Concrete is made by solidifying a mixture of cement and sand with gravel in proportions that are predetermined.
Concrete properties change depending on the proportions and quality of the ingredients used in the mix.
Concrete is very strong in compression but weakens in tension. Concrete is, therefore, weak in bending and shear. It is therefore recommended to avoid using plain concrete in situations where compressive strength, weight, and other important factors are required. Tensile stress should be absent or very low.
Concrete: Its Properties
Concrete mix design is based on the following properties:
(i) Weight
The unit weights for plain concrete and concrete reinforced with crushed natural stone or sand are 24 KN/m3 each.
The Compressive Strength
The compressive strength is affected by age, cement, and water-cement ratio. The strength at 28 days is a good indicator of the concrete’s characteristic strength. Strength at seven days will be about two-thirds of the strength at 28 days for ordinary Portland cement. This is a good indicator to retain exact strength.
The strength of the muscles increases with age.
The strength of the cement increases after 28 days. The amount of strength gained depends on the type and grade of cement, environmental conditions, and so forth.
The tensile strength is the fourth characteristic of concrete
When the designer wishes to estimate the tensile from the compressive strength of the concrete, the following formula must be used.
Flexural strength, for = 0.7fck
Elastic Deformation N/mm2
The modulus depends primarily on the elastic properties in the aggregate and less on the curing conditions and aging of the concrete. It also depends on the proportions of the mix and the type and quality of cement. The modulus is usually associated with the concrete’s compressive strength.
Ec=5000fck N/mm2
Here, Ec is the short-term static modulus elasticity in N/mm 2
Concrete shrinkage (vi)
Shrinkage is a time-dependent deformation that is usually compressive. Concrete shrinkage is affected by the concrete ingredients, the size of the concrete member, and the environmental conditions. The total shrinkage is determined by the amount of water in the concrete when it’s mixed at a certain temperature and humidity. In the absence of test results, the estimated value for the total shrinkage is 0.0003.
<(viii) Creep concrete
Concrete creep is a time-dependent deformation that occurs when it continues to deform under compression. When the stresses are released, some of the creep strains will recover.
The following factors affect the creep of concrete
- Concrete has many properties
- Water/cement ratio
- Humidity and temperature for curing
- Humidity during use
- Age of concrete when first loaded
- Stress and its magnitude
- Surface-volume ratio (surface area) of the member
It is common to examine aggregates with a combined grading, combining fine and coarse materials in proportions intended for the concrete mix. This allows an estimation of the concrete mix’s performance. When the combined aggregate gradation (percentage retained against sieve size) is plotted, these deficiencies can be easily identified and corrected. To achieve the “ideal” aggregate gradation, alternate aggregate sources or supplementary blending can be used.
Thermal expansion of concrete
Thermal expansion is important because concrete can be used in a variety of climates, including those with extreme heat or cold. The coefficient of thermal contraction is determined by the type of cement, the aggregate, the cement content, the relative humidity, and the size of the section.
Concrete Workability and Durability
Concrete’s workability and durability are both very important. Workability is a measure of how easily fresh concrete can be arranged and compacted. The concrete mix must be able to maintain consistency and should not separate the aggregates. It should also flow easily into the form. Where sections are thin, or reinforcement is dense and impenetrable, a mix with high workability will be required. Water content is the main factor that affects the workability of the mix. Mixtures can increase the workability but decrease the strength. Workability is also affected by the size of the aggregate, its shape and grading, the ratio between coarse and fine aggregates, and the ratio of total to cement.