Concrete tensile behaviour

Question 1

Why is the tensile behavior of concrete important?

Answer 1

1. Cracking and ultimately failure is caused by tensile strains. (This holds for concrete in compression as well as tension.)
2. For the transmission of concentrated tension loads, e.g. from:
   • reinforcement,
   • anchorages.
3. Determination of some design quantities, e.g. minimum reinforcement ratio for crack control, etc.

Question 2

How (& why) does concrete behave differently in tension than in compression?

Answer 2

1. Cracks form perpendicular to the tension strains.
2. Concrete tensile strength is about 1/10th of its compressive strength.
3. Tension stress-strain behavior is roughly linear up to 0.9*fct. (Different progression of crack formation compared to compression.)
4. Micro-cracks accumulate to form a macro-crack in the post-peak region.
5. As a macro-crack forms micro-cracks in the surrounding region close.

Question 3

How is tension strength tested?

Answer 3

1. Three types of tests are typically used to test the tensile strength of concrete: direct tension, splitting, bending.
2. The direct tension test is the most difficult to conduct and is seldom performed in practice.
3. The longer the region of uniform tensile stress, the more realistic (lower) the average tensile strength value.
4. Tension strength formula (figure). C depends on:

• direct = 0.24
• splitting = 0.27
• 4-pt. bending = 0.45
• 3-pt. bending = 0.50

Question 4

A consistent description of tension behavior

Answer 4

1. Stress-strain (s-e) behavior in the post-peak region (strain-softening) is dependent on the length over which the displacements are measured. This is due to strain localization in the measurement direction and the definition of strain De=Dl/l.
2. Stress-crack opening (s-w) consistently describes tension behavior in the post-peak region.
3. A continuous stress-strain (s-e) curve for behavior in tension is possible if the post-peak curve is “regularized” using the concrete fracture energy GF.

Question 5

How do internal and external influencing factors affect the tensile behavior of concrete?

Answer 5

The two primary quantities of interest in tension are the tension strength (fct) and the fracture energy (GF). Assuming that all other concrete mix proportions remain the same the effects are shown in the table.