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

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.

CLASSIFICATION OF CRACKS

VERTICAL CRACKS

• Develops due to shrinkage, expansion or thermal movement of brick, mortar and concrete.

• Do not endanger the safety of the building but unsightly

• Impression of faulty workmanship

HORIZONTAL CRACKS

• Develops mainly at the junction of brick masonry with RCC slab

• Weakens the construction and requires heavy repairing resulting high cost involvement

STRUCTURAL

• Faulty construction

• Overload

• Extensive cracking of RCC beam

• Incorrect Design

NON STRUCTURAL

• 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

PATTERN OF CRACKS

CLASSIFICATIONS:

• Straight

• Toothed

• Stepped

• Random

• Crazing - Occurrence of closely spaced fine cracks at surface of a material

STRAIGHT CRACKS

TOOTHED CRACKS

STEPPED CRACKS

RANDOM CRACKS

CRAZING

CAUSES OF CRACKS

A building component develops cracks whenever stress developed in the component exceeds its strength.

External applied force

• Due to dead, live, wind or seismic load

• Formation of settlement

Internal induced stress

• Due to thermal movements moisture change and chemical actions

• In building component leading to dimensional changes

• Horizontal movements.

• Due to volume change within a component resulting either expansion or contraction

• Compressive Tensile Shear in Building composition

SUBJECT TO CRACKING

•Masonry•Concrete•Mortar

WIDTH OF CRACKS

•Thin – <1mm•Medium –1 to 2 mm

•Wide – >2mm

PRINCIPAL CAUSES OF CRACKS IN BUILDING (Non structural)

• Thermal variation

• Chemical reaction

• Moisture movement

• Elastic deformation

• Creep

• Foundation movement & settlement of soil

• Vegetation

• Manufacturing defects

Details in next few slides

THERMAL MOVEMENTS

FACTORS AFFECTING

•Temperature variation•Dimensions•Coefficient of expansion•Physical properties of the materials

COEFFICIENT OF THERMAL EXPANSION (10-6 / ⁰C)

• Clay Brick & Brick work 5-7•Cement mortar & concrete 10-14•Sand lime bricks 11-14•Fly ash bricks 13-17

SOME FACTORS INFLUENCING IN THERMAL CRACKING

a.Color & surface characterization (high reflectivity coefficient) reduces heat load on the roof

b.Thermal conductivityc.Provision of an insulating or protective layerd.Internally generated heat

OTHER FACTORS INFLUENCING IN THERMAL CRACKING

• Loss of heat by radiation into the atmosphere depends on the proportion of exposed surface to volume of the component. For instance, if under certain conditions in a 15 cm thick fly ash brick wall, 95% of heat is lost to the air in 1.5 hours under similar circumstances; same amount of heat will be lost in about one week when the wall is 1.5 m thick .

• Generally speaking thermal variations in the internal walls are not much and this does not cause cracking. It is mainly the external walls, especially thin walls exposed to distinct solar radiation and the roof which are subject to substantial thermal variations and are this liable to cracking.

• Horizontal crack at the support of an RCC Roof slab due to Thermal movement of slabs.

CHEMICAL REACTIONS

MOVEMENT DUE TO CHEMICAL REACTION

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.

SEVERITY OF SULPHATE /CHLORIDE ATTACK

a.Amount of soluble sulphate/Chloride presentb.Permeability of bricks, concrete mortar

c.Proportion of C3A present

d.Duration for which the building components in quantity remains damp.

MOISTURE MOVEMENT

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.

ReversibleIn the first instance, moisture present in the intermolecular space (absorbed moisture) dries out, causes some reduction in volume & shrinkage. This is reversible in nature.

Irreversible

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.

DRY SHRINKAGE VS FLY ASH – SAND-LIME COMPOSITION

FLY ASH-SAND-LIME BRICKS VS CLAY BRICKS

ELASTIC DEFORMATION

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.

CREEP

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 .

FOUNDATION MOVEMENT & SETTLEMENT

Fly ash bricks are not responsible for this type of failure

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

SHRINKAGE

SHRINKAGE OF FLY-ASH BRICKS:The Factors

a) LIME / FLY ASH CONTENT b) WATER CONTENTc) AGGREGATESd) ACCELERATORSe) CURINGf) PRESENCE OF EXCESSIVE FINESg) HUMIDITYh) CEMENT AS A COMPONENTi) TEMPERATURE

DETAILS GIVEN IN THE NEXT FEW SLIDES

LIME/ FLY ASH CONTENT

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.

WATER CONTENT

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.

AGGREGATES

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

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.

CURING

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

PRESENCE OF EXCESSIVE FOREIGN FINES

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.

HUMIDITY

Shrinkage is much less in coastal areas where relative humidity remains high. Low relative humidity causes plastic shrinkage.

CEMENT AS A COMPONENT

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.

TEMPERATURE

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.

MEASURES FOR CONTROLLING CRACKS DUE TO SHRINKAGE

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.

MEASURES FOR CONTROLLING CRACKS DUE TO SHRINKAGE(Cont.)

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 sandIn 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.

DRYING SHRINING OF FLY ASH-SAND-LIME BRICKS(Comparison)

FRESH MIXTURE BRICK

STRENGTH 75-85 kg/cm2 3 days

95-105 kg/cm2

15 days

107-112 kg/cm2

30 days

DRYING SHRINKAGE

0.084%

SORPTION 21%

DEAD MIXTURE BRICK

STRENGTH 40-50 kg/cm2

3 days

52-55 kg/cm2

15 days

55-60 kg/cm2

30 days

56-62 kg/cm2

45 days

DRYING SHRINKAGE

0.128%

SORPTION 34%

MORTOR SHRINKAGEMORTOR SHRINKAGE (sand: cement=5:1)

Rate of 190.585 10 days

190.535 16 days

190.500 18 days

190.465 21 days

190.455 23 days

190.435 30 days

SHRINAGE 0.079%

MORTOR SHRINKAGE (sand: cement=6:1)

SRENGTH 190.275 10 days

190.155 15 days

190.160 18 days

190.130 25 days

190.090 30 days

SHRINAGE 0.097%

MANUFACTURING DEFECTS

SOURCES OF SUCH DEFECTS – Role of fly ash bricks

• To use proper and standard raw materials

• Heterogeneity in mixing causes unequal stress development and subsequently cracks in bricks and in structure

• If moisture is high in the mixture the coarse aggregate will settle down and lime will come up on the surface during compaction resulting less strength development

• If the filling of the materials in the mould box is not proper, unequal pressure will develop on the brick mixture and improper compact will be formed. So crack will develop in the brick itself and in the construction

• If numbers of bricks are more in one cycle then deviations in compaction will be much more resulting different quality bricks

• Old mixture in manufacturing bricks to be avoided

SOME CONSTRUCTIONS

CONSTRUCTION ON PROCESS WITH FLY ASH BRICKS

COMPLETE CONTRUCTION WITH FLY ASH BRICKS

A 3-Storied building constructed by Fly ash bricks

A BEAUTIFUL CONSTRUCTION WITH FLY ASH BRICKS

CONCLUSION

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.

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