Generally concrete structures are designed for sizing and strength of structural members and not for durability. Durable design of concrete members as much important as design for strength, to give the structure long life.
Safety of structural members does not only depend on its strength, but also on its durability. As the concrete ages, it is exposed various kinds of changes which may affect its strength, suitability, stability. So, design for durable concrete structures must be considered.
Design Engineers consider design of concrete structures to assess:
- The size and strength of structural components and concrete strength grade to meet safety and serviceability limits.
- The amount, size and distribution of reinforcements for strength and control cracks to an acceptable size.
It is anticipated that with good site control and good workmanship, the structure and its components shall last indefinitely. They have forgotten to take into account the environmental loads while designing the structures (Structures are designed adequately for Dead Load (DL) and Live Load (LL) and sometimes for erection loads).
Environmental factors affect durability of structures. Hundreds of bridges and structures are collapsing or showing signs of deterioration with corroding reinforcement – all within 25 years of construction.
It is, therefore, necessary for the designer to develop a feel for the problem and design the structures to satisfy safety, serviceability and durability requirements (structural and non-structural loads caused by environment).
Fig: Effect of Low Durability of Concrete
Table of Contents
Design of Concrete Structures for Durability
As durability depends upon permeability. It is important to control parameters affecting permeability.
Therefore, it is necessary to lay down permissible limits for parameters like minimum cementitious material content, maximum water cement ratio, maximum crack width and extent of cover to reinforcement etc. depending upon the exposure condition. Following values for these parameter are recommended.
i) Minimum cementitious material content and maximum water cement ratio:
Recommended values are given in table-1.
Table-1: Minimum cementitious materials and maximum water-content ratio for PCC, RCC and Prestressed Concrete
The areas covered under different exposure conditions shall be asunder:
Mild: Concrete surfaces protected against weather or aggressive conditions.
Moderate: Concrete surfaces sheltered from severe rain or freezing while wet, concrete continuously under water.
Severe: Concrete surface exposed to severe rain, alternate wetting and drying or occasional freezing or severe condensation. Concrete exposed to aggressive sub soil / groundwater or coastal environment.
Very severe: Concrete surface exposed to seawater spray, corrosive fumes, severe freezing.
Extreme: Concrete surface exposed to abrasive action. Surface of members in tidal zone.
* Min. Cementitious material content is for 20mm maximum size aggregate (MSA)
Add extra cementitious material:
- For 10 mm MSA = +20 Kg/m3
- For 40 mm MSA = – 10 Kg/m3
Mini mum grade of concrete mix should be M-40 for pretensioned PSC bridges.
Note: The maximum content of cementitious material should be 500 Kg/m3.
ii) Minimum clear cover:
Recommended values are given in table-2. The clear cover shall mean cover from the outer most metal /steel, binding wire or its end.
Table-2: Minimum clear cover for concrete structures under different environmental conditions:
While designing, it should be ensured that cover does not exceed 2.5 times dia of reinforcing bar. If cover is more, chicken mesh may be provided in cover concrete to keep the concrete in position.
iii) Flexural Crack Width:
It is necessary to control the crack width to protect steel rebar against corrosion. The crack width is controlled by reducing shrinkage, and distributing the reinforcement over the zone of maximum concrete tension, uniformly, and using smaller dia bars.
Cement with low heat of hydration and not too fine should be used. Recommended value of maximum crack width to be considered during design are given in table-3.
Table-3: Maximum crack width to be considered during design