Cracks and Repairs R.C.C. Structures

C R A C K S A N D R E PA I R S R.C.C. STRUCTURES

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CRACKS IN BUILDING Structural cracks « d ue to incorrect d esign faulty construction « Overloa d ing «

Non Structural cracks

Moisture changes « Thermal movement « Elastic d eformation « Creep « Chemical reaction « Foun d ation movement an d settlement of soil « Vegetation «

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NON STRUCTURAL CRACKS: MOISTURE CHANGES B uil d ing materials expan d on absorbing moisture an d shrink on d rying. These are generally

reversible. Shrinkage in concrete or mortar d epen d s on a number of factors. ´

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Cement concrete: Richer the mix greater is the d rying shrinkage. Water content: More water in mix in d uces greater shrinkage . Aggregates : Large aggregates with goo d gra d ing has less shrinkage for same workability as less water is use d 4.10/ P/ 3

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Curing: If proper curing starts as soon as initial set has taken place an d continue d for 7 to 10 d ays shrinkage is comparatively less

Excessive fines: More fines in aggregate requires more water for same workability an hence more shrinkage. Temperature: Concrete ma d e in hot weather nee d s more water for same workability see Fig ² 8 an d hence results in more shrinkage. Temperature: Concrete ma d e in hot weather nee d s more water for same workability hence results in more shrinkage.

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INITIAL EXPANSION An example of cracks of wall

d ue

to initial

expansion of bricks is given in fig

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SOME MEASURES FOR CONTROLLING SHRINKAGE ´

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Shrinkage in plastering can be re d uce d by ensuring proper a d hesion. The plastere d shoul not be stronger than the back groun d . Shrinkage cracks in masonry can be minimize d by avoi d ing use of rich cement mortar an d by d elaying

plastering till masonry has d rie d after proper curing an d has un d ergone most of its initial shrinkage.

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THERMAL MOVEMENT ´

. The cracking of a typical structure d ue to thermal movement is given in fig

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IN CASE OF FRAMED BUILDINGS DUE TO THERMAL MOVEMENT FRAMES ARE DISTORTED AND CRACKS

MAY APPEAR AS SHOW N IN FIG

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PREVENTION OF THERMAL CRACKS To prevent thermal cracks expansion joints, control joints an d joints in case of change of shape an d d irection of wing in a structure are to be provi d e d

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MOVEMENT JOINTS Typ e o f Struc tur e

a b

M ov ement o f J o ints

RCC roo f slab

P rov ide 20 to 25 mm wide, j o int a t 10 to 20 M apar t S uppor ts f or RCC slabs 4P rov ide slip j o int b etween slab a nd b e ar ing w all. to 6M length

c

RCC f ra med s truc tur e, P rov ide 25 to 40 mm wide ex pa n s io n J o ints a t o ther loa d a nd b earing 30 to 45 M inte rval s truc tur e

d

J u n c tio n b etween old a ndP rov ide v e r tical slip j o ints . new s truc tur e

e

Co m pou nd w alls

Ex pa n s io n j o int 5 to 8mm wide a t 5to 8M inte rval a nd c h a nge o f dir e c ti o n .

f

Co n cr ete pav ement

g

C h a jj a

P rov ide 20 to 25mm wide j o ints a t 25m to 40m inte rval with co ntrol j o ints a t 5 to 8m. In cross dir ec tio n co ntrol j o ints h av e to b e prov ided a t 3 to 5 m inte rvals . P rov ide ex pa n s io n j o int 5 to 8mm wide a t 4 to 6 M inte rval.

h

RCC Ra iling

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P rov ide ex pa n s io n j o ints 5 to 8mm wide a t 6 to 4.10/ P/ 12 9m interval.

ELASTIC DEFORMATION

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CREEP B uil d ing

items such as concrete an d brick work when subjecte d to a sustaine d loa d not only un d ergo elastic strain but also d evelop gra d ual an d slow time d epen d ent d eformation known as creep or plastic strain. The creep in brick work may stop after 4 months but the same in concrete continue upto a year or so. The creep in concrete may be 2 to 3 times of the elastic d eformation an d hence has to be fully care fully consi d ere d .

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AVOIDANCE REDUCTION OF CRACKS DUE TO ELASTIC STRAIN, CREEP AND SHRINKAGE ´ ´ ´ ´ ´

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Water cement ratio is to be controlle d . Reasonable pace of construction a d opte d . B rick work over loa d bearing RCC members shoul d be d one after removal of shutting giving a time gap B rick walls between columns shoul d be d eferre d as much as possible. Plastering of areas having RCC an d brick members shoul d be d one after sufficient time gap say one month or suitable groves provi d e d in junction. Shutting shoul d be allowe d stay for a larger perio d say 30 d ays or so for cantilevers which are boun d to d efect appreciably. 4.10/ P/ 15

MOVEMENT DUE TO CHEMICAL REACTION ´

Certain chemical reaction in buil d ing materials result is appreciable change in volume of resulting pro d ucts an d internal stresses are set up which may result in outwar thrust an d formation of cracks .

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Soluble sulphate reacts with tricalcuium aluminate in cement an d hy d raulic lime an d form pro d ucts which occupy larger volume an d en d s in d eveloping cracks. An example of cracking of a floor d ue to coming in contact of the sub base ma d e of brick khoa with heavy sulphate content an d water can be seen in fig

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PREVENTION ´

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If sulphate content in soil is more that 0.2 % or in groun d water more than 300 ppm use rich mix of concrete ant mortar has to be a d opte d . Avoi d bricks containing too much soluble sulphates (more than 5 %) an d use rich mortar in such cases. Use expansion an d control joint at closure intervals

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CORROSION OF REINFORCEMENT Corro d e d reinforcement expan d s an d cracks the concrete cover. To avoi d this phenomenon rich m of concrete using proper quality of water an d a d equate cover shoul d a d opte d .

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FOUNDATION MOVEMENT AND SETTLEMENT OF SOIL B uil d ing

on expansion clays are extremely crack prone. The soil movement in such clay is more appreciable upto a d epth of 1.5 to 2M an d this cause swelling an d shrinkage an d results in crack in the structure. The cracks d ue to settlement are usually d iagonal in shape. Crack appearing d ue to swelling is vertical Fig

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CRACKING DUE TO VEGETATION Large trees growing in the vicinity of buil d ings cause d amage in all type of soil con d itions. If the soil is shrinkable clay cracking is severe

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REPAIR

The repair for cracks may be un d ertaken after ascertaining the rea for the appearance of the crack. A few basic principles if followe d wi more effective 1.

Ren d ering of minor crack less that 1m wi d e may be d one aft observing the crack for some time an d then sealing it with weak mo of cement, lime an d san d .

2. Cracks where wi d th change with season shoul d be fille d up with el fillers like silicon or polyurethene compoun d .

3. Where sheer crack are observe d shear keys ma d e of RCC concrete at least 1.5 percent steel vein forcemeat may be provi d e d at 1 to 1 intervals.

4. If cracks are d ue to movement of soil in black cotton once, preventi moisture penetration in the surroun d ing areas has to be ensure provi d ing a waterproof blanket aroun d the plinth. The masonry below groun d level shoul d also be separate d from the a d joining soi replacing the existing soil with coarse grain material. 4.10/ P/

STRUCTURAL CRACKS Structural cracks mainly occur

d ue

to:

a)

Defective d esign an d d efective loa d assumptions an d perception of behavior of the structure.

b)

In correct assessment of bearing capacity of foun d ation soil an d soil properly.

c)

Defective d etailing of joints of components like roof with brick wall corner joints of walls

d)

Defective d etailing of structural reinforcement.

e)

Lack of quality control

d uring

d etailing

of steel

construction.

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