Concrete Technology

Concrete Strength and Importance of Water

by Bastukar | Mar 29, 2016 | Concrete Technology

Today I will discuss about importance of water in concrete at different stages of it . I’ve seen that most of the workers, contractors if not all and some of the Engineers tends to ignore the importance of Water at various stages of concrete. The main reason behind this I think is that they are not aware of the fact that water plays a dramatically important role in concreting operations, as it is a kind of maker or breaker. Also students tends to take the matter lightly, but they must understand that actually it is a single most important factor which can change the quality of concrete even no matter how much extra cement you put into for the compensating purpose.

CONCRETING OPERATION – PERFECT COLOUR AND TEXTURE

Let us first understand when and how water comes in contact with the concrete. The concrete itself is a conglomeration of Water, Binder, Fine Aggregate and Coarse Aggregate. Binder in case of Cement Concrete which one is in use in modern day construction is Cement, which may be of any type like OPC, PPC, PSC etc. as Fine Aggregate generally Sand is used, and as Coarse Aggregate Crushed Stone or Stone Chips is used. Now Water in concrete comes in two different stages,

During the mixing of the raw ingredients of concrete, that means while producing the Green Concrete.
Secondly, after the concrete has hardened.

Before going into main course let us first understand few key notes:-

Water-Cement Ratio:

It is the ratio of weight of water to the weight of Cement in a Concrete mix. Suppose we have made 1 cu.m. of Concrete in which 380 kg of cement and 228 kg of Water is used, then the as the above definition states it can be understood that Water-Cement Ratio will be (weight of water/ weight of cement) = (228/380) = 0.60

Bleeding :

In simple words, Bleeding means channeling out of water from the concrete to the surface after it is placed, and during this bleeding cement in the form of thin slurry is also dragged out.

Segregation :

Segregation means loosing of homogeneity of mass in green concrete by separating out of the Coarse and Fine aggregate from the Cement-Water Matrix.

Hydration Process :

It is the process by which the cement particles forms Calcium Silicate Hydrate (C-S-H) from the constituents of cement like C₃S, C₂S etc. in the presence of water, that is the cement gains Strength gradually in a diminishing rate of strength gaining with the passage of time.

Curing:

In short Curing is a procedure of preventing the Concrete from getting dried out of moisture.

Now we are ready for learning the whole procedure deeply but without any extra crappy theories, so don’t worry, it is my guarantee that You will understand 100% sure.

CONCRETING IN LAYERS FOR PERFECT COMPACTION

Importance of Water during Mixing or Production :-

Hey first you go a little up on the page and read the note on Water-Cement Ratio again, I know you hurriedly read it last time isn’t it?.

So, there was a person named Abram, who made out a law by experiments on Concrete. According to him (which is 100% correct so read carefully) The Strength of a Concrete is Inversely Proportional to the Water-Cement Ratio for a given Temperature and Time, other things being constant. Hummmm, don’t worry I’ll explain it out in much easier way.

The above law states that Strength of Concrete will increase if the Water-Cement Ratio is Decreased and Strength of Concrete will Decrease if the Water-Cement Ratio is Increased. Again in other words, if we need to make a Concrete which will be strong then we must put in less amount of water during mixing of concrete, and if we want a weak concrete then we may increase the water content.

But remember one thing over here, that other things are unchanged, like if you will increase the cement content upto certain limit the strength will definitely increase even if you don’t lower the Water-Cement Ratio.

Now the question is why this happens? Why would you believe in someone’s law no matter who the guy he is without knowing the reason behind that.

The most influencing reasons are occurring of Voids, Bleeding and Segregation of Green Concrete if Water in excess is used. Excess of water will cause Bleeding of Concrete and as the water during the phenomenon of bleeding form small tube like channels within the freshly laid concrete, later on hardening these tubes will remain as voids, similar to like low bone density in humans, they will cause easy braking up of the hardened concrete mass, that means lesser will be strength. If water is used in excess than what is required then is will give fluidity to the green concrete to a higher level that the Cement-Water Matrix will be very thinner and have lesser adhesive power to put together all the ingredients of concrete with homogeneity, as a result segregation will take place, and the concrete will no more remain a concrete due to the separation of Cement Slurry from the Aggregates. It may be compared with the fact that if water is mixed with paper adhesive (gum) the bonding strength is reduced.

But in field, there are numerous problems which may arise due to low water content during mixing, especially in tropical conditions, they are as follows :

Very Harsh and dry mix is obtained, which cannot be placed easily in between forms and reinforcements.
Bad Surface is Obtained showing undulated surfaces and too much Stone Chips exposed Surfaces, which becomes very porous to the water in future and causes leakage of water from roof.

The above two situations can easily be tackled, just follow the guidelines below :

Use of Proper Water-Cement Ratio as per Your National relevant Code

Use Cement-Water Slurry in the same Water Cement Ratio that of the concrete to wash the internals of the mixing drum before each continuous batch of mixing, so that cement slurry is not absorbed at the actual time of batching to the dry surface of mixing drum.

Required mix proportions must strictly be maintained.

Bulkage of sand is to be calculated each day at site before starting the casting operation, and required suitable adjustments are to be made, generally this varies from 18%-33% by Volume.

Mix all the ingredients atleast for 80 Seconds, the time must be counted from after the last material has been dumped into the mixer. For producing best mixing in Tilting Drum Mixer takes 2 minutes.

Water up the base and saturate it before commencing of casting operation where the green concrete will be discharged from the mixer directly near the mixer.

Moistened the Formworks 30 minutes prior to casting so that they do not suck up the initial moisture during the casting operation.

Always use Black or Green Alkathene Paper on formworks, it not only prevents the moisture from the concrete to escape away, also gives good finishing to the concrete surface, specially to Beams and Roofs.

Synchronize the whole casting process in such a manner that concrete after discharging from the mixing drum, it is carried and properly placed in the form, and well compacted and surface is finished off within 30-45 minutes.

Place and compact the Concrete in forms in layers suitably not exceeding 150mm thick each, don’t fill full depth all at once, for an example if a beam of overall depth 450mm is being cast, then it is good to fill the whole beams preferably in three layers i.e. 150mm+150mm+150mm = 450mm in this way concrete will be easily compacted and harshness, honeycombing will be avoided.

If one follows all of the above guidelines one will be able to obtain Workable and Homogeneous Concrete even with strict control of water content.

Importance of Water After Concreting is Done :-

Now after the concrete casting is over, both the initial and final setting is passed, it becomes very necessary to retain the moisture in concrete or to continuously keep the concrete moistened so that the hydration process of the cement within the concrete goes on without any discontinuity.

After casting is over, the freshly laid concrete runs out of water or gets dried due to the followings:

Water being used up in the Hydration Process of Cement.
Water is absorbed from the concrete by the wooden Formworks.
Water evaporates due the effects of Wind and Sun.

Now, for the hydration process to be completed satisfactorily Water around 23% by the weight of Cement is required, i.e. Water-Cement Ratio of 0.23. But practically Water-Cement Ratio is kept much higher so as to give the required flow ability to the concrete. Again due to the above mentioned losses of water from the concrete the amount of water required becomes unavailable, as the whole strength gaining time is much longer and water continuously gets lost at a fairly higher rate. As a result of which the concrete becomes dry from inside and out, and also does not get the water required for completion of the hydration process, so the strength gain also disrupts, and the concrete does not attain the strength for which structural design has been done for, and the structure becomes unsafe.

The concrete increases strength for an indefinite period of time at a decreasing rate. In other words Concrete gains most of the strength within first few days, and then it gets slower and slower with the passage of time. It has been observed that Concrete attains about 50% of its 28 days strength in first 3 Days, and 70%-80% of its 28 days strength in first 7 days, and 100% in 28 days, and is continued indefinitely with gradually slower rate of strength gain.

Most of the initial strength gain is due to the hydration of the fast reacting particles of cement, which are C₃S (Tri-Calcium-Silicate), C₃S is mostly responsible for first 3-7 days strength, then less reactive C₂S (Di-Calcium Silicate) hydration start. Later on the gain of strength of concrete is mostly due to the hydration of C₂S.

From the above discussion it can be easily understood that Hydration in the initial stage is much more important than on the later stage, as most of the strength is gained within the first 7 days, therefore it is very much essential and foremost important to keep the concrete hydrated or moistened in this period of time, any drying out of concrete at this stage will cause damage to a level of such an extent that the whole structure may become unfit, and this initial damage cannot be repaired later on by over supplying of water.

The Concrete which has been freshly laid must be treated like a new born baby. A baby in his/her first 6 months are more prone to germs and viruses than later on, that is why that period is very critical for their parents, and they are taken great care of. Similarly Concrete in their initial stage especially within first 14 Days must be taken great care of with no drying out of slab or else it will turn disastrous. Believe it or Not this part of Concreting Operation mostly ignored by 90% of the people.

To cope up with all of these successive scenarios, Curing is to be done. Curing does not essentially mean Watering from outside, everybody mistakenly thinks so. Rather Curing means preventing the Concrete from getting dried, weather by supply the water from outside which is a common practice and restoring the moisture required for hydration process or by stopping the moisture from getting out of the concrete, which can be achieved by applying curing agents to the green concrete surface, this procedure is also known as Membrane Curing.

I hope You enjoyed the article, and now have a clear and good understanding of the Topic. Please do share Link to this article to the fellow Civil Engineers, Colleagues, Friends and in your Circle of People on Social Media, and help me to grow this Website. Wish You All Have A Very Good Day.

Any Comments or Criticism will be highly appreciated.

If you like this Article then I am sure you will like the following well researched articles too, come on check them out also

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Design of RCC Column in Limit State Method under 5 Minutes

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Concrete Mix Design Easy Guide As Per IS 10262-2009 With Example

by Bastukar | Mar 5, 2015 | Concrete Technology

Concrete Mix Design or Mix Design of Concrete is a procedure of determining the relative quantities and proportions of different ingredients of the concrete i.e. Cement, Water, Fine Aggregate, Coarse Aggregate, with the object of producing homogeneous fully compactable concrete having required Strength and Properties with economy.
It is manufactured in Concrete Batching Plant, or sometimes in Mobile Concrete Batching Plant. The Concrete Batching Plant automatically calculates the predefined quantity of materials needed to produces concrete accurately measured by their weight and thoroughly mixed, in the Concrete Batching Plant the concrete produced is of homogenous quality and controlled concrete.

Now a Days with the advancement of concrete technology, the mix design of concrete is in a boom due to the requirements of High Grades of Concretes having required amount of workability, durability and other properties but the main factor is that these all things are achieved by mix design also with economy, that is there is nothing more in quantities of any ingredients of concrete than that required for producing particular type of concrete grade. Nominal Mixes can be adopted only upto the grade of M20, but above m20 grade, a concrete must be mix designed as per code provisions. Though most of us uses Nominal mix of 1:1:2 for M25 Grade, but it shouldn’t be done. There are various methods of concrete mix design, some of them are as follows :-

Indian Standard I.S: 10262-2009 method.
Arbitrary Proportioning or Trial and Error method
American Concrete Institute, ACI Committee 211 method
Indian Road Congress, IRC 44 method
Road Note 4 method
Maximum Density and Minimum Voids method
High Strength Concrete Mix Design method
Mix Design of Pumpable Concrete
British DoE method etc.

Today in this Article I will illustrate the method of Mix Design of Concrete with Example and providing a very easy and engaging Step By Step Approach as you have found in my other Articles. I will use The Latest Revision of The Indian Standard Code of Practice for Concrete Mix Design that is I.S.: 10262-2009 for the below illustration.

STEP 1 – Understand the Requirements of the Problem or Situation :-

First of collect the data and write down the following things

a) Grade of Concrete to be designed

b) Amount of Slump Required.

c) Exposure Type it will have in its service life
d) Any other special requirements like delayed setting etc.

e) Air Content

f) Will it be Reinforced or Plain Concrete

For our example let us consider M30 Grade of Concrete with 75-100mm Slump which will be exposed to mild exposure conditions and there is delayed setting is required by using Superplasticizer and with 2% Air Content and will be used in Reinforced Concrete Structure.

STEP 2 – Collect Data About the Available Materials for the Production of Concrete :-

Perform Tests where ever required and collect the following data for concrete producing materials which will be used

a) Type and Grade of Cement

b) Specific Gravity of the Cement

c) Nominal Maximum size of the coarse aggregate

d) Grading Zone of fine aggregate

e) Specific Gravity of the coarse and fine aggregate

f) Moisture Content in coarse and fine aggregate

g) Specific Gravity of the Admixture to be used

h) Shape of the Aggregate

In our example let us consider the following data : we are using 43 Grade OPC of Specific Gravity 3.15, the Nominal Maximum size of the coarse aggregate is 20mm, Sand is of Zone-II , the specific gravity of coarse and fine aggregates are 2.67 and 2.62 respectively. Moisture content in the coarse and fine aggregate are 1% (Including 0.5% Absorbed) and 4% (Including 1% Absorbed) respectively. The specific gravity of the admixture is 1.145 and Aggregates are Angular.

STEP 3 – Computation of Target Mean Strength of Concrete :

All concretes are composed of various materials like cement, sand, stone chips and water mainly, which vary from time to time and batch to batch, and also after concrete is placed it undergoes different atmosphere in different cases, and concrete will also have different types of workmanship, compactions and other conditions at different cases, thus In production and casting of concrete nothing is constant, almost everything is variable, so we have to provide a suitable allowance for this variation, thus we need to design the concrete for some higher strength than the grade which is required so that it can compensate all those variables. For this reason we have to design the concrete for Target Mean Strength fck’ or ft. The Target Mean Strength is computed as follows :

fck’= fck + (k x s)

Where, fck’ = Target Mean Strength N/mm²

fck = Characteristics Compressive Strength of Concrete in N/mm²

k = A Statistical Co-efficient

s = Standar Deviation N/mm²

Now in our example fck = 30 N/mm²for M30 Grade of concrete, k = 1.65 as per I.S. 456-2000 for 5% Tolerance, as adopted in the definition of Characteristics Compressive Strength, The value of ‘s’ first time design of M30 grade Concrete is 5 N/mm² [I.S.: 456-2000 Page 23 Table-8 or I.S.: 10262-2009 Page-2 Table-1]

Therefore, fck’ = fck + (k x s) = 30 + (1.65×5) = 38.25 N/mm²

Therefore we have to design the concrete for 38.25 N/mm² strength.

STEP 4 – Determination of the Free Water-Cement Ratio :

Now from the curve of Compressive Strength vs Water-Cement Ratio we have to find out the required Free Water-Cement Ratio for the strength 38.25 N/mm² from the particular curve of the grade of cement to be used, or it can also be found out from the generalized curve of BIS, and also consider the value of Maximum Free Water-Cement Ratio from the I.S.: 456-2000 [Page 20 Table-5] and take lower of those two values.

COMPRESSIVE STRENGTH vs WATER-CEMENT RATION CURVE

From the above curve it is found out that the Water-Cement Ratio corresponding to the 38.25MPa and 43 Grade OPC is or from the Generalized curve of BIS is found out as 0.40 and as per I.S. 456-2000 is 0.45, Hence we must take the Free Water-Cement Ratio as 0.40

STEP 5 – Determination of the quantity of Water required per cu.m. of Concrete :

Now the quantity of water required for the production of concrete is to be determined as per I.S.: 10262-2009 Page-3 Table-2 and adjustments are to be made if required as per the code provisions. The basic selection of water is based on the Nominal Maximum size of the aggregate which is in our case is 20 mm so from the table we get Water content corresponding to the 20 mm Maximum Nominal size aggregate is 186 ltr. or 186 kg. But these values are for 25-50mm slump, but we need 75mm-100mm slump. Therefore we have to do the adjustments. Here the slump difference between the standardize and required value is (100mm – 50mm) = 50mm and as per I.S.: 10262-2009 we have to increase the water content given in the table by 3% for each increase of 25mm slump above 50mm (standardize value). Therefore in our case the required increase I water content will be ((Slump Difference/25)*3)% = ((50/25)*3)% = 6%. Thus we need to increase the water content by 6%. Therefore the adjusted water content is (186 + 6% of 186) = (186+((6/100)*186)) = 197.16 kg [See I.S.: 10262-2009 Page- 2 Clause – 4.2]. In our case the aggregates are angular hence no adjustments are needed for water content. But we are using superplastisizer, hence the water content can be reduced by 20% minimum or as per manufacturers specification. Thus reducing the water content by 20% we get the final water content as (197.16 – ((20/100)*197.16)) = 157.73 kg.

STEP 6 – Determination of Cement Content :

Now as we know that Water-Cement Ratio = (Weight of Water/Weight of Cement) , therefore we get

Weight of Cement = (Weight of Water/Water-Cement Ratio)

In our case it will be Weight of Cement = (157.73/0.40) = 394.33 kg per cu.m. of concrete

As per I.S.: 456-2000 Page-20 Table-5 we get for Reinforced Concrete of M30 Grade the Minimum Cement Content is 320kg per cu.m. of concrete, and Maximum cement content for any grade of concrete is 450kg per cu.m. of concrete. Hence our calculated value of cement content satisfies both the minimum and maximum cement content criteria hence okay. If our value would have fall below the minimum vale then we had to take the grater of the calculated and the minimum value of cement content

STEP 7 – Calculation of Quantities of Coarse and Fine Aggregates :

The percentage of coarse aggregate of the total volume of aggregate have to be determined as per I.S.: 10262-2009 Page-3 Table-3 corresponding to the Nominal Maximum size of coarse aggregate ad Zoning of fine aggregate. In our case we have considered Nominal Maximum size as 20mm and Fine Aggregate as Zone-II, therefore the percentage of coarse aggregate by volume of total aggregate is 0.62 or 62%. This values of table are for Water-Cement Ratio of 0.50, and for other Water-Cement Ratios adjustments are to be made as Percentage of Coarse Aggregate is increased by 0.01 or 1% for every 0.05 decrease in Water-Cement Ratio from 0.50, and Percentage of Coarse Aggregate is decreased by 0.01 or 1% for every 0.05 Increase in Water-Cement Ratio from 0.50. In our case the Water-Cement Ratio is decreased by (0.50-0.40) = 0.10 hence the volume of Coarse Aggregate have to be increased by 2% or 0.02. Therefore the adjusted percentage of coarse aggregate is (0.62+0.02) = 0.64 or 64% of the Total Volume of Aggregate.

Now we have to compute the Total Volume of Aggregate by the following

Va = 1.0 – [v + {C/(Scx1000)} + (W/1000) + {S/(Ssx1000)}]

Where, Va = Total Volume of Aggregates in cu.m.

v = Air Content expressed in Decimal [ e.g. 2%=0.02]

C = Mass of Cement in kg

Sc = Specific Gravity of the Cement

W = Mass of Water in g

S = Mass of Admixture in kg

Ss = Specific Gravity of the Admixture

Considering Admixture content as 2% by weight of Cement In our example we get,

Va = 1.0 – [0.02 + {394.33/(3.15×1000)} + (157.73/1000) + {7.89/(1.145×1000)}]

= 0.69 cu.m.

Therefore Volume of Coarse Aggregate required = 64% of 0.69 cum. = (64/100)x0.69 = 0.442 cu.m.

Now we know that

Total Volume of Aggregate = Volume of Coarse Aggregate + Volume of Fine Aggregate

Or Volume of Fine Aggregate = Total Volume of Aggregate – Volume of Coarse Aggregate

Therefore in our example Volume of Fine Aggregate = 0.690 – 0.442 = 0.248 cum.

Now we need to to convert their volume into mass as weight batching will be done.

Mass of Any Substance = (Specific Gravity of that Substance x 1000) x Volume of that Substance

Therefore we get,

Mass of Coarse Aggregate Required = (2.67×1000) x 0.442 = 1180.14 kg

Considering 55% 20mm Aggregate = 55% of 1180.14 = 649.1 kg

Considering 45% 10mm Aggregate = 45% of 1180.14 = 531.04 kg

Mass of Fine Aggregate Required = (2.62×1000) x 0.248 = 649.76 kg

STEP 8 – Arranging the Proportions of Different Ingredients :

Now the Proportions are to be computed of different ingredients of concrete as per the materials calculated in the previous steps.

Mix Proportions by Mass :

Cement = 394.33 kg [1]

Water = 157.73 kg (Water-Cement Ratio =0.40)

Fine Aggregate = 649.76 kg [1.648]

Coarse Aggregate = 1180.14 kg [2.993] (20mm = 649.1kg [1.646] 10mm = 531.04 kg [1.347] )

Admixture = 7.89 kg [2% by weight of Cement]

Hence Cement : Fine Aggregate : Coarse Aggregate = 1 : 1.648 : 2.993

STEP 9 – Final Adjustments for Moisture Contents in Aggregates :

The Aggregates which are used may contain some moisture in absorbed or both in absorbed and free surface moisture form. If necessary corrections in the quantity of mixing water and in the content of aggregates will not be adjusted then the Water-Cement Ratio in actual will increase and the Aggregate Content in actual will decrease due to the free water in the aggregates which will increase the water content, and at the same time which would occupy the volume of aggregate. Thus we have to make necessary corrections for this are to be made so that the calculated water content and aggregate content does not increase and decrease respectively.
In our example the coarse aggregate has 1% moisture in which 0.5% is absorbed into the aggregate it self, therefore the free water is (1%-0.5%) = 0.5%. And in the case of fine aggregate it has 4% moisture in which 1% is absorbed in it self, therefore free moisture is (4%-1%) = 3%.

It means that if we use the calculated value of coarse aggregate then it will include 0.5% of water by its weight and in case of fine aggregate the inclusion of water will be 3% of the weight of fine aggregate.

Total amount of inclusion of water from aggregates are (0.5% of Coarse Aggregate + 3% of Fine Aggregate) = [{1180.14x(0.5/100)}+{649.76x(3/100)}] = 25.39 kg

Therefore amount of water actually to be added at the time of mixing is = 157.73 – 25.39 = 132.34 kg

It also means that if we use the calculated value of coarse aggregate then it will not include 1% of coarse aggregate by its weight which have been occupied by water and in case of fine aggregate the non-inclusion of fine aggregate will be 4% of the weight of fine aggregate.

Thus the actual amount of Coarse Aggregate required at the time of missing will be (1180.14 + 1%) = [1180.14 + (1180.14*(1/100)}] = 1191.94 kg comprising of 55% of 20mm = 655.78 kg and 45% 10mm = 536.37 kg

Thus the actual amount of Fine Aggregate required at the time of mixing will be (649.76 + 4%) = [649.76 + {649.76*(4/100)}] = 675.75 kg

Therefore the Final Mix Proportions After all the Adjustments are as follows :

Cement = 394.33 kg [1]

Water = 132.34 kg (Free Water-Cement Ratio =0.40)

Fine Aggregate = 675.75 kg [1.714]

Coarse Aggregate = 1191.94 kg [3.023] (20mm = 655.78 kg [1.663] 10mm = 536.37 kg [1.360] )

Admixture = 7.89 kg [2% by weight of Cement]

Hence Cement : Fine Aggregate : Coarse Aggregate = 1 : 1.714 : 3.023

Now Trial Cubes are to be prepared and tested to see the results.

I hope I have been able to make the topic clear, easy and understandable. Please do share the Link of MyCivil – Civil Engineering Redefined which is www.mycivil.engineer . And do leave Your Comments regarding this article. Any suggestions will be highly appreciated. Thank You and Have A Good Day.

If you like this Article then I am sure you will like the following well researched articles too, come on check them out also

Concrete Mix Design Step by Step Guide with Example

Top 5 Mistakes at Site You are Doing causing Failure of Concrete

Design of RCC Column in Limit State Method under 5 Minutes

Septic Tank Design for Any Building – Fastest Method

How to Choose Different Foundations Based on Site Conditions

Come Connect with me On Social Media

LinkedIN

Facebook Fan Page

Twitter

Quora

Field Quality Control of Concrete and Deadly 5 Mistakes

by Bastukar | Sep 26, 2014 | Concrete Technology

What is Quality Control of Concrete ? What is Controlled Concrete ? How the the concrete should be mixed in Concrete Mixer ? . In the Construction Engineering The Top 5 Constructional Mistakes and Concrete Problems which can bring disastrous results if ignored are not the huge ones but very common in every construction work, which gets out of the notice of the Engineer-In-Charge in most of the cases. As I feel most of the bad construction takes places not due to the unwillingness of the Engineers, but mostly due to the lack of awareness about those small things which they take lightly and thinks they are of no major importance and will not affect the ultimate construction. I do partially agree with them to the fact that it will not harm the ultimate construction. But How long? How long that improperly made structure be durable and sustainable to the causes of failures? Though this constructional errors may not hamper it all or partially the structure in short run, say about for 10 to 15 years after the construction but in the long run it will cost a massive amount for its restoration due to the degradation through which it will undergo in the future. No one of us like You and me would like to construct a permanent structure which only lasts for 15 years safely, do we? So for assuring the safety to the structure, its durability to withstand all probable condition in future and for preventing the costly restoration works we must take care strictly to the following things, which in general larger part of us don’t really bother about. Here I will discuss about those Top mighty mistakes and their consequences if not corrected at the time of occurring as Quality Control of Concrete is foremost important both in Controlled or uncontrolled concrete production.

1) Clear Cover Of Column, Beam and Slab :-

Clear Cover means the clear distance between the exterior face of the reinforcements towards the nearest surface of the RCC Section and the exterior face of that section. In most of the general cases the values of clear cover are 50mm for Foundation, 40mm for Column, 25mm for Beam, 15mm for Slab and Stair.

Now I’ve seen In most cases that people tend to ignore this cover value about weather it is maintained correctly or not, in cases I found that there may be unequal cover provided on column sides like in one side mason has given 20mm cover and on the other side it is 75mm. This occurs mainly due to the not properly using Cover Blocks for maintain the cover. In case of Beam, Slab the same thing occurs, the mason either may have increased the cover or may have decreased the value than what is actually to be provided. In either way it’s harmful and may bring catastrophic results.

EFFECT OF MOISTURE PENETRATION

As a result of the of the above the consequence will follow like this way, if we provide insufficient cover then after 5-15 years later depending upon the exposure the insufficient cover will be easily penetrated by the moisture as the cover is insufficient to resist the penetration and also due to the fact that if cover is too little then the concrete cover at the face of reinforcement will be so thin that it will have a tendency to crack and hence will ease the entrance of moisture, which in turn will corrode the reinforcement then the structure will lose it’s capacity of taking loads as rusting in the Steel is made of loose particles hence the Bond between the steel and the concrete will be totally lost, and will cause a failure of the structure under the Load. If the cover is increased than what has been specified by the Designing sheets then there will lie a huge risk of flexural failure of the structure due to the fact if the cover is increased then it means the effective depth of the structure will be decreased also, due to this decrease in the effective depth in the section, the Moment of Resistance of the section will get reduced drastically even with a small increase in the cover than what is specified. Therefore the structure will not be capable of taking up the Designed Bending Moment. So we now see that the ignorance of the cover must be avoided, and the specified cover in the designing sheet must be strictly followed to.

2) Addition Of Extra Water For Better Workability :-

This is one of the biggest silly mistake that sometime happens. This specially takes place in areas where weather is Hot, and the Water from the Green Concrete (Fresh Concrete) is evaporating at a very fast rate. In those conditions Fresh Concrete becomes dry and less workable and harsh. For this reason it becomes very difficult to place the concrete and compact it properly, and as a cure to this many a people adds water to the concrete to make it workable again. This will cause catastrophic results, as addition of further water to the concrete will change the Water-Cement Ratio of the concrete, and the increase in the Water-Cement Ration beyond what is specified for that particular mix will reduce the strength as we know from the Abrams Law of Water-Cement Ratio that The Strength of Concrete is Inversely Proportional to the Water-Cement Ration of Concrete Mix, other factors remaining constant. Therefore this addition of water for the sake of improving the workability will not improve the quality of concrete, rather it will destroy it absolutely.

It is my earnest requests to everyone please don’t do this, please don’t let others to do this also in front of you. I know how hard it is concreting in the hot zone due to the rapid evaporation of moisture from the Fresh Concrete, but we can steel make it our way, Don’t We? We can use Admixture to solve this. Water Reducers can be used as Admixture, like PCE that is Poly Carboxylic Ether. But dosing must be carefully done or all the concrete will become useless mass. Now You may say that for small constructions like Residential Buildings, other small castings where Admixtures may not be available, then What to do there? This can be solved by synchronizing the rate of production of concrete with the rate of placement of concrete. Don’t make concrete of too much quantity that it cannot be used at that rate, hence there is being stored a heap of concrete which is taking time to be consumed on and dries up. Match the rate of production with the rate of consumption and keep the mixing plant as close as possible to the spot of placing of concrete. In this way you will surely be able to solve out the problem of drying out of fresh concrete and becoming it less workable.

3) Free Fall Of Concrete :-

Concrete is the conglomeration of Binding Material (Cement), Fine Aggregate (Sand), Coarse Aggregate (Stone Chips) with right proportion of water based on the Water-Cement Ratio. As the Specific Gravity of different ingredients of concrete varies hence when concrete is dropped at the time of placing of concrete the different ingredients tends to fall at different rate under the action of Gravitational Pull. Therefore all the ingredients has a tendency to get separated from each other, but if the fall of concrete is not from much height then these ingredients does not get separated from each other due to the binding of cement paste. But if the Concrete is dropped from a greater height then the cement paste fails to hold them as a homogeneous mixture and all the ingredients separates and gets deposited in layers according to their specific gravity, hence the Stone Chips silts at the bottom, then over it sand silts then over it cement slurry of the greater portion.

CONCRETE SEGREGATION

This phenomenon is known as the Segregation of Concrete. Therefore in this case the concrete does not remains actually concrete, and losses almost its all properties and strength, and such a structure collapses under Design Load. This can be prevented by dropping the concrete from a maximum height of 1.5m. I’ve seen in casting of column, many people are casting the full length of column at a time, which means a full length of column say about 2.7m is being caste at a time, hence the concrete dropping from the top is falling through a height of more than 1.5m, as a result segregation takes place at the bottom of the column, and the whole column becomes unstable under the loads and have greater chance of collapsing. And one more thing, Patching up the segregated column with Rich Mortar say of proportion 1:3 will not cure it, this patch will only hide the Scar beneath. Only cure is to dismantle the column and reconstructing it.

4) Improper Compaction :-

Compaction is a very essential process in concreting practice. Proper compaction makes a concrete dense, and hence the harden concrete results in having greater density which in turn gives higher durability to the concrete, making it impervious to moisture and other harmful chemicals. As the density increases with good quality compaction hence it gives greater strength, and also the honeycombing can be avoided.

HONEY COMBING DUE TO BAD COMPACTION

If Compactions is not good then durability of the concrete will be reduced and also strength will be reduced. Improper Compaction causes air voids in the concrete which reduces the strength of the concrete. Only About 5% Voids can reduce the strength of the concrete up to 30%. Hence this becomes very necessary to do proper compaction to the concrete. The Concrete in Beam, Lintel and in other type of section where Depth is 150mm or more in those cases Concrete should be placed in layer and after compaction of a layer the next layer should be laid and the compaction should be done and then the next layer and compaction again so as to achieve a Compact and Homogeneous Concrete.

5) Too Early Removal Of Formworks :-

This is another thing which is being done all around mostly in case of the construction of Residential Complexes. A concrete structure gains strength with age, and the design strength which we target for is achieved at 28 days. So logically a structure should be given support by means of props for full period of 28 days, but the fact is that after a structure is ready and has gained its design strength even then at that time it is not subjected to full design load as the whole structures including finishing works cannot be complete within that period and hence we can flexibly remove the props when a structure gains about 70% – 80% of the design strength. This much strength is generally gained at 7 Days, but before this the props must not be removed. As for this reason the minimum period for which props are to be provided is 7 days, no matter how short is the span and how unimportant it is. But in actual practice the Time frame for which support of the props are to be provided depends upon the type of structure and the span of it. Generally For Slabs props must be provided for 7 Days in case where the span is up to 4.5m and for span over 4.5m the time of supporting is 14 Days. In case of Beam for span up to 6m the time for which props are to be provided is 14 Days and for span over 6m the time for supporting is 21 Days. For all the structures the side shuttering may be removed after 16 hours – 24 hours.

So the main problem in removal the formworks early is that the structure will not have enough strength to resist the design loads which will then cause failure of the structure.
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