The serviceability limit states are discussed in BS8110: Part 1, section 2.2.3. The code states that account is to be taken of temperature, creep, shrinkage, sway and settlement. The main serviceability limit states and code provisions are as follows;
(a) Deflection : The deformation of the structure should not adversely affect its efficiency or appearance. Deflections may be calculated, but in normal cases span-to-effective depth ratios can be used to check compliance with requirements.
(b) Cracking : Cracking should be kept within reasonable limits by correct detailing. Crack widths can be calculated, but in normal cases cracking can be controlled by adhering to detailing rules with regard to bar spacing in zones where the concrete is in tension.
In analyzing a section for the serviceability limit states the behavior is assessed assuming a linear elastic relationship for steel and concrete stresses. Allowance is made for the stiffening effect of concrete in the tension zone and for creep and shrinkage.
FAILURE IN CONCRETE STRUCTURE
Failure in concrete structures can be due to any of the following factors;
- Incorrect selection of materials
- Error in design calculations and materials
- Chemical attack
(a) INCORRECT SELECTION OF MATERIALS
The concrete mix required should be selected to meet the environmental or soil conditions where the concrete is to be placed. The minimum grade that should be used for reinforced concrete is grade 30. Higher grades should be used for some foundations and for structures near the sea or in an aggressive industrial environment. If sulphates are present in the soil or groundwater, sulphate-resisting Portland cement should be used. Where freezing and thawing occurs air entertainment should be adopted. Further aspects of materials selection are discussed below.
(b) ERROR IN DESIGN CALCULATIONS & MATERIALS
An independent check should be made of all design calculations to ensure that the section sizes, slab thickness etc. and reinforcement sizes and spacing specified are adequate to carry the worst combination of design loads. The check should include overall stability, robustness and serviceability and foundation design.
Incorrect detailing is one of the commonest causes of failure and cracking in concrete structure. First the overall arrangement of the structure should be correct, efficient and robust. Movement joints should be provided where required to reduce or eliminate cracking. The overall detail should be such as to shed water.
Internal or element detailing must comply with the code requirements. The provisions specify the cover to reinforcement, minimum thickness for fire resistance, maximum and minimum steel areas, bar spacing limits and reinforcement to control cracking, lap lengths, anchorage of bar etc.