CracksandRepairsofRCStructures裂缝与结构维修汇总

1、Cracks and Repairs of R.C StructuresCracks in buildingStructural cracks-due to incorrect design-faulty constnjction-OverloadingNon Structural cracks-Moisture changes-Thermal movement-Elastic deformation-Creep-Chemical reaction-Foundation movement and settlement of soil-VegetationNon Structural crack

2、s:Moisture Changes Building materials expand on absorbing moisture and shrink on drying These are generally reversibleShrinkage in concrete or mortar depends on a number of factorsCement concrete：Richer the mix greater is the drying shrinkage Water content：More water in mix induces greater shrinkage

3、Aggregates :Large aggregates with good grading has less shrinkage for same workability as lesswater is usedCuring：If proper curing starts as soon as initial set has taken place and continued for 7 to 10 days,shrinkage is comparatively less.Excessive fines：More fines in aggregate requires more water

4、for same workability and hence moreshrinkage.Temperature：Concrete made in hot weather needs more water for same workability and henceresults in more shrinkage Some Measures for controlling shrinkageShrinkage in plastering can be reduced by ensuring proper adhesion. The plastered should not bestronge

5、r than the back ground 2Shrinkage cracks in masonry can be minimized by avoiding use of rich cement mortar and by3delaying plastering till masonry has dried after proper curing and has undergone most of its initialshrinkage.Thermal Movement The cracking of a typicalFig. H Cracking in Top Most Storey

6、 of a Load Bearing StructureIn case of framed buildings due to thermal movement frames are distorted and cracks may appear asshown in figure below.Fig/J(rat jiinf； tn Claddmf* and Cross U alh of a Frame(/ Strueiurc厂CRACKuIIJ Y一 I M-zrzz卜吟14Arching up and Cracking of Coping Stones of a Long Garden Wa

7、ll4F% I、Horizontal Crack at (he Base of Brick Masonry Parapet or Masonrx um-lron Railing)Supported on a Proteamg RCC SlabPrevention of Thermal CracksTo prevent thermal cracks expansion joints, control joints and joints in case of change of shape anddirection of wing in a structure are to be provided

8、.r ONEROW OF LOOSELY PLACED FLOOR TILESSMOOTHLY RENDERED SURFACE懈勰從黠耸叙缶J。和ELEVAHONr HORIZONTALCRACKS AT THEUNCTION OF BRICK MASONRYWITH RCC SLABMASONRYPAPAPFTCRACKMASNORY-CUWIRONSECTIONSECTIONFLOORINGJOINT IN PLASTER AT CEILINGAND WALL JUNCTION17A Joint in floor slab5PRECAST CONCRETE TILES WITHOUT B

9、EDDING MORTAR BUT VERTICAL JOINTS TO BE IN MORTARm GAP FILLED WITH MASTIC COMPOUNDSLIP JOINT17B Joint in roofGeneral guide lines to provide movement jointsType of StructureMovement of JointsaRC roof slabProvide 20 to 25 mm wide, joint at 10 to 20 M apartbSupports for RC slabs 4 to 6MlengthProvide sl

10、ip joint between slab and bearing wall.cRC framed structure. other loadand bearing structureProvide 25 to 40 mm wide expansion Joints at 30 to 45 MintervaldJunction between old and newstructureProvide vertical slip joints eCompound wallsExpansion joint 5 to 8mm wide at 5to 8M interval and change ofd

11、ireclion.fConcrete pavementProvide 20 to 25mm wide joints at 25m to 40m interval withcontrol joints at 5 to 8m. In cross direction control joints have tobe provided at 3 to 5 m intervals.hRC RailingProvide expansion joints 5 to 8mm wide at 6 to 9m interval 6Elastic DeformationCreep Building items su

12、ch as concrete and brick work when subjected to a sustained load not only undergoelastic strain but also develop gradual and slow time dependent deformation known as creep or plasticstrain. The creep in brick work may stop after 4 months but the same in concrete continue upto a year orso. The creep

13、in concrete may be 2 to 3 times of the elastic deformation and hence has to be fully care fullyconsidered General measures for avoidance reduction of cracks due to elastic strain,creep and shrinkageWater cement ratio is to be controlled Reasonable pace of constnjction adopted Brick work over load be

14、aring RCC members should be done after removal of shutting giving a time gap Brick walls between columns should be deferred as much as possible Plastering of areas having RCC and brick members should be done after sufficient time gap say onemonth or suitable groves provided in junction.Shutting shou

15、ld be allowed stay for a larger period say 30 days or so for cantilevers which are bound todefect appreciably.Fig.27 Horizontal Cracks in a艸吧Supports Due io Excessive Dejleiion of a Slab ofLarge Span7Movement due to chemical reactionCertain chemical reaction in building materials result is appreciab

16、le change in volume of resultingproducts and internal stresses are set up which may result in outward thrust and formation of cracks Soluble sulphate reacts with tricalcuium aluminate in cement and hydraulic lime and form products whichoccupy larger volume and ends in developing cracks An example of

17、 cracking of a floor due to coining incontact of the sub base made of brick khoa with heavy sulphate content and water can be seen in figurebelow.f代厶PreventionIf sulphate content in soil is more that 0.2 % or in ground water more than 300 ppm use rich mix ofconcrete ant mortar has to be adopted Avoi

18、d bricks containing too much soluble sulphates (more than 5 %) and use rich mortar in such cases Use expansion and control joint at closure intervalsCorrosion of ReinforcementCorroded reinforcement expands and cracks the concrete cover To avoid this phenomenon rich mix ofconcrete using proper qualit

19、y of water and adequate cover should adopted Foundation movement and settlement of soilBuilding on expansion clays are extremely crack prone The soil movement in such clay is moreappreciable upto a depth of 1.5 to 2M and this cause swelling and shrinkage and results in crackiCr(h kina end ( pheuvin

20、r Due心Su!ph(ne Hon8in the structure The cracks due to settlement are usually diagonal in shape. Crack appearing due toswelling is vertical Figure.Cracking due to vegetation Large trees growing in the vicinity of buildings cause damage in all type of soil conditions.If the soil is shrinkable clay cra

21、cking is severe SP 25- 1984RepairThe repair for cracks may be undertaken after ascertaining the reasons for the appearance of 什 w crack A few basic principles if followed will be more effective 91. Rendering of minor crack less that lm wide may be done after observing the crack for some time andthen

22、 sealing it with weak mortar of cement, lime and sand.2. Cracks where width change with season should be filled up with elastic fillers like silicon orpolyurethene compound 3. Where sheer crack are observed shear keys made of RCC concrete with at least 1.5 percent steel veinforcemeat may be provided at 1 to 1.5m intervals.4. If cracks are due to movement of so