Chemistry of Concrete

8/4/2019 Chemistry of Concrete

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DELHI INTERNATION AIRPORT

DELHI INTERNATIONALAIRPORT PROJECT

The Chemistry Of Concrete

By

SANJEEV GUPTA


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DELHI INTERNATIONAL AIRPORT

(1)Is Concrete a work of art or engineering?It was an art in early days of usage. Then it wasengineering and architecture. Then it became technology.Today, it is chemistry. Concrete is made out of severalmaterials 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 basicingredients, aggregate, cement and water. The chemicalsare e.g. retarders to control the setting time, plasticizersfor workability, slump retention admixtures for Ready Mixed

Concrete and mineral admixtures like micro silica, fly ash orslag for durability. Chemical and physical properties ofingredients decide the concrete properties, both in plasticand hardened form.


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(2)What is the most important aspect to beconsidered for concrete construction?Many aspects are important for good concreteconstruction: ingredient proportioning, batching,mixing, transporting, placing, compacting and curing.

But to point out one important aspect, then it is theproportion in which you mix the ingredients, resultingfrom what is called 'Concrete Mix Design.

Why?The ingredients influence all properties of concreteincluding strength and durability.


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(3)What are the considerations for concretemix design?

Concrete mix design depends on: (1) Environment i.e.the surroundings 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 hardenedconcrete can be given shape, compacted etc., andeconomics. Also, design a concrete mix from the best

locally available materials.


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(4)How do you get correct concrete mixdesign?

Concrete mix design is a step-by-step procedure.Calculations and filling of worksheets arenecessary to work in a systematic manner.

However, full knowledge of all physical andchemical properties of ingredients is always

necessary.


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(5) Is there any method for concrete mixdesign?

There are several. So called 'Road Note No 4' &'DOE' (Department of Environment, UK) are twowidely used. We have developed our own

method. It is a simplified version of these twomethods. Other methods followed are the Bureauof Indian Standard, American Concrete Instituteetc.


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(6) How do you get smooth finish of concretesurface?

Through right mix design for proper cohesivenessand workability, correct formwork, placingcorrectly without segregation and adequately

compacting to remove entrapped air


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(7) What precautions should be observed whilepouring or placing the concrete?

* Place concrete at or, as near as possible, at thefinal position.* Place in uniform layers. Concrete should not

segregate.* Place concrete as quickly as possible. Compactit at the same time.* Compact the earlier layer fully before placing

the next layer.


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(8) What about the pumping of concrete which iscommon nowadays?While pumping concrete, we have to take many

precautionary measures. Some are:

* Concrete mix should be suitable for pump ability 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 effectivesignaling system between the pump operator and theconcrete placing crew


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(9) What is your recommendation forcompaction?Compact at the same pace as placing concrete.While inadequate compaction is harmful, overcompaction is bad. Stop compaction once

entrapped air is driven out.


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(10) What about curing?Curing affects strength, durability, water-tightness,wear-resistance etc. Neglected curing causes crackingof concrete. This reduces the durability and causesleakage and corrosion. So, this last step must receivegood attention.The hardening of concrete depends on the presence ofwater. The chemical action must continue till therequired strength is gained. The hydration process israpid in the first few days and, therefore, concrete

must have sufficient water within it. Remember, whatyou do not CURE you have to ENDURE.


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(11) What is temperature control of concreteand why is it necessary?In massive concrete structures (thickness ofmore than 1 m) chances are that due to heat ofhydration of cement, differential of temperaturesbetween concrete core and concrete surface

exists. This causes thermal stresses that cancause cracks in concrete. It is important that thetemperature of concrete in hot ambience is low atthe time of placing. This is done by using flakedice instead of water. In massive structures likedams, the aggregates are also chilled.


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(12)What points are to be remembered forquality construction?First, have quality construction materials. Next,reject the first concrete member that isunacceptable during construction. Compromisewill ensure poor quality. If you demolish

unacceptable concrete, precautions will be takento do quality job. Carry out the material andconcrete testing at site. You cannot controlquality of concrete work if the lab is located

elsewhere. Most importantly, the topmanagement must have full commitment toquality.


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Chemistry of Concrete