Construction materials and techniques have changed with time. The 20^th century can rightly be called the century of the reinforced cement concrete structures and that continues to be even today.

      RCC is used extensively because of the ease of construction and the ability of the concrete to form any shape. RCC construction also gives speed to the work.

       But, there is an important issue in using RCC. The issue is regarding the deterioration of the structures. Recently, heavy deterioration had been reported from all over the world. This is also experienced in India. The deterioration is due to the corrosion of the reinforcement steel bars.

Why do RCC structures deteriorate?

The main reason : Structures are not built to durability parameters.

Some other reasons :

• Deficiency in design
• Use of nominal mixes such as 1:2:4
• High ‘water-to-cement’ ratio
• Substandard cement
• Sand with high silt content

          We need to build durable RCC structures from concept to completion. But that is not yet done because, first there is no awareness and second we have a misconception that durable structures cost a fortune.

What is durability?

          Durability is the ability of the structure to withstand the deterioration caused under influence of environment throughout its desired life, without the undue maintenance.

Actions for durability

• First: be aware that RCC structures are to be durable and then work towards it. Think durability. Talk durability.

• Design to the applicable BIS codes including the earthquake factor. Take good points from American, British and other international codes.

• Design for a minimum strength of M-30. This is necessary from durability consideration.

• Introduce the concept of ‘life-cycle-cost’ at planning stage. Remember that there is high cost of restoration later in its life. Dharma Pattana, a Global Pagoda, coming up at Gorai, Mumbai, to accommodate 10,000 mediators at a time, a column-free space, is designed for life of 500 years, with LCC concept.

Make structure impermeable

• Water should not stagnate at any part.
• Water should not seep into the building from any location
• Give access to all units of the structure for inspection, maintenance and restoration
• Use materials with life longer than the life of the structure.

Concrete

• Use blended cement only. Increase the percentage of the supplementary material. Use the ‘water-to-binder’ ratio 0.35Design and specify a particular concrete mix. Forget 1:2:4,1:3:6 etc mixes.

• Use graded, good quality sand with permissible silt content

• Use water suitable for construction work. Test the water for chlorides and sulphate. These have a major influence on the durability.

• Apart from usual tests, test the concrete at site for in-situ permeability.
• Specify mixing in mixers only. Goodbye to hand mixing

• Hacking of concrete develops micro-cracks that help deterioration of structure. Through hacking freshly placed concrete, we make the structure deteriorate from day one.

• Make reinforcement uncongested for the concrete to flow freely and around every reinforcement bar.
• Start curing as early as possible. Make curing a separate item of work in the bill of quantities.

Formwork

• Design the formwork to take the service load and make it rigid and water tight, consistent with the safety.

• Give attention to construction joints so they do not become a passage for water seepage. Keep the construction joints as few as possible.

• Foundations cannot be inspected during the lifetime. Provide suitable additional protective coat.

• The conventional systems of waterproofing are bituminous system and brickbat coba with or without china mosaic chips. But, they have short life. They all have some inherent problem in the system. Water proofing system that uses polymer flexible membrane becomes a part of the structure and is without any joints.

• Make chajjas sloping so that water does not stagnate and there is no vegetation growth. Use external plaster that is impermeable and crackles-type. Give a good bond with the masonry.

• Use anchor fasteners for all joints. Forget M.S.nails.

• Plumbing forms 12 to 20 percent of the cost of the building and is one of the major sources of leakage. Be careful. Use long life pipes, pressure test the joints and keep joints accessible. Remember G.I pipe life is about 15 years. So, for a structure life longer than that, use alternative material, say, copper etc.

What next?

For structure durability some other steps will also help. They are: 

• Development control regulations should make it mandatory to incorporate measures to ensure durability to structures.
• Code provisions need to be modified to promote durability.
• Engineering students should have courses in durability of structures, in their syllabus.
• Financing and development agencies should insist on adoption of durability measures.

Design to last

          If built with lifecycle cost consideration and with awareness, any structure will last the life for which it is designed. Think durability. Talk durability.