| Topic : Concrete Design Structures |
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Construction Planning & Management |
construction of multistory buildings |
Construction Intelligence |
1 more ...
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Activity:
442 views;
last activity : 05 17 2012 12:50:41 +0000
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Temperature Changes
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FINAL SETTING TIME OF CEMENT IS 600MIN.AS SOON AS THIS TIME FINISH WE SHOULD START CURING.IF NOT THEN CEMENT STARTS HYDRATION
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Fine cement and fine aggregate
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At the time of failure of the concrete
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Increased W/C ratio
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Shrinkage
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Failure to resist tension
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Poor Workmanship.
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Inadequate structural supports
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hi
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proper w/c ratio,workmanship and surface curing(Temperature control)
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FINAL SETTING TIME OF CEMENT IS 600MIN.AS SOON AS THIS TIME FINISH WE SHOULD START CURING.IF NOT THEN CEMENT STARTS HYDRATION
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Due to improper curing
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Low Quality Cements, Mix ration & Temperature
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Low Quality Cements, Mix ratio & Temperature
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Not following the quality control measures as provided by different IS codes.
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Water Cement Ratio
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changes in concrete (why does concrete crack
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settlement in founding soil
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Dear friends, temperature change is a major cause of cracks in large concrete applications. This is because of the temperature variations between concrete that is poured first and subsequent layers of concrete. These temperature variations can cause some portions of the slab to expand while others are shrinking, which leads to cracks. The more extreme the difference in temperature, the more dramatic and deep the cracking will be. |
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yes i am agree with u Anjali and mr.Vimalan ,,, Temperature is invariably big factor responsible for cracking of concreate
yes i am agree with u Anjali and mr.Vimalan ,,, Temperature is invariably big factor responsible for cracking of concreate
In building construction,cracks in concrete occurs due to various reasons like transverse cracks occurs due to improper staying and supporting,beams come under this problem due to insufficient reinforcement,when thick element is placed over thin element(lintel and roof beam) cracks occur and uneven supports causes major structural cracks,but at the time of placing,vibrating and compacting the temperature of all concrete comes into consideration especially for ready mix concrete at day time even though admixtures are added,major surface cracks occurs.This problem can be sort out by applying self curing compounds immediately
FINAL SETTING TIME OF CEMENT IS 600MIN.AS SOON AS THIS TIME FINISH WE SHOULD START CURING.IF NOT THEN CEMENT STARTS HYDRATION
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IMRAN KHAN G.M PROJECTS |
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yes i am agree with u Anjali and mr.Vimalan ,,, Temperature is invariably big factor responsible for cracking of concreate
yes i am agree with u Anjali and mr.Vimalan ,,, Temperature is invariably big factor responsible for cracking of concreate
Curing is required to meet with the hydration temperature during setting of cement.But in High strength concretes like M50 or more there will be minor cracks on the surface. To avoid this immediately sprinkle(spray) the water on the surface even within half an hour of casting,So that this immediate cracks can be avoided.
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Added to all the ideas if the cement is very fine (like 53 grade) or the fine content is too high then especialy in case of elements like slab where thickness is less and area exposed to sun & wind is high, minor non-structural cracks get developed. Addition of fly-ash and curing ASAP may prevent this type of cracks. Use of PPC or PSC may also help. |
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Addition of mineral aggregate like fly ash does help in reducing plastic cracks since the cement gets reduced and thus the heat of hydration.
If surface area of concrete is large i.e. slab, due to wind velocity, exposer to sun light & heat sudden loss of moist / water is rapid. This causes 'surface plastic cracks'. In such cases, finishing of surface should be done twice with help of trowel, followed by immediate curing after initial setting of 50 minute. The causes of structural cracks could be several & well argued on this page. Grade of cement or addition of fly ash or GGBS could add to the strength due to secondary reaction but dose not help in minimizing cracking phenomena.
It is proved that the more exposed area of concrete the more the chances of heat of hydration and exposure to temperature variation during the time of final setting of concrete. The higher grade of cement is finer then the lower grade and same for heat of hydration.
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When the stresses in the outer layers of the concrete beam or slab reaches ultimate values of the characteristic strength of the concrete, concrete fails. In the case of the under reinforced concrete the concrete reaches the failure stress first than steel and then cracks. |
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In addition to poor mix design and workman ships and other environmental factors….. A Reinforced concrete structure is a composite structure composed of cement concrete and reinforcement bars, as we know steel reinforcement is strong in tension and concrete is strong in compression, but can take tension to some extent 1/10 of its compressive strength, however if tensile stress is more than allowable limits of concrete tensile strength it becomes crack, thus whole tension is taken by steel.
It is often seen that micro cracks occur at at wall beam junctions along the line of joint. This due to difference thermal coefficients of Masonry and concrete.
Cracks are even seen at lintel beam spreading diagonally from the beam into masonry. In all these cases Craft paper is places at beam masonry juctions with a longitudinal groove so that the expansion is facilitated without getting cracks spread to the surrounding areas.
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Increase in water content and less cement content will allow Bleeding. And Hence the accumulated water will lead to.... But exactly i think the gases such as CO2,SO2, SO3, etc from atmosphere react with the water and initiallly making it ACIDIC and then used for mixing mortar ultimately results in CRACKS. In other words also shrinkage ,creep, modulus of elasticity,.etc many other factors lead to formation of CRACKS.  |
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Concrete, like all other materials, will slightly change in volume when it dries out. In typical concrete this change amounts to about 500 millionths. Translated into dimensions this is about 1/16 of an inch in 10 feet. The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when the volume of the concrete changes due to shrinkage. |
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Concrete by charector is to crack when there is unadjusted tension. However out of other reasons,a rich cement, poor detailing ofreinforcement, inadequate cover to reinforcement, variable load conditions etc also responsible for cracking of concrete
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Ther are many reasons why concrete cracks....my expirience shows that the major reasons are poor curing,imbalanced water cement ratio,poor compacting of concrete,water used for concrete mix, leaving steel elements like binding wires, nails, links exposed, using rusted steel.......yes and other material charachteristics...different contraction and expansion values.
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in case of RMC while mix designing many things effecting concrete are considered, but improper pouring may result in cracks. if it is temperature variation then cracks will appeare frequently.
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The concreting in large construction is basically form works.If these are not properly thought out it aggravates cracks due to shrinkage,temperature changes,spalling,rusting.While conditions can be simulated to control these defects,the inadequate structural supports will spoil the party. |
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The concrete is a structural material.The concrete also experiences expansion and contraction similar to welding.The formula is self explanatory:Cement+H2O+Aggregate=Hardened cement+Energy(Heat).If concrete structure is big,the heat and temperature variation causes cracking problem.The cement is made of lime(caO),Alumina(Al2O3),silica(SiO2),and iron oxide.The properites of cement depends on relative amount of these compounds.Adding water in the dry cement starts chemical reaction(hydration),while the mixture of cement,water and Aggregate is fluid,it can be poured into mould(called form work).The compund gradually hardens into the desired shape.Here the concrete experiences three type of forces namely,temperature variation,chemical reaction, and live load,the general pulling and pushing of material,So the concrete cracks per se as a out come of of so many forces.So only viable thing remains form works and protection of concrete just as jigs and fixture are needed to tackle welding.i therefore conclude my argument that cracks will be there but follow the proper design and construction practices.
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The concrete cracking is mainly due to temperature change.as we know the hardening of concrete is an exothermic reaction which means huge quantity of heat is generated at its initial setting, so the core of concrete get heated up and the same heat has to be transferred out side which results in the formation of cracks. so curing plays an important role in reducing cracks.cracks can be controlled to an extent with proper w/c ratio, workmanship and surface curing. |
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FINAL SETTING TIME OF CEMENT IS 600MIN.AS SOON AS THIS TIME FINISH WE SHOULD START CURING.IF NOT THEN CEMENT STARTS HYDRATION
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This is due to lack of curing and second thing is that the quality of the cement will also matters
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This is due to lack of curing and second thing is that the quality of the cement will also matters
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These days the cement quality has become worse that the concrete gets cracked in 3 to 5 years at every place in India especially regions like Kerala and Tamilnadu. Since the labour charge is high, workers who do not have any experience in concrete mixing would do that job and finally ends up with a wrong proportion of cement,water and soil mix. The other reason is that Concrete buildings are not suitable for Indian climate especially where the temperature goes extreme every season ( Delhi, Kerala, TN).
Regards,
Anna George.
Web Analyst.
Nichepro Technologies.
Bangalore.
http://www.nichesuite.com
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These days the cement quality has become worse that the concrete gets cracked in 3 to 5 years at every place in India especially regions like Kerala and Tamilnadu. Since the labour charge is high, workers who do not have any experience in concrete mixing would do that job and finally ends up with a wrong proportion of cement,water and soil mix. The other reason is that Concrete buildings are not suitable for Indian climate especially where the temperature goes extreme every season ( Delhi, Kerala, TN).
Regards,
Anna George.
Web Analyst.
Nichepro Technologies.
Bangalore.
http://www.nichesuite.com
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Whatever the ideas have been delivered by our friends above are very correct why concrete get cracked before its designed life. It is frequently seen in the field that even big contractors are reluctant and most of the time deliberately do not like to follow the quality control measures supplied by the codes.
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Standard water cement ratio is not followed on site
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Concrete, like all other materials, will slightly change in volume when it dries out. In typical concrete this change amounts to about 500 millionths. Translated into dimensions-this is about 1/16 of an inch in 10 feet (.4 cm in 3 meters). The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when the volume of the concrete changes due to shrinkage. |
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It is observed that if the founding soil comes under uneven settlement, vertical as well as diagonal crack develops in the structure resting on the said founding soil |
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I want to know which is better steel or concrete |
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