approximately 0.0003 of the original dimension. The permanent dimension changes due to loading over a long period are termed as a creep. Its value depends upon the stress of concrete strength, the age of the concrete at the time of loading, and the duration of the loading. The ultimate creep strain may be estimated from the values of the creep coefficient. The creep coefficient is defined as the ultimate creep strain divided by the elastic strain at the age of loading. These values are listed in Table (Green Concrete ).
Age of Loading
|7 days||28 days||1 year|
The size of the concrete may change due to thermal expansion also calculated. The coefficient of the thermal expansion depends upon the nature of the used cement and the type of aggregates, cement content, relative humidity, and the size of the sections of the structural elements. The below-given table shows the coefficient of thermal expansion of green concrete with different types of course aggregates.
Types of Aggregate
|Coefficient of Thermal Expansion/C°|
(1.2 to 1.3) × 10– 5
(0.9 to 1.2) × 10– 5
(0.7 to 0.95) × 10– 5
(0.8 to 0.95) × 10– 5
Durability: The environment of forces such as weathering and chemical attack and heat & freezing & and thawing try to destroy concrete. The duration of this test exists of concrete without getting adversely affected by these forces known as durability. Generally dense and strong concretes have better durability. Cube crushing strength alone is not a reliable guide to durability. Concrete should have adequate cement content and should have a low water-cement ratio in concrete.
I permeability: This is the resistance of concrete to the flow of water through its pores. Excess water during concrete leaves many continuous pores leading to permeability. Since the permeability reduces the durability of concrete, it should be kept very low by using a low water-cement ratio, dense and well-graded aggregates, good compaction, and continuous curing at low-temperature conditions.