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The Chemistry of concrete
C.M. Dordi, Vice President (Technical Services), Gujarat Ambuja Cements Ltd, has more than 30 years of experience in technical services, construction supervision and quality control of concrete structures. He has worked on many prestigious projects, in India and abroad. Dordi has, to his credit, 89 technical papers, 85 books and 400 lectures on concrete and allied subjects.
In this interview Dordi gives useful insights into quality concrete construction.
Is concrete a work of art or engineering?
Neither. It was an art in early days of usage. Then it was engineering and architecture. Then it became technology. Today, it is chemistry. Concrete is made out of several materials besides cement, water and aggregates.
Chemistry?
Yes, chemistry. Concrete today is no longer a 1:2:4 issue. Concrete has many chemicals added to it besides the basic ingredients, aggregate, cement and water. The chemicals are e.g. retarders to control the setting time, plasticizers for workability, slump retention admixtures for Ready Mixed Concrete and mineral admixtures like micro silica, fly ash or slag for durability. Chemical and physical properties of ingredients decide the concrete properties, both in plastic and hardened form.
What is the most important aspect to be considered for concrete construction?
Many aspects are important for good concrete construction: ingredient proportioning, batching, mixing, transporting, placing, compacting and curing. But to point out one important aspect, then it is the proportion in which you mix the ingredients, resulting from what is called 'Concrete Mix Design'.
Why?
The ingredients influence all properties of concrete including strength and durability.
What are the considerations for concrete mix design?
Concrete mix design depends on: (1) Environment i.e. the surrou-ndings of the structure: onshore, offshore, below ground etc (2) Physical and chemical properties of the mix ingredients (3) Beauty of the structure (surface finish) (4) Strength and durability and (5) Cohesiveness and workability i.e. when not hardened concrete can be given shape, compacted etc., and economics. Also, design a concrete mix from the best locally available materials.
How do you get correct concrete mix design?
Concrete mix design is a step-by-step procedure. Calculations and filling of worksheets are necessary to work in a systematic manner. However, full knowledge of all physical and chemical properties of ingredients is always necessary.
Is there any method for concrete mix design?
There are several. So called 'Road Note No 4' & 'DOE' (Department of Environment, UK) are two widely used. We have developed our own method. It is a simplified version of these two methods. Other methods followed are the Bureau of Indian Standard, American Concrete Institute etc.
How do you get smooth finish of concrete surface?
Through right mix design for proper cohesiveness and workability, correct formwork, placing correctly without segregation and adequately compacting to remove entrapped air
What precautions should be observed while pouring or placing the concrete?
* Place concrete at or, as near as possible, at the final position.
* Place in uniform layers. Concrete should not segregate.
* Place concrete as quickly as possible. Compact it at the same time.
* Compact the earlier layer fully before placing the next layer.
What about the pumping of concrete which is common nowadays?
While pumping concrete, we have to take many precautionary measures. Some are:
* Concrete mix should be suitable for pumpability i.e. good workability and good cohesiveness.
* All concrete ingredients should be readily available.
* Pipe layout should have minimum number of bends.
* Pump installation should be correct in the first place.
* And, most importantly, you must have an effective signalling system between the pump operator and the concrete placing crew
What is your recommendation for compaction?
Compact at the same pace as placing concrete. While inadequate compaction is harmful, over compaction is bad. Stop compaction once entrapped air is driven out.
What about curing?
Curing affects strength, durability, water-tightness, wear-resistance etc. Neglected curing causes cracking of concrete. This reduces the durability and causes leakage and corrosion. So, this last step must receive good attention.
The hardening of concrete depends on the presence of water. The chemical action must continue till the required strength is gained. The hydration process is rapid in the first few days and, therefore, concrete must have sufficient water within it. Remember, what you do not CURE you have to ENDURE.
What is temperature control of concrete and why is it necessary?
In massive concrete structures (thickness of more than 1 m) chances are that due to heat of hydration of cement, differential of temperatures between concrete core and concrete surface exists. This causes thermal stresses that can cause cracks in concrete. It is important that the temperature of concrete in hot ambience is low at the time of placing. This is done by using flaked ice instead of water. In massive structures like dams, the aggregates are also chilled.
What points are to be remembered for quality construction?
First, have quality construction materials. Next, reject the first concrete member that is unacceptable during construction. Compromise will ensure poor quality. If you demolish unacceptable concrete, precautions will be taken to do quality job. Carry out the material and concrete testing at site. You cannot control quality of concrete work if the lab is located elsewhere. Most importantly, the top management must have full commitment to quality.
You have published a booklet titled Concrete Verses. Any couplet for our readers?
"We must forget our outdated practices and make a fresh start / Proper techniques, practices and training will make structures to last."
The Chemistry of concrete
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