concrete compressive behaviour

Question 1

Why is the compressive behavior of concrete important?

Answer 1

1. Concrete is primarily used to transfer compressive loads.
2. Compressive strength is easy to test.
3. Concrete is typically classified by its compressive strength.
4. Compressive strength can be related (indirectly) to other material properties that are not so easy to measure directly, e.g. elastic modulus, tensile strength, etc.

Question 2

Describe Concrete compression testing

Answer 2

• The compressive strength is always obtained with a particular specimen size and mould (molde): Cylinder or cube (figure a).
• The test is carried out by applying pressure on the specimen’s surface area (Figure b) causing the specimen to deform (displacement and bulge).

Question 3

Describe Compresnssive strenght and Strain (formula) and make the stress-strain curve (diagram.

Answer 3

From the Concrete compression test, the characteristic compressive strength curve is obtained.

Question 4

Concrete classification. Explain C20/25

Answer 4

The compressive strength of concrete cubes at the age of 28 days “ß28”, is used to classify the concrete into its so-called strength classes or grades.

Question 5

Average (mean) concrete compressive strength (formula and diagram)

Answer 5

Question 6

Behavior of specimen under compression load

Answer 6

The specimen of concrete under compressing loading develops internal compressive and tensile forces that are distributed along the specimen as shown in Figure a.

These internal forces induce the deformation of the specimen. The specimen experiences a deformation in both directions (parallel and perpendicular to the loading) causing the specimen to crack. Cracking patterns develop at increasing levels of uniaxial loading is shown in Figure b.

Question 7

Typical crack pattern through normal strength concrete

Answer 7

Energy dissipation during crack propagation leads to nonlinear strainsoftening behavior

Question 8

Describe the two types of cracking

Answer 8

• Micro-cracks – They influence only marginally the stress-strain compression relationship of concrete
• Macro-cracks - These cracks develop under loading and are manifested in concrete in different forms depending on the strength relationship between the hardened concrete paste and the aggregate

Question 9

Typical stress-strain curves for different concrete strengths (diagram)

Answer 9

In Figure 3-6 the stress-strain compressive behavior of specimen with different concrete strength is shown. For all the specimens the strain (ε) at peak load can be taken approximately as 0.002. In compression the stress-strain curve behaves as a straight line up to approximately 0.3*fc then the behavior becomes increasingly non-linear up to the peak load. After peak loading the concrete displays a strain-softening behavior

Question 10

What are the elastic modulus (E) and Poisson’s ratio (n) for concrete?

Answer 10

• Elastic modulus (E): The elastic modulus of elasticity (Young modulus) of the concrete is the combination of the modulus of the aggregate and the cement paste. The modulus of elasticity might be defined as the ratio of normal stress to corresponding stress for tensile of compressive below the proportional limit of a material (Figure a).
• Poisson’s ratio (ν): The Poisson’s ratio is the ratio, when a sample object is stretched, of the contraction or transverse strain (perpendicular to the applied load), to the extension or axial strain (in the direction of the applied load). When a material is compressed in one direction, it usually tends to expand in the other two directions perpendicular to the direction of compression. Poisson’s ratio (n) for typical concretes ranges between 0.18 to 0.24.

Question 11

Definition of concrete elastic modulus (E) in a diagram

Answer 11

Question 12

What is the effect of confinement on concrete compressive behavior?

Answer 12

• Confinement can significantly increase the compressive strength and ductility of concrete.
• Reinforced concrete is typically confined by using reinforcement steel.
• The steel can be used internally in the form of steel reinforcement (i.e. spiral reinforcement in reinforced concrete columns) or externally in the form of a steel jacket bonded to the external face of a column.
• The fiber reinforced polymer (FRP) is generally applied externally by wrapping it around the
• structure in strengthening measures.
• Also, in practice, concrete used to transfer compressive loads is frequently confined by surrounding concrete

Question 13

How does specimen geometry affect compressive strength and behavior? (size effects & boundary conditions)

Answer 13

• 1. In general, concrete compressive strength decreases as specimen size increases.
• 2. The relation between geometry and behavior can be described in terms of the statistical size effect, deterministic size effect, and the boundary conditions.
• 3. Specimens with a large aspect ratio are less susceptible to boundary shear effects.
• 4. The influence of boundary shear on the peak load can be reduced using brush bearing or Teflon loading platens.
• 5. It is important to know how the concrete was tested when considering its compressive strength and stress-strain curve.

Question 14

How do internal and external influencing factors affect the compressive behavior of concrete? (Composition, Curing, Testing)

Answer 14

Assuming that all other concrete mix proportions remain the same.