4) 5% A concrete mix was found to be unworkable (not resistance to freeze-thaw c

Question

4) 5% A concrete mix was found to be unworkable (not resistance to freeze-thaw cycl fluid enough in fresh state) and have poor es. Describe how two additives may assist in improving the concrete mix properties without affecting the concrete strength (hint: How do the additives work? Will you have to change the w/c ratio if you use the additives?). 5) 4% a) Consider the figure below depicting the variation of strength with time of concrete with different g methods. Why is strength of specimens with longer periods of moist curing higher? b) Typically formwork is left in place on structural members for about 7 days. What would be the impact to the strength of the member if the formwork were removed after the first day? 072

Explanation / Answer

Q4)Since the concrete mix is of poor workablity, the additives to be added it are:

i)Plasticizer or Superplasticizer(whichever are avaliable):-There are basically the mixture of the inorganic and organic substances which allows higher workablity at the same water/cement ratio.The plasticizers acts like a defloculating agent so it get absorbed over the cement particle,thereby makes the entrapped water free(generally 15% water which are entrapped in the voids of the cement particles).The dose of the plasticizers are (0.1-0.4)% by the weight of the cement at which they permit the reduction of water by (5-15)% at the same workablity.Examples for plasticizers are lingosulphate,Polyglycol Ester,etc.

In case of Superplasticizer only thing different from plasticizer is that it reduces the reduction in the water content upto 30%,so there is higher workablity at the same water content.Example of superplasticizer are Modified Lingo sulphate,Sulphonated Malanie Formyldehyde,etc.

ii)Air Entraining:-These are the additives that entraps millions of air bubblesbetween the voids of the aggregates which works as flexible ball bearing that slips over each other, thereby modifying the property of the concrete:

a)higher workablity b)higher resistance to frost action

c)higher resistance against segregation and beeding d)higher durablity

Some of the examples are Natural wood resins,Plant and animals Fatty acids,Steric acids,etc.

For the 2 additive above no need to increase the water content to have the same workablity but to have higher workablity you need to increase the water content.

Q5)i)When the concrete is in the curing period, it requires water for the hyderation of its boghes compound.

Boghes Compounds a)Tricalcium Aluminate:-It undergoes hyderation within 24 hours of addition of water.This is reponsible for the flash setting.This releases the maximum heat of hyderation.

b)Tetra calcium Alumino Ferrate:-It undergoes hyderation within 24 hours of addition of water.this is considerd to be the worst cementing property.

c)Tricalcium Silicate:-It goes hyderation within 7 days of addition of water.It is responsible for the early development of the strength and also increases the resistance against the freezing and thawing.Best cementing properties.

d)Dicalcium Silicate-It undergoes hyderation within a yearof the addition of water.So is responsible for the development of progressive strength.And it also increases the resistance against chemicals.

So all the boghes compounds(which includes the tricalcium Silicate that is reponsible for the early development of the strength) require water for its hyderation and if the water is not present for the complete hyderation of the cement's boghes compound then the strength of the cement will not develope satisfactorily.

Q5)ii)The concrete gains its strenght with time and we provide framework to the freshly prepared concrete(Green Concrete)

As you can see from the above table the percentage of the strength gained by the concrete after 1 day and 7 day is 16% and 65%.So the formwork that is provided should be there for atlest 7 days so that the concrete gains its (60-65)% of its strength.

And if you remove the formwork after just one the the concrete will not be having sufficient strength to support its own weight and eventually it will collapse.

age strength gain in percentage 1 day 16% 3 day 40% 7 day 65% 14 day 90%

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