Hardened concrete is concrete that must be sufficiently strong to withstand the structural and service applied loads and it must be durable enough for the environmental risk for which it is designed.
It is the strongest and durable construction material.
Characteristic strength, drying shrinkage, modulus of elasticity, permeability, and resistance to chemical attack are the rigid and time-dependent properties of concrete that can affect structural performance.
Properties of hardened concrete:
The strength of concrete is defined as the maximum stress it can withstand or the maximum it can yield.
Cubes, cylinders and prisms are 3 types of compression test specimens.
Flexible tensile tests are used to estimate the load at which concrete members can crack, while compressive strength is taken as the maximum compression load that it can take per unit area.
Factors affecting the strength of Hardened Concrete:
1. Influence of the constituent materials:
Chemical composition, Fineness & types of cement (e.g. RHPC, SRPC, LHPC). increases the cement strength.
Water cement ratio (w / c) is required for the hydration process, the lower w / c, the greater the compressive power and vice versa.
The compressive strength of hardened concrete increases with the reduction of the size of coarse aggregates.
Due to the fact that smaller size aggregates provide a larger surface area for bonding with the mortar matrix.
The bond strength (greater roughness, higher bond strength) is significantly affected by the improvement in mechanical strength than the roughness of the aggregate surface.
In particular, the effect of penetration (e.g. accelerator, retarder, plasticizer, etc.) depends on the exact nature of the admixture.
2. Degree of Compaction:
Proper compaction increases strength.
When the concrete is compressed, it has very small pores, resulting in very high strength.
The increase in strength of hardened concrete is probably influenced by the amount of voids in the concrete, i.e. entrapped air, capillary perforation, gel orifice, or entrained air.
3. Influence of curing:
Curing is used to promote the hydration of cement and to control the temperature & moisture speed from the concrete.
The longer the duration of treatment, the greater its ultimate strength.
- Early Age: Below 7 days of age.
- Later Age: Over 28 days of age.
4. Influence of test conditions:
- Sample size and shape: Cube, cylinder, and square prism.
- Specify the moisture content and temperature.
- Method of loading.
2. Deformation under load:
It is a stress-strain relationship under normal loading and continuous loading as follows:
Under normal loading:
The first effect of applying a load on concrete is to produce an elastic deformation i.e. as the load increases the deformation increases.
Under continuous loading:
Continuous application of tension causes a slow deformation called creep, the increase of deformation is not proportional, as the deformation decreases as time passes.
When the applied load is released, the hardened concrete does not fully recover its original shape.
Under repeated loading and unloading, the deformation increases at a given load level.
It is defined as resistance to its deteriorating processes that may occur as a result of interactions with its environment (external) or between constituent substances or their reaction with contaminants (internal).
Ability to withstand harmful environmental effects over a long period of time.
The absence of durability is due to the environment to which the concrete is exposed i.e. external or internal causes.
Hardened Concrete has a tendency to become porous due to the presence of voids formed during or after placing of concrete.
Penetration by the substance may adversely affect the durability, i.e. Ca(OH) 2 leach out and corrosion occurs due to lack of air and moisture.
The liquid retaining structure is important in relation to water tightness.
To produce concrete of low permeability, complete compaction and proper curing are necessary while low permeability is important to enhancing resistance to frost action, chemical attack, and protecting embedded steel against corrosion.
The permeability of cement paste varies with the progress of hydration or with age, the permeability decreases as the gel slowly fills the original waterlogged space.
For the same w / c ratio, the permeability of the paste with coarse cement particles is higher than that of fine cement.
In general, the higher the strength of cement paste, the lower the permeability.
Factors influencing permeability of hardened concrete are as follows:
i. W/C Ratio.
iii. Method of compaction.
v. Soundness & porosity of the aggregate.
vi. Age. (permeability decrease with age)
vii. Grading of aggregate.
viii. Types of structures.
It is caused by the disposal of solids and the loss of water free from plastic concrete (plastic shrinkage), by the chemical combination of cement with water and by the drying (drying shrinkage) of concrete.
Shrinkage depends on the amount of drying that can take place.
Affected by the humidity and temperature of the surrounding air, the rate of airflow on the surface and surface area to volume of concrete.
Also read: Self Healing Concrete, Autoclaved Aerated Concrete & Green Concrete
Workability and related properties of fresh concrete are essential for transport and laying, the properties that determine the quality of hardened concrete are strength, durability & dimensional stability.