Cracks

Cracks in Building

Er Gurudev Singh, IDSE

SE

Director (Design)

ADG (D&C) Pune

INTRODUCTION

• Cracks in building are the common building defects.

• A building component develops cracks whenever stress in a building component exceeds its strength.

• Cracks

Structural - affects strength

Non-Structural - affects beauty/ appearance

• Thin cracks, even though closely spaced & greater in number are less damaging to the structure as compared to fewer Nos. of wide cracks.

Causes of cracks(a) Moisture changes(b) Thermal variations(c) Elastic deformation(d) Creep(e) Chemical reaction(f) Foundation movement & settlement of soil(g) Vegetation(h) Future extention (Horizontal/Vertical)

Flexural tension cracks in wall masonry

Shear crack in masonry wall

Cracking due to expansion of brickwork

Cracks in wall

Settlement Crack

Shear Crack

Shrinkage Crack

Measures for controlling cracks due to shrinkage

(a) Use of bricks after 2-3 weeks on receipt from kiln

(b) Avoid use of rich cement and very fine sand

(c) Delay plaster; till masonry dries and after curing

(d) Use of precast components

(e) Laying floor in small panels

(f) Use of temp reinforcement in concrete

(a) Moisture changes

Most of the building materials are porous in their structure. That’s why they expand on absorbing moisture & shrink on drying. These reversible movement (cyclic in nature) causes cracks in building component/material.

(b) Thermal variations

All materials, more or less, expand on heating & contract on cooling. Magnitude of movement varies for different materials. When there is some restraint to movement of a component of a structure, internal stresses are set up in the component, resulting in cracks due to tensile or shear stresses.

Daily changes which are rapid, have much greater damaging effect on account of movement than seasonal changes, which are gradual.

Thermal movement depends upon :

(i) Colour, Surface characteristics and Dimensions

(ii) Thermal conductivity/Co-eff. Of expansion and Temperature variation

(iii) Insulation and protective layer

(iv) Internally generated heat

(v) 1 m wide strip of slab of 100 mm thick heated through 22 deg C requires force of 500 kN to restain.

Horizontal crack at the base of brick masonry parapet supported on a projecting RCC slab

Vertical cracks at corners in the side walls of a

long bldg due to thermal movement

(c) Elastic deformation

(i) Unevenly loaded walls have wide variation in stress. Excessive shear strain develops cracks in walls.

(ii) When a beam or slab of large span undergoes excessive deflection & there

is not much vertical load above the supports, end of beam/slab curl up causing cracks in supporting masonry.

(iii) When two materials, having widely different elastic properties, are built side by side, under the effect of load ,

shear stress is set up at the interface of the two materials, resulting in cracks at the junction.

Diagonal cracks in cross walls of Multi-storied

Load Bearing Structures

Vertical cracks in Multi-storied Load Bearing Structures

having window openings in load bearing wall

Horizontal cracks in a wall at supports due to excessive deflection of a slab of large span

Horizontal cracks above the RCC slab in top most storey due to arching and expansion of slab

(d) CreepCreepDue to creep in concrete, there is substantial increase in

deformation of structural members,which may be 2 to 3 times the initial elastic deformation. This deformation sometimes results in formation of crack in brick masonry of framed & load bearing structures.

– Creep increases with increase with increase in water & cement content, w/c ratio & temperature.

– Decreases with increase in humidity & age of bldg

– Use of admixtures & pozzolona increases creep.

– High surface to volume ratio of concrete increases creep

Horizontal cracks in brick panels of a framed structure

Vertical cracks in brick panel wall of framed Structures due to expansion of brickwork

Cracking of a partition wall supported on RCC slab/beam. When length to height ratio of partition wall

is large and there is no door opening

Cracking of a partition wall supported on RCC slab/beam. When length to height ratio of partition

wall is large and there is central door opening

(e) Chemical reaction (i) Corrosion of reinforcement in concrete

(ii) Alkali-Aggregate reaction

i.e. Alkali-Silica expansion

(iii) Sulphate attack

Sulphate reacts with C A in presence of moisture & form products of much bigger volume

(iv) Carbonation

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(f) Foundation movement & settlement of soil

Differential settlement

- unequal bearing pressure

- bearing pressure excess of safe bearing capacity

Cracks that occur due to foundation movement of a corner on an end of a building are usually diagonal in shape (wide at top & decreases in width downwards).

(g) Vegetation

(i) Dehydrating action of growing root which causes soil to shrink & cause foundation settlement. (crack wide at top & narrow at bottom).

(ii) Cracks in wall due to expansive action of root growing under the foundation.

(iii) Upward thrust on a portion of the building, when old trees are cut off and the soil that had been dehydrated earlier by roots, swells up on getting moisture from some source, such as rain.

These crack (ii & iii) are wide at the base & narrow down as they pass upwards.

Cracking of a compound wall due to growing roots under the foundation

Roots of fast growing tree under the foundation of compound wall may topple down the wall

Trees growing close to a building on shrinkable soil may cause cracks in the walls

Cracking due to expansion of soil, if construction is taken up soon after removal of trees

(h) Future extension

Horizontal extension

Foundation of a old building undergoes some settlement as load comes on the foundation & the new building is constructed without giving any separation joint.

Vertical extension In case of framed construction, all frame-

work should be completed before taking up masonry work of cladding & partitions, which should be started from top storey downwards.

If we follow this principle, then where is the scope for vertical extension of building!

But, still it is being catered & done.

Prevention of CracksPrevention of Cracks

(i) Time lag between removal of brick from kilns & their use in masonry should be 2 to 3 weeks.

(ii) Avoid using rich cement and very fine sand.

(iii) As far as possible try to use weak mortar for masonry work. Plaster work should be delayed till masonry has dried after proper curing & has undergone most of its initial shrinkage.

(iv) Slip joint should be introduced between slab & its supporting wall. Ceiling plaster should be made discontinuous by a groove about 10 mm in width.

(v) Provision of vertical expansion joint, if the block of building longer than 20 m (in hot & dry regions).

(vi) Provision of horizontal expansion joint of 10 mm depth between the top of wall panel & soffit of beam. This gap may be filled with mastic compound, finished with some sealant or filled with weak mortar. If structurally necessary, lateral restraint to the panel at the top should be provided by using telescoping anchorages.

Elevation

Schematic sketch of telescopic anchorages for panel walls

Schematic sketch of telescopic anchorages for panel walls

(vii) The subgrade below the entire area of the building shall preferably be of the same type of the soil. Wherever this is not possible, a suitably located separation or crumple section shall be provided.

(viii) Stair halls shall preferably be separated from the rest of the building by means of separation or crumple sections to eliminate the bracing effect on the floors.

(ix) Temperature reinf. should be increased by 50 to 100 % of the min amount for the member which are exposed to sun (sun-shades, fins, facia, railings, canopies, balconies etc.)

(x) Over flat roof slab, a layer of insulating material along with a high reflectivity finish should be provided so as to reduce heat load.

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(xi) Try to avoid very fast pace of construction. Allow a time interval of atleast 2 weeks between

removal of centering & construction of partition or panel wall over it.

(xii) Removal of centering & imposition of load should be deferred as much as possible (at least one month) so that concrete attains sufficient strength before it bears the load (for cantilever beam/slab).

(xiii) All frame-work should be completed before taking up masonry work of cladding & partitions which should be started from top storey downwards.

(xiv) Provide upward camber in floor slab/beam so as to counteract deflection.

(xv) Concrete grade should be of min M25 to match with the strength of Fe 415.

(xvi) Concreting should be commenced from the point, loading upon which results max deflection.

Simply supported/ continuous -center towards end

Cantilever - free end towards fixed end

(xvii) Provision of horizontal extension should be made , as per fig. shown :-

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Structural provision of horizontal extension with an expansion joint, for RCC framed structure

(xviii) Na O & K O content of cement should be less than 0.6% & should be less than 3 kg per cubic metre of concrete. If it is unavoidable then mix a suitable proportion of pozzolanic material (especially silica fume) for making concrete.

(xix) Do not let trees grow too close to building.

(xx) If some large trees exist close to a building (within a distance of the ht. of the tree) & these are not causing any problem, as far as possible, do not disturb these trees if soil under the foundation happen to be shrinkable clay.

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(xxi) If, from any site intended for new construction, vegetation including trees is removed & the soil is shrinkable clay, do not commence construction activity on that soil until it has undergone expansion after absorbing moisture & has stabilized.

(xxii) Length of return wall should not be less than the length of 3 bricks.

(xxiii) Use of precast components.

(xxiv) Laying of floor in small panels.

Corrosion of steel

- Use CRS bar, Fusion bonded epoxy coated bar, TMT bar.

- MCI (Migratory corrosion inhibitors)

M C I

MCI are vapour phase inhibitors which by virtue of their very high vapour pressure & their affinity for steel, penetrate through concrete to the steel. It provides a monomolecular layer of inhibitor protection which reduces or stops the corrosion, electrochemically, thus areas of steel not exposed or deeply embedded are also provided protection. Further the MCI systems are effective even in the presence of high percentage of chlorides within the concrete.