IEM PI Section A – Design experience related 2

(For PI section A, candidate will have to write an essay, which the question was earlier prepared by interviewers based on candidate’s Report on Training and Practical Experience. The following were predicted questions, by Mr. BAY based on his submitted report to principal interview and his trial exercise prior to sitting of PI.)

Design Experience Related Question

What are the design concerns for non-suspended ground slab and rectification method? Is the suspended ground to follow same design criteria? What the work procedure to strive for minimum cracking problem?

Introduction

Non-suspended ground slab is supported directly by earth throughout its area. The following aspects are design concerns for non-suspended ground slab.

1)      Drying Shrinkage 

Drying shrinkage is characteristic behavior of concrete. If the ground slab is restrained from movement, there will be locked-in stresses inside ground slab.

The strain in plain concrete, in accordance to BS8110: Part 2 is recommended 500x10-6 for calculation. This strain represents 1.5mm movement per 3m length. The resulted stress is 10N/sq.mm for concrete of Ec 20kN/sq.mm. This stress is often the cause of ground slab cracking.

2)      Thermal expansion

Similarly to drying shrinkage, the difference in temperature of the ground slab during day and night will also result the ground slab to expand and contract. The repetitive built-up and release of stresses is one cause of floor cracking.

3)      Creep effect of concrete

Creep is also characteristic behavior of concrete. Though it will reduce the adverse impact of concrete shrinkage, we do not rely on this in practice.

4)      Movement joint

Like loading, we provide reinforcement to resist stresses arisen from drying shrinkage, thermal expansion, etc. However, it is always not the practical and successful approach to control slab cracks.

The more appropriate method is to allow the ground slab to move. Hence, movement joints shall be stipulated at estimated location. In addition, saw cut line or crack inducers could control the crack pattern into more pleasant ways.

5)      Subgrade condition and modulus of subgrade reaction (K)

The earlier paragraphs are concerns related to characteristic behavior of concrete. The solution is provision of rebar and creation movement joint and saw cuts.

The slab thickness should also be checked with forces induced by the loading. Provision of thicker sub-base and well-compacted crusher run definitely could make stronger ground slab structure. It is because greater thickness of granular sub-base results greater value of modulus of subgrade reaction, K. Greater K value will result smaller moment forces of the ground slab.

The above are concern during design stages.

Works during construction are also very important to ensure good quality of non-suspended ground slab. During construction, the soften earth shall be replaced with suitable fill material and compacted. The whole area shall also be rolled and compacted by rollers and compacter. Top rebars shall be checked place closed to top surface as practical as possible. Joint and crack inducer line such saw cut shall be started the 2^nd days of concrete casting.