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.
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 Compaction 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.