Concrete and Reinforcing Steel

18 important questions on Concrete and Reinforcing Steel

What are the main components of hardened concrete?

Cement paste, aggregate (sand + gravel), water

How is compression strength of concrete found?

Using compression test on a 4x8" cylinder

Why is NSC heterogeneous?

Different strengths of cement paste and aggregate
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Why is the HSC stress strain response curve linear and brittle?

Similar strengths of aggregate and paste, when concrete cracks it goes right through both

What is the typical compression strength of beams and slabs?

30 - 40 MPa

Why does concrete density influence the elastic modulus?

As density increases, there is less pore space which reduces crack propagation and overall strength

The modulus of elasticity intersects the non-linear stress strain curve at which level?

At 40% stress

At what compression stress is the linear approximation given by modulus of elasticity an unsafe estimate?

60% stress level

How is tension strength measured?

Either in direct tension or through a 3 point bending test

Why is bending tension higher than direct tension strength

Bending tension tests submit less area of the concrete to load. Under direct tension, more volume of concrete is subjected to stress and there is a more likely chance of their being a flaw.

For 30 MPa concrete what are the bending and direct tension strengths?

- Bending = 0.6*sqrt(30)
- Direct tension = 0.33*sqrt(30)

What physical phenomena cause creep and shrinkage?

The loss of adsorbed water molecules from cement paste

How large can creep strain become as a portion of elastic strain?

3*initial deformation

What three factors influence creep AND shrinkage

- Maturity at time of loading
- Humidity level/temperature
- Strength of paste
- Aggregate to cement ratio

What factor only influences creep and not shrinkage?

Level of stress/magnitude of load

Why do reinforcing bars have deformations

Prevent slip, improve bond between steel and concrete

What is the linear portion of the stress strain curve of steel?

Elastic deformation

What is strain hardening?

The strengthening of steel beyond the elastic range due to increased dislocations

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