Why does concrete crack?

We assume concrete as an isotropic material but this property can not be achieved 100% and in real, concrete is a 3-phase material. The three phases are metal chips, sand-cement mortar and the third one is the junction of these two. Hair cracks start at this junction first because this is the weakest part of the three phases. This happens due to number factors of which water cement ratio and poor workmanships are the most dominating factors.

Yes i agree with miss Anjali Bhatia....the main cause of concrete failure is due to variation in temperature..... the w/c ratio can be controlled!!!

All well designed and well constructed structures can take care of the bending moment and the deflection, but in most cases the thermal behaviour cannot be accurately predicted. So thermal stresses are usually not well catered for.

The challenges posed by the length of the building structure can be effectively tackled by providing expansion joints,but cracks do occur even in well designed and well constructed small structures like residences. The reason for this is the thermal stresses caused by seasonal changes like extreme summer, high humidity and extreme winters. This is further compounded by heavy rains which leads to leakages.

Structural joints of members constructed using materials of different thermal co-effecients like a brick wall & RCC beam; joints between precast and cast-in-situ members and the ones involving existing and new concrete are the usual vulnerable areas for crack formation. Proper joint detailing, appropriate and adequate secondary reinforcements are the way out when such joints cannot be avoided.

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.

yes i accept this point. because the crack is made due to Heat at the of Hydration process. This Heat is make Cracks in Concrete Surface

_{Tricalcium silicate + Water--->Calcium silicate hydrate+Calcium hydroxide + heat}

I support the idea because the curing plays the main role in formation of cracks either within the limit or beyond limit. The workmanship and quality control plays very important role. The use of vibrators and centering shuttering works plays major role to keep the cracks under control. Besides, the cement we use varies a lot in respect of setting time as well as initial strength.

FINAL SETTING TIME OF CEMENT IS 600MIN.AS SOON AS THIS TIME FINISH WE SHOULD START CURING.IF NOT THEN CEMENT STARTS HYDRATION & CHEMICAL REACTION TAKES PLACE IN THE FORM OF CRACKS.

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.

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.

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.