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Concrete Mix Design Step by Step Guide with Example

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.
I hope You have enjoyed this article, I will be posting on my next article about Common mistakes made in Brickwork. If you feel this article is useful then do share the link of this post in your network. Join Techno Genome Today for the Growth of this Blog, make it large “For Us For Them”. Any Comment will be Highly Appreciated. Keep Visiting MyCivil – Civil Engineering Redefined for More Articles which may change your Thought @ www.mycivil.engineer .

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Prestressed Concrete Structures A Brief Description With Example

by Bastukar | Sep 22, 2014 | Structural Analysis

Prestressed Concrete Structure is the one of the most advancement in the Civil Engineering Construction Field, particularly in the Concreting Operation. In the modern day every heavily loaded structures such as Bridges, Flyovers, Railway Sleepers in all these structure where loads are so heavy that Normal RCC will not be enough there prestressed concrete is used. In this article I will share with you the main concept that is the backbone of the Prestressing technique and procedure for the Industrial manufacturing of Prestressed concrete, also after reading this article thoroughly you will understand why one should resort to Prestressed concrete, Advantages of Prestressed concrete over the normal RCC structure. First of all let us understand what the science is being the backbone of this prestressed concrete structures.

PRE STRESSING ACTION TO COUNTERACT THE TENSILE STRESSES

Why Prestressed Concrete Structures becomes necessary ==>

As you and me already know that a structure under the action of different loads acting on it, the structure takes the shape of bending. The loads acting on a structure causes it to bend in between the supports with shape having convexity downwards that is what we called as positive bending, and near the support the bending occurs in a reverse shape , that is the convexity is on a upward direction, this we called as the negative bending of the structure. Now due the nature of this kind of bending of the structure the one side of the structure about the Neutral Axis of the section the particles there goes on squeezing that is compressing, and the other side of the Neutral Axis the particles tries to go away from each other, hence tension between the each particles develops. Now the main problem arises because concrete is strong enough t withstand the compressive stresses but it cannot withstand the tensile stresses to a reasonable amount, hence we provided reinforcements to take up this tension.

But what if the loads are very high, I really mean very very high, as in the case of Bridges, Railways Sleepers? will it be able to withstand that high super high amount of tensile stresses?
Yes it can, but the sections will be huge and hence this will not be practical to construct normal RCC on these cases. Apart from this fact any RCC structure will defiantly have cracks (micro cracks) which may not be a matter of concern in case of normal structure, but in case of heavily loaded structure this cracks are not to be allowed, especially in case of water retaining structures. Due to all of these reasons we need to use Prestressed concrete structure.

If we study a Concrete structure we will notice that it is susceptible to failure mainly due to the Tensile stresses. Hence if we can make any arrangements by which there will be no Tensile stresses in the structure or a very little amount of tensile stress which is manageable then we can definitely the solve the problem, But How?

The main concept is that when a structure is loaded it results in bending and thus both the tensile and compressive stress develops now as the Tensile and Compressive are of two exactly opposite nature of stress so we have to make arrangements to counteract this Tensile stresses, and I know you have guessed it right, Yes we will introduce Compressive Stress in the Tensile zone of the structure with atleast equal amount of that of the tensile stress, so this Compressive stress will counteract and omit the tensile stresses completely and in the whole section there will be no tensile stress and the whole section will become a compressive section. I know, you now know the conclusion don’t you? Yes now as the whole section becomes compressive therefore there is no risks of tensile failure, and about the compressive stress the concrete itself can handle it like captain America.

But Wait, How Can We Introduce Fully Compressive Sections? ==>

Simple, we can do this by Prestressing the section. So how to do so is what you want know now. For making prestressed concrete we have to use High Strength concrete made of OPC 53 grade, as the stresses are really high in section due to prestressing it is a must to use High Strength concrete of High Grade, otherwise the section will fail in high compressive stresses. Prestressing is done by means of inserting High Tensile Steel Tendons (similar to cable) in a bunch or in other form inside the section throughout its length about its periphery and exiting from the each end. This tendons are stressed within their Elastic Limit by pulling them at both ends of the structure under the action of machine., then from the both the ends of the structure at the exit points of the tendons arrangements are made by means of wedge so that the tendons are rigidly locked and if the machine releases the tendons, the tendons cannot slip away from the wedge and becomes fixed. As the tendons are pulled from both the ends, tensile stresses are developed, in the tendons, and after releasing the tendons it tries to go back to its original normal position from its stretched condition, but due to the locking of the tendons at it both ends of the structure, the tendons cannot go back to its initial position, it tries squeeze itself but cannot, due to this squeezing or compression effect and as it cannot go back for being locked with the structure, it is always squeezing the structure along with itself, and hence the compressive stresses develops in that zone on which side Prestressing tendons are provided, and due to loading the amount of tensile stresses developed in that zone is counteracted by the compressive stress developed in the structure due to the prestressing. And thus the whole structure becomes compressive and no tensile stresses are remaining in the structure.

I hope this article helped you in understanding the Prestressed Concrete Structure concept. On the next article I will discuss about How the Prestressed concrete structures are manufactured and Advantages of the Prestressed concrete over RCC and link it to this post. For more Civil Engineering Info, Facts, Technological discussion keep visiting MyCivil – Civil Engineering Redefined. And do share this page in your circles on social network, make this blog large, as you know people like you are the only asset to this blog. Feel free to comment on this post, any comments will be highly appreciated.

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Ready Mix Concrete A Approach To Modern Concreting

by Bastukar | Sep 21, 2014 | Construction Practices

Ready Mix Concrete is one of the most advancement in concreting industry today. The concrete whose constituents are weigh batched at a central batching plant, mixed either at the plant itself or in Transit Mixer, and then transported to the construction site and delivered in a condition ready to use is what called as Ready Mixed Concrete or RMC.This enables the place of the manufacture and use of concrete being separated and connected by suitable transportation method. This type of concrete is useful in congested sites or at diverse work places and saves the consumer from the headache of acquiring materials for preparing concrete. RMC is made under factory conditions and in a controlled state hence this is more durable and economical in terms of proper utilisation of the ingredients of concrete. The RMC can be produced according to the required properties such as Strength, Workability, Setting Time etc.

DISCHARGE OF READY MIXED CONCRETE FROM THE TRANSIT MIXER

Proportioning of the Ready Mixed Concrete :-

The Ready Mixed Concrete proportioning aims at obtaining an economical and practical combination materials to produce concrete with required properties for its intended use, such as workability, strength, durability and appearance. The following basics of a good concrete mix should be considered :-

Concrete aggregates are required to meet appropriate specification.
Fly Ash and other supplementary cementing materials which enhances the properties of concrete are normally added to the RMC. Least amount of water which can produce required workability must be used.
Admixtures are commonly used in small quantities varying from 0.5% to 2% for improving properties of both fresh and hardened concrete such as setting time, workability, rate of strength development etc. mainly Water Reducers are used like Naptha Base, PCE or Polyvinyl Carboxylic Ether.

Production Of Ready Mixed Concrete :-

First of all Concrete Mix Design is done to obtain the required mix proportions for the required strength and workability. Ready Mixed Concrete can be produced using any efficient concrete mixer including paddle mixer, free-fall mixers and truck mixers, but forced action mixers are generally preferred. Ready-made self-compacting concrete may take longer time to obtain complete mixing than for normal concrete due to reduction of frictional force and to fully activate the superplasticizer.
Admixtures should not be added directly to the dry constituent materials but dispensed together with or in the mixing water. Different Admixtures should not be blended together prior to adding.
The preferable procedure is to load the aggregates first together with cement, and this immediately followed by the main mixing water and superplasticizer.

Classification of Ready Mixed Concrete :-

1)Transit Mixed Concrete :-

Concrete mixed at job site while travelling to the job site the drum is turned at agitating speed that is slowly. After arriving at the job site, the concrete is completely mixed. The Drum of the Transit Mixer is turned for 70 to 100 revolutions, or about five minutes at mixing speed.
Concrete mixed in the yard or central batching plant the drum is turned at high speed or 12-15 rpm for 50 revolutions. This allows a quick check of the batch. The concrete is then agitated slowly while driving to the job site.
Concrete mixed in transit the drum is turned at the medium speed or about 8 rpm for 70 revolutions while driving to the job site. The drum is then slowed to agitating speed.

2) Shrink-mixed Concrete :-

Concrete that is partially mixed at the central batching plant and then discharged into the drum of the truck mixer for completion of the mixing is called Shrink-mixed Concrete.

3) Central-mixed Concrete :-

In this case concrete is fully mixed and prepared in the Central Batching plant and then carried to the job site by the truck mixers.

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Shallow Foundation Of Different Types And Their Selection

by Bastukar | Sep 21, 2014 | Foundation Engineering

Hi it has been long time since I’ve posted on my Techno Genome. It was due to my busy schedule, but now I’m back with bag full of things for every Civil Engineers out there.

Today I will discuss about Foundation, Specifically about Shallow Foundation. After follwing this post the readers will be able to answer and understand about the followings :-

What is Foundation?
What is the Deep Foundation and Shallow Foundation?
What are the different types of Shallow Foundation?
What type of Shallow Foundation is to be provided in which case?

ISOLATED COLUMN FOOTING

First of all let us understand what is Foundation and why these are required to be provide. To be precise a foundation can be defined as the lower most integrated part of a structure, which is in direct contact with the soil, and which safely transmits the whole load of the structure with the full stability of the structure to the soil beneath. So what does this all means? Its functions are as same like as that of our feet. A structure cannot be suspended in the air by it’s own, can it? so we have to give opposite and equal reaction to the direction from which the loads are coming. This done by providing the foundation, which transfers all the loads coming from the structure to the hard soil layer at certain depth, A foundation also gives lateral stability to the structure so that it does not Slide or Overturn.
There are two types of Foundation mainly, a) Shallow Foundation b) Deep Foundation. This two type of foundation is based on a criteria that is as follows, If the Breadth of the lower most portion of the foundation is grater than the depth of the lower most portion of the foundation from the top of the soil, that is Ground Level then it is called as a Shallow Foundation. And is the Breadth of the lower most portion of the foundation is lesser than the depth of the lower most portion of the foundation from the top of the soil, that is Ground Level then it is called as a Deep Foundation.
In most of the cases of light and moderately loaded structures like Residential Buildings up to 5 storied, and other general type of works shallow foundation is to be provided, but the soil must be not of poor quality with highly compressible soil and low bearing capacity. and in case of heavily loaded structures like High Rise Buildings, Bridges and Flyovers etc. deep foundation is to be provided. Deep foundation are the only most suitable method of providing foundation in case of poor soil condition. In this article I will only discuss about the Shallow Foundation, On a article sooner I’ll discuss about the Deep Foundation so keep visiting Techno Genome.
Actually there are many a types of shallow foundation which are to be provided depending upon the site condition and type of loading and pattern. Namely :-

Isolated Column Footing :- Isolated Column Footing is a very common type of foundation, and is provided in case of Framed Structure and Combined Framed and Load Bearing Structure for providing the foundation of a single column only.For each and every individual column a seperate Isolated Column Footing is to be provided with. This type of foundations can be of two types as a footing having uniform depth at all points of the footing, and a footing being slopped from the face of the column towards the edge of the footing. Sloped footing are much economical than footing with uniform depth.
Combined Footing :- This footing are used when the footing area of one column overlaps the footing area of the other column. In this type of foundation Two or more Columns are provided within a single Footing, which maybe Rectangular or Trapezoidal depending upon the loading pattern on the columns to be provided with Combined Footing. If all columns are having almost equal load on them then Rectangular Footing is used and if the loading differs largely then Trapezoidal Footings are used, the width being greater to smaller from the side of column having greater load the column supporting comparatively lesser Loads.
Continuous Strip Footing :- This is the oldest type of footing I think. This type of foundation is used for providing foundation to Wall, which may or may not be load bearing wall. Continuous Strip Footing are of two types, one made of P.C.C. and the Other is made of R.C.C. If you are planning to provide a continuous strip footing in the form of stepped from main wall thickness at plinth to the foundation width at the bottom of footing then P.C.C. are most economical, but in case of slightly compressible soil and higher loading intensities, R.C.C. Continuous Strip Footing should be used
Strap Footing :- Strap Footing are nothing but an improvised type of Isolated Column Footing, in this type of foundation each isolated column footings are connected at the pedestal level with the adjacent isolated column footings on all four sides by Strap Beams. by connecting all isolated column footings with strap beams they all become bonded together hence they will resist the Differential Settlements due to the compressibility or lowering of the pore water pressure in the subsoil.
Raft Foundation :- Raft Foundation are one of the best type of shallow foundation, and which are very effective where the bearing capacity of soil is low and there are chances of Differential Settlements. This type of foundation is generally provided for framed structures, and where the load on each column are are almost same. In this type of foundation a Hard solid base is prepared over which Primary and Secondary Beams are constructed according to the column position, so that the primary and secondary beams intersects each other at the positions where columns are located, and thus the columns are erected at the Nodal Points or points of intersection of the primary and secondary beams.
Mat Foundation :- Mat Foundation are similar to that of the Raft Foundation, this is suitable where the soil is compressible and having low bearing capacity and there are very risk of differential settlement. The load on each column should be almost equal. In this type of foundation an Inverted Slab Beam is constructed monolithically, this means that the slab is at the bottom most part and above it the beam is casted at the same time. As in the case of the Raft the beams are position as primary and secondary beams as required and the columns are then erected from the Nodal Points.
Grillage Foundation :- This type of Foundation are used generally only for providing foundation to the Steel Column, Column Under Huge Loading, and in the case of the Machine Foundation. In this Foundation R.S.J. and Concreting is done.
Inverted Arch Type Foundation :- This type of foundation are mostly suitable for providing foundation for Arcade. In this, Brick or Concrete Arches are provided having Convexity Downwards, between all adjacent Columns. The Foundation Keeps its stability under the Arch Action.
Cantilever Type Foundation :- This type of foundation is used when no sufficient space is available to provide the foundation for a column directly beneath it, but the column must placed in that particular position. In this case a cantilever beam is projected from the nearby column and at the free end of that Cantilever Beam The Column is Supported. Sometimes a Balancing Load is also applied on the other end without fixing the beam with the nearby Column.

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