Properties of Hardened Concrete Flashcards
What are the major concerns about hardened state of concrete?
Strengths of hardened concrete
–Compressive
–Tensile
–Shear
Elastic behaviour
–Stress-strain curve
–Elastic modulus
–Poisson’s ratio
Dimensional stability
–Drying shrinkage
–Creep
–Thermal expansion (not covered)
Durability
What is strength of hardened concrete?
The ability of a material to resist stress without failure. The failure of concrete is due to cracking.
Under direct tension, failure caused by propagation of single major crack.
In compression, failure involves propagation of multiple cracks, leading to disintegration known as ‘crushing’.
It is the property generally specified in construction design and quality control.
What are the different tests for compressive strength of concrete?
Cube test: Cube and cylinder cast in-situ (estimate potential strength)
Core test: Cores are drilled (determine actual strength)
What are the factors influencing cube test?
Cube size
Moisture condition
Loading rate
Uneven loading
Restraint effect
Describe the influence of cube size on compressive strength of concrete.
150 mm cubes fail at stresses ~5 % lower than those of 100 mm cubes because of ‘weak link’ theory, since the larger quantities of material present in a large cube is more likely to contain a very weak region.
Large size also gives less variability in test results, which is more important when large size aggr are used (40 mm), in which case 150 mm cubes are always recommended.
Describe the influence of moisture conditions on compressive strength of concrete.
Test results for cubes depend on their moisture
condition, a cube dried just before testing will give
a higher strength. So since ‘wet’ state is
most reproducible condition, cubes should
always be tested wet.
Describe the influence of loading rate on compressive strength of concrete.
Decreasing the loading rate gives lower cube strengths because of the lower inertia effect of the concrete and the increased contribution of creep to failure.
Describe the influence of uneven loading on compressive strength of concrete.
Uneven loading will lead to lower compressive strength because it will create stress concentrations on the cube’s surface. These may be due to particles of
loose material on the cube surface, or irregularities in the surfaces of the loading platens, because of wear.
Describe the influence of restraint effect on compressive strength of concrete.
Restraint effect comes from the large friction between the ends of the sample and the loading platen, which alters the uniaxial compression condition (into a
quasi-triaxial compression condition). It increases compressive strength. Using cylinder samples can reduce the restraint effect.
What is the proper operation of cube test?
Concrete cube centered in machine to minimize eccentricities in loading arrangement.
Machine checked for accuracy in calibration loading.
Describe tensile strength.
Cracks form and propagate a lot easier in tension than in compression. So tensile strength is much lower than compressive strength.
Concrete tensile stresses occur as a result of shear, torsion, and other actions, and in most cases member behaviour changes upon cracking.
Improvement by the introduction of randomly oriented fibres into the concrete mix (e.g. polypropylene, glass and steel fibres).
What are the indirect tests for concrete tensile strength?
Splitting cylinder test: Cylinders 300 mm long and 150 mm in diameter
Flexure test: Determine flexural strength and modulus of rupture, which is the maximum tensile bending stress in a plain concrete test beam at failure.
What is shear strength?
Shear strength is about 35 - 80% of its compressive strength, so even if its subjected to pure shear, it fails because of principal tensile stresses.
What is Poisson ratio?
It is defined as the ratio of transverse strain to longitudinal strain.
The ratio for concrete is very small and neglected (taken 0).
It increases considerably at very high load level.
Typically = 0.20
What is dimensional stability of concrete?
” “ of a construction material refers to its dimensional changes over a long period of time.
If the change is so small that it will not cause any structural problems, the material is dimensionally stable. For concrete, drying shrinkage and creep are two phenomena that compromise its dimensional stability.
Tensile stresses induced by some form of restraint (partly or wholly) to these movements may result in concrete cracking.
