Fresh vs Hard Properties of Concrete | Explained
Concrete is an artificial building material obtained by mixing cement, fine aggregate, coarse aggregate, and water in suitable proportions. Cement act as a binding material and it forms a paste with water. It holds coarse and fine aggregates together to form a solid mass. The concrete mix is used for constructing slabs, roofs, retaining walls, pillars, dams, etc.
Concrete is a mixture of cement, aggregate, and water, the properties of constituents affect the properties of concrete. The quantity of cement and water affects the strength of concrete. The size and shape of aggregate affect the strength and workability of concrete. Compaction (the method of removing voids between the aggregate) also influences the properties of concrete.
There are two stages for concrete
· freshly prepared concrete i.e. in plastic stage
· hardened concrete (after attaining its full strength)
Components of Concrete
The components of concrete are as follows:
Ø Cement: Cement is the most important ingredient of concrete. It is used to bind the fine and coarse aggregates together. Most of the concrete is prepared with ordinary Portland cement. Other special varieties of cement-like waterproof cement, rapid hardening cement, and high alumina cement are used under specific circumstances.
Ø Fine aggregate: Fine aggregates are inert or chemically inactive material. River sand, crushed stone, crushed gravel, etc. are used as fine aggregate. Fine aggregates are used to make the concrete denser by filling the voids of coarse aggregates. It is used to make the concrete mix economical and it reduces the shrinkage of concrete on hardening.
Ø Coarse aggregate: Uncrushed gravel, broken bricks crushed stone, etc. are used as coarse aggregate. They make concrete strong and tough.
Ø Water: Water is used for mixing concrete. It should be potable (fit to drink). Water acts chemically with cement to form a paste for binding the aggregates. It also enables the concrete to flow into molds. Water required for hydration of one part of cement is about 0.3 part of water, but for lubricating, the aggregate extra water is required.
Hence, the water-cement ratio for the medium and high strength concrete is 0.6 and 0.35 respectively. Insufficient quantity of water makes a concrete mix harsh and unworkable whereas an excess quantity of water causes bleeding and segregation of concrete.
The strength of concrete is inversely proportional to the water-cement ratio. That is if we decrease the water-cement ratio we can increase the strength of concrete. But for water-cement ration less than 0.35 it is very difficult to work with than concrete.
Water cement ratio depends upon the following factors:
· Quality of cement
· Quality of aggregate
· Internal moisture content
· Atmospheric temperature
· Size and age of test specimen
Sometimes ingredients other than the above are added in concrete for improving or imparting certain properties and these ingredients are known as admixtures. The addition of admixtures improves durability, workability, water-resisting power, the strength of concrete, setting time of concrete, reducing shrinkages, imparting color, reducing bleeding, etc. Commonly used admixtures are lime, alum barium oxide, soap, aluminum sulfate, calcium chloride, mineral, and organic oils, bitumen, etc.
Properties of freshly prepared concrete
The properties of freshly prepared concrete are as follows:
· Workability: Workability is the ease with which concrete can be mixed, transported, and placed in position.
· Segregation: Segregation can be defined as the separation of the constituents of a concrete mix during the transportation of prepared concrete.
· Bleeding: Bleeding is defined as the separation of water or mortar from the freshly prepared concrete. This is due to the high water content in the mix. This will cause the formation of porous, weak, and non-durable concrete at the top of the placed concrete.
Properties of hardened concrete
Properties of hardened concrete are as follows:
· Strength: Good quality concrete in a hardened state should possess desired crushing strength. The compressive strength of concrete at 28 days after casting is known as the designed strength of concrete.
· Durability: It is the ability of concrete to resist weathering agents, chemical action, seawater, fire, and wear. Durability depends upon the quality of aggregate and water-cement ratio.
· Shrinkage: Concrete experiences shrinkage during hardening. This depends upon the constituents of concrete, atmospheric temperature, and size of the structure. It can be minimized by using the designed quantity of water-cement ratio and by proper curing.
· Creep: Creep may be defined as the time-dependent elongation of concrete structure under the external loads. It depends on the water-cement ratio, nature of aggregate and its grading, the humidity of the air, intensity and duration of load, and age at the time of loading.
· Fire resistance: It has good fire resistance properties and acts as a good insulator.
· Density: According to BS EN 1922-1-1, the weight density of concrete should be approximately 2500kg/m3.
· Bond strength: The strength of the bond between the steel reinforcement and the surrounding concrete is a very important factor for RCC structures. Bond strength is determined by pullout tests i.e. the load required to cause a slip of 0.25mm divided by the area of contact between the steel bar and concrete.
· Elasticity of concrete: Concrete behaves elastically only up to 10 to 15% of its ultimate strength. Up to that loading, condition stress is directly proportional to strain. Modulus of elasticity of concrete depends upon the strength, water-cement ratio, moisture content, quality of aggregate, and age of the structure.
· Permeability: Concrete consists of so many pores. Permeability of concrete is a function of porosity, size, and continuity of pores. The porosity of cement gel is about 28% but its permeability is 7×10-14 cm/sec. It is due to the fine texture of hardened concrete. Factors that affect the permeability of concrete are water-cement ratio, quality of aggregate, curing, uniformity of concrete, etc.
· Thermal properties of concrete: Basic thermal properties of concrete are thermal conductivity and diffusivity, specific heat, and coefficient of thermal expansion. These properties of concrete are required for the study of insulating properties of concrete, sweating of concrete in different climates, temperature conditions in mass concrete.
· Resistance to wear and tear: Concrete floors and road and runway pavements are subjected to abrasion and impact, which cause wear of the surface. The wear resistance of concrete can be improved by using a lower water-cement ratio, lowest slump, good quality graded aggregate, etc.
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Testing of concrete
Compressive strength test of concrete
The compressive strength of concrete depends on the water-cement ratio, grading of aggregate, the proportion of constituents, type of aggregate, mixing efficiency, etc. The strength of the concrete usually increases with the age.
Among all other properties of concrete, the compressive strength of concrete is the most important property and this value is used for the design of structures. Hence for the design, we must know the strength of concrete.
For compressive strength testing, first, prepare 12 cubes of concrete with 15cm sides. 6 cubes will be cured for 7 days and the rest of them will be cured for 28 days. The cured specimen is loaded in a compression-testing machine at a standard rate of 315 kN/minute. Note down the load at which the specimen ceases to break.
Compressive strength can be calculated using the formula given as follows:
Similarly, we can also find the compressive strength of concrete for 28 days.
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2. Pablo Tamayo; Joao Pacheco; Carlos Thomas; Jorge de Brito and Jokin Rico, Mechanical and Durability Properties of Concrete with Coarse Recycled Aggregate Produced with Electric Arc Furnace Slag Concrete, Applied Sciences, 2020, doi:10.3390/app10010216
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4. Tehmina Ayub; Sadaqat Ullah Khan and Fareed Ahmed Memon, Mechanical Characteristics of Hardened Concrete with Different Mineral Admixtures: A Review, The Scientific World Journal, Hindawi Publishing Corporation, 2014 http://dx.doi.org/10.1155/2014/875082