Concrete is the mixture of different construction materials like cement, sand, aggregate, etc. The basic materials among them are cement, sand, and aggregate.
While preparing the concrete, we have to design it according to the purpose and strength required for the building or bridge. A structure like a culvert and bridge will require more strength than a residential building and drainage slab.
So, a civil engineer has to design concrete according to the important factors of the structure. To design concrete one should have deep knowledge of the properties of fresh and hardened concrete.
Fresh concrete is those which are prepared newly while hardened concrete is those whose final setting time has finished and is ready to bear the load on it. The knowledge of the properties of fresh concrete is necessary to know the mixing, placing, transporting, and setting of the concrete in formwork such that it provides us better strength and workability while construction.
The knowledge of dry concrete is necessary to prevent the concrete from different aspects like frost, dirt, and temperature, and also to predict the life of the concrete.
Properties of Fresh Concrete
The stage of concrete before its initial setting time is known as fresh concrete. To prepare fresh concrete we have to mix cement, sand, and aggregate with the required amount of water in it.
- Slump loss
- Plastic shrinkage
- Setting Time
The ease of placing and pouring concrete in the formwork is known as the workability of concrete. The workability of concrete should be low as far as possible to avoid segregation and bleeding in concrete.
The workability of concrete depends upon the water-cement ratio in concrete. More the water-cement ratio greater the workability and vice versa.
2) Segregation/ Bleeding
Segregation and bleeding in concrete are very dangerous for concrete. The separation of ingredients used in the concrete like cement slurry, aggregate and sand from concrete while casting or placing in formwork is known as segregation.
To avoid segregation we should not pour the concrete from a height of more than 1.5 meters and should be over compacted.
Bleeding is the upcoming of water from concrete to the surface of the concrete cast is known as bleeding. Bleeding causes a lack of hydration in concrete which makes the concrete weak.
So, to avoid bleeding concrete should not be over compacted and the water-cement ratio should be maintained in fixed proportion according to design.
Harshness is the resistance offered by concrete to its surface finish. Harshness in concrete is caused due to the mixing of lower quality of fine and coarse aggregate and cement in the concrete. Sometimes it may be due to a low water-cement ratio than required in concrete.
4) Setting Time
Setting time of concrete is the time at which concrete is set into its plastic state. It is nothing but a different phase of the concrete transformation. There are various factors that affect the setting time of concrete.
Types of cement, fineness of cement, water cement ratio, and weather conditions are the main factors that affect the setting time of concrete.
There are two setting times of concrete. The initial setting time is around 30 minutes and the final setting time is of 24 hours after the casting of concrete. After the setting time concrete is not workable i.e the shape of concrete can not be altered.
5) Hydration of Concrete
When fresh concrete is prepared and poured into a formwork, the concrete begins to come in a plastic state. During this stage, the heat is generated within the concrete. This is called heat of hydration.
The heat of hydration is very important for concrete. The heat of hydration takes place in presence of moisture in the concrete. So if there is a lack of water or moisture in concrete then cracks will generate in concrete.
Hence, to overcome these defects proper curing is required for concrete. According to IS Code, the concrete should be cured for at least 10 days to 14 days after the initial setting of concrete.
Properties of hardened Concrete
When concrete is prepared, it is very weak to bear the load. As its age increases its strength also increases. Concrete takes it’s 99.9% of its strength in 28 days. Proper curing should be done for acquiring the required strength in concert after de-shuttering of concrete for 10 days to 14 days.
The strength of the concrete is the maximum amount of load it can carry without failure. Concrete is designed according to the strength required for the structures. High-quality concrete provides high strength.
The strength of concrete is judged by its different capacities of concrete. The most important test are the compressive strength test, tensile strength test, and flexural strength test. Concrete is weak in tensile strength and strong in compressive strength so, they are made strong in tensile strength by providing steel rebar.
a) Compressive Strength of concrete
- The compressive strength of concrete is measured per unit area.
- It is also called the crushing strength of concrete.
- The compressive strength of concrete is usually determined by crushing the concrete cube of the standard dimension of about 15cm x 15cm x 15cm.
- The test is carried out in 3 days, 7 days, 14 days, and 28 days of the concrete cube.
- Crushing strength can be calculated by Cs=Load / Area.
Strength gained by concrete as its age increases
b) Tensile strength of Concrete
- Tensile strength of concrete can be calculated with the help of a splitting tensile strength test on concrete structure.
- Due to its brittle nature, Concrete is weak to bear tensile force. so it is not expected to bear tensile stresses.
- Concrete generates cracks when subjected to tensile stress. so to overcome this we have to provide steel rods in concrete.
- Hence, it is necessary to design the structure very carefully to bear tensile force.
c) Flexural Strength
This test is done on the concrete without steel rods to test the bending moment capacity of concrete.
To test the flexural strength of concrete a beam is prepared of size 15 cm x 15 cm x 70 cm when the size of aggregate is less than 38 mm. and of size 10 cm x 10 cm x 50 cm when the size of aggregate is less than 19 mm.
Creep is the deformation of concrete under continuous loading for a long period of time. When concrete is loaded for short time then it again restores its original shape but when it is subject for a longer period of time, it can not restore its original shape. This deformation is known as creep.
Shrinkage in concrete is also of three types.
- Plastic shrinkage
- Drying shrinkage
- thermal shrinkage
Initial shrinkage happens just after the freshly prepares concrete is placed in formwork till its initial setting time. Shrinkage in concrete happens due to volumetric change due to evaporation of water, and bleeding of water from concrete.
Shrinkage in concrete more than assumed may cause cracks in concrete and make it weak to bear the load. So, proper curing of concrete is required for the complete hydration of concrete.
Drying shrinkage may cause due to contraction of gel structure if the moisture in concrete becomes less.
Finally, thermal shrinkage may occur due to falling temperatures below 20 degrees celsius. Thermal shrinkage can be neglected on its own account because the shrinkage is very less as compared to plastic shrinkage.
Expansion in concrete may occur due to thermal action as well as a chemical reaction in concrete. Sometimes we pour the concrete into different layers. At that time when we pour concrete on the previous layer when they are not completely set, then chemical expansion occurs in the lower layer.
This happens because the heat of hydration accumulates under the top layer and causes observable expansion. These types of expansion are very dangerous and may cause cracks in concrete.
5) Permeability of Concrete
The ability of concrete to let passing the fluid material through it is known as permeability of concrete. There should not be permeable concrete in the structure. Permeable concrete reduces the durability and strength of concrete.
The permeable concrete allows the harmful solution to enter the concrete, which reacts with concrete ingredients, cement chemicals, and steel rods and spoils the quality of concrete as well as steel rods.
Permeability in concrete may cause leaching of concrete, efflorescence, and sulfate attract in concrete and make it weak in strength as well as durability.
The durability of concrete is the strength of concrete to carry the load on it for a long time. Durability in concrete is a very important property as it decides the age of the structure to be workable.
The durability of concrete depends upon several factors like environment, void in concrete, permeability, and quality of ingredients in concrete.