CRACKS IN BUILDINGS : THE ROLE OF FLYASH BRICKS AND THE REMEDIES. Prof. (Dr.) Ananta Kumar Das Department of Chemical Engineering, Durgapur Institute of Advanced Technology and Management, Durgapur. INTRODUCTION.
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CRACKS IN BUILDINGS : THE ROLE OF FLYASH BRICKS AND THE REMEDIES
Prof. (Dr.) Ananta Kumar Das
Department of Chemical Engineering,
Durgapur Institute of Advanced Technology and Management, Durgapur
Cracks in buildings are of common occurrence and are developed whenever stress in the component exceeds its strength. This stress caused by external forces such as dead, live, wind, seismic loads and foundation settlement; or induced internally due to thermal movements, moisture changes, chemical actions, weathering actions resulting in shrinkage or expansion of the bricks, mortar, concrete or due to corrosion of reinforcement etc. render the structure unsafe. The other causes may be due to fault of structural design. Fly ash brick, a building component has a little effect on these cracks that can be avoided if it is manufactured and used properly.
Internal induced stress in building materials
Penetration of moisture and weather actions through the masonry works
Resulting in shrinkage in bricks, mortar, concrete
Resulting corrosion of reinforcement & hence increase in volume
External applied force
Internal induced stress
Due to thermal movements moisture change and chemical actions
In building component leading to dimensional changes
Due to volume change within a component resulting either expansion or contraction
Compressive Tensile Shear in Building composition
SUBJECT TO CRACKING
WIDTH OF CRACKS
Thin – <1mm
Medium –1 to 2 mm
Wide – >2mm
Details in next few slides
Soluble sulphates, which are sometimes present in ground water react with excess lime of fly ash bricks, form gypsum and calcium aluminate sulphate which occupy much bigger volume than that of the original constituents. This expansion reaction results in weakening of masonry & plaster and then formation of cracks.
The severity of sulphate attack depends upon amount of soluble sulphate present, permeability of fly ash bricks, concrete mortar, proportion of C3A present and duration for which the building components in quantity remains damp. If pure water free of sulphate is used for manufacturing of fly ash bricks then sulphate attack can be avoided. Similarly chloride content in water enhances the cell formation in case of RCC and again propagates the cracks due to increase in volume of Fe3O4.
The chemical reaction proceeds very slowly and it may take about two to more years before the effect of this reaction becomes apparent.
Initial shrinkage:- Initial shrinkage in fly ash –sand-lime bricks is about 50% greater than that due to subsequent wetting and drying from saturation to dry state.
After capillary water is lost, CaSiO3 gel crystallizes and gives up some moisture (absorbed moisture) and individual molecules undergo reduction in size, resulting in shrinkage which is irreversible nature.
Most of the cracking in these materials occurs due to shrinkage at the time of initial drying.
In the first instance, moisture present in the intermolecular space (absorbed moisture) dries out, causes some reduction in volume & shrinkage. This is reversible in nature.
Hydration of lime after brick formation causes a reduction in the volume of the system of silica – lime – water to an extent of 0.5% of the volume of the dry compact. This is the plastic strain which aggravates due to loss of water by evaporation that causes surface cracks. If the proportions of each components are properly mixed this problem will be negligible for making fly ash bricks.
The increase of strain of a compact with time under sustained stress is termed creep, the shrinkage and creep occurs simultaneously.
The rate of creep decreases with time and the factors influencing creep are similar to shrinkage which are described later. In case of fly ash bricks Creep is negligible if the bricks are matured .
VEGETATION-The growth of unwanted plants in the construction makes the construction weak. So any growth of plant is to be stopped. Not related to fly ash bricks
b) WATER CONTENT
f) PRESENCE OF EXCESSIVE FINES
h) CEMENT AS A COMPONENT
DETAILS GIVEN IN THE NEXT FEW SLIDES
Higher the lime, greater the drying shrinkage. Conversely larger the volume of aggregate, lesser the shrinkage for bricks, increasing the volume of aggregates by 10%, reduction of shrinkage by 50%.So proper composition to be maintained.
Greater the quantity of water used in the mix, greater the shrinkage. Thus a wet mix has more shrinkage than a dry mix which is otherwise similar. So better vibration / high pressure gives less shrinkage. On the other hand variation in the strength will occure.
By using largest possible maximum size of aggregate in brick and ensuring good grading – requirement of water is reduced. Aggregates that are porous and shrink on drying result in higher shrinkage
Accelerators like CaCl2, MgCl2 is added for faster reaction towards silicate bonding but use in high percentage over 0.5 to 2, shrinkage could be more. In case of steam curing the addition of accelerators has no noticeable impact.
Proper curing should be done started as soon as initial set has taken place and it is to be continued for at least for 7 days, then drying shrinkage will be less, because when hardening takes place under moist environments, then there is initially some expansion which offsets a part of subsequent shrinkage. Steam curing at the time of manufacturing reduces the liability to shrinkage as high lime results in pre-carbonation
Like Silt, clay, dust should not be more than 2-4% in aggregates because it increase surface area resulting high water requirement and resistant to bonding in chemical reaction.
Shrinkage is much less in coastal areas where relative humidity remains high. Low relative humidity causes plastic shrinkage.
Rapid hardening cement to be avoided because it has greater shrinkage than ordinary Portland cement of higher proportion of CaSiO3 & lower proportion of alkalis like sodium oxide and potassium oxide to be used. PPC cement is preferable.
Otherwise difference in strength development between fly ash bricks and cement mortar will cause cracks.
If the temperature of the mix is lowered from 38⁰C to 10⁰C, it would results reduction of water requirements and hence lower shrinkage. It is thus follows that in a tropical countries like India, brick work done by fly ash bricks in mild winter would have much less tendency for cracking than that done in hot summer. So the aggregates and mixing water should be shaded from direct sun.
On account of drying out of moisture content in building materials/ components.
Shrinkage in a material induces tensile stress when there is some restraint to movement where the stress exceeds the strength, cracking occurs, this relating the stress. Cracks get localized at weak sections such as door and window opening or staircase walls.
Avoiding use of rich cement mortar in masonry made of fly ash bricks. Delaying plaster work till masonry has dried after proper curing. Shrinkage is made to take place without any restraint.
Coat of plastering on masonry is restrained from shrinkage to some extent by its adhesive bond to non shrinking background, the later having already undergone shrinkage.
Shrinkage of a rich and strong mortar is known to extent sufficient force to tear off the surface layer of weak bricks.
In summer 1 cement: 6 sand
In winter 1 cement: 5 sand
If a wall exceeds 5-7m length, provide control joints at weak sections.
Curing of masonry should be done sparingly to avoid body of the blocks getting wet.
Avoid excessive welting of masonry at the time of plastering so that moisture doesn’t reach the body of the blocks.
It is found that fly ash-sand-lime bricks are the only alternative to burnt clay bricks to avoid pollution in the atmosphere, to save huge energy consumed in brick kiln by burning coal and to save huge fertile land. If the product is manufactured with proper technological method and construction is done with proper way it will be the only alternative to conventional clay brick. This is the prime importance of the manufacturers, users and engineers to find out the solution how the ecofriendly bricks can be consumed for all construction purposes for the sustainability of the Society and our Earth.