properties of concrete
concrete is a composite material composed of cement, coarse aggregate, fine aggregate, and water. admixture can be used to enhance concrete properties. for example, superplasticizers added to increase concrete workability and reduce W/C. GGBS and silica fumes added to increase concrete strength and durability. The weakness of concrete compensated by adding steel. concrete is good at resisting compression but weak in tension. therefore steel added to resist tension. the following is the properties of concrete
- compressive strength: concrete possess excellent compressive strength. the compressive strength of concrete depends on many factors such as:
- W/C: the increase of w/c will reduce the concrete compressive strength. the decrease of strength result from the evaporation of water from concrete. and this will increase concrete porosity and reduce strength. Superplasticizer used usually to increase workability and reduce W/C ratio.
- pozzolans: adding silica fume and GGBS to the concrete mix will reduce the amount of cement used. also, it will increase the strength of concrete significantly
- handling, placing and compacting of concrete: skilled workmanship will help in producing dense concrete with high strength and durability.
- curing: curing is a vital factor. the purpose of curing is to prevent concrete from losing water. water is very important to continue hydration reaction to reach the desired strength.
concrete strength tested at 28 days. cylinder (150mmx300mm or 100mmx200mm) or cubes (150mmx150mmx150mm) used to test concrete compressive strength. samples of concrete are submerged in water or kept in a room with constant temperature with 100% humidity.
the ultimate strength of concrete range from 15 Mpa to 140Mpa. the vast majority of concrete used in construction strength ranges from 20 Mpa to 50 Mpa.
- static modulus of elasticity: concrete has no clear modulus of elasticity. value of modulus of elasticity depends on concrete strength, age, type of loading and proportion of concrete ingredients. there are several definitions for modulus of elasticity:
- the initial modulus: can be determined by measuring the slope of the stress-strain curve at the origin.
- The tangent modulus: is the slope of the curve tangent at some point ( the point can be taken as 50% of ultimate strength)
- the secant modulus: is the slope of the line drawn from the curve origin to a point between 25% to 50% of ultimate strength
AASHTO 18.104.22.168 provides an equation for calculating the modulus of elasticity.
this equation valid for concrete unit weight between 0.09 and 0.155Kcf and compressive strength up to 15KSI.
For SI units
this equation valid for concrete unit weight between 1440 and 2500Kg/m3 and compressive strength up to 105Mpa.
- Dynamic modulus of elasticity: dynamic modulus of elasticity is higher than the static modulus of elasticity by 20% to 40%. The using of dynamic modulus of elasticity in seismic analysis seems appropriate.
- Poisson's ratio: Poisson's ratio is the ratio of lateral strain to axial strain. for concrete when a cube is compressed. the size of the cube will contract in longtidunal direction and expand in the lateral direction. for concrete Poisson's ratio is the ratio of lateral expansion to longitudinal contraction. The value of Poisson's ratio ranges from 0.11 to 0.21. most designers doesn't consider Poisson's ratio in their design.
- Shrinkage: evaporation of concrete water will result in a contraction of casted concrete. this contraction will generate shrinkage cracks. these crack will reduce concrete shear strength and durability. shrinkage crack may act as a passage for harmful chemicals. for example, ingress of chloride will cause reinforcing bars to rust. shrinkage continue for many years. but 90% of shrinkage occurred at the first year. shrinkage depends on the following factors:
- W/C: the increase of W/C will increase the shrinkage
- volume to Surface area: the larger the volume to surface area of the member the lesser water evaporation and less shrinkage will occur.
- Temperature: shrinkage is more for hotter weather.
- Humidity: the increase in humidity will reduce the shrinkage of concrete.
- the maturity of concrete: The shrinkage decrease with increasing of concrete age.
- Concrete strength: higher concrete strength means less shrinkage
Shrinkage of concrete can be reduced by maintaining proper curing of concrete and increasing the time between casting and loading.
- Creep: Creep is a time-dependent deformation under permanent load. when a structural member loaded. at the time of loading instantaneous elastic deformation occurs. if the load kept for a long period. the deformation of the member will increase with time. the increase of deformation under sustained load known as creep or plastic flow. 75% of creep will occur in the first year of loading. if the load removed. member will recover most of the elastic strain and small portion of creep strain. maintaining a load on a member for a long time can reduce member ultimate strength by 15-25%. creep depends on the following factors:
- W/C: the increase of W/C will increase the creep
- volume to Surface area: the larger the volume to surface area of the member the lesser water evaporation and less creep will occur.
- Temperature: creep is more for hotter weather.
- Humidity: the increase of humidity will reduce the creep of concrete.
- the maturity of concrete: increase concrete age will reduce creep.
- Concrete strength: higher concrete strength means less creep
creep of concrete can be reduced by maintaining proper curing of concrete and increasing the time between casting and loading.
- Tensile strength: Tensile strength of concrete estimated to be 8-14% of concrete compressive strength. concrete filled with a huge number of fine cracks. these tiny cracks will impair concrete ability to resist tensile loads. The cracks will close under compression forces. tensile strength varies proportionally with the square root of fc'. it is difficult to measure the tensile strength with direct tensile test due to the difficulty in the gripping test specimen and aligning the load. there are two indirect tests used to compute concrete tensile strength: modulus of rapture and split-cylinder test. tensile strength is important when cracks and deformation of the beam are considered.
- Shear strength: it is difficult to achieve a pure shear failure in concrete at the laboratory. the shear strength estimated to be one-third to fourth-fifths of ultimate compressive strength.