Cracks and Repairs of R.C Structures裂缝与结构维修.doc

Cracks and Repairs of R.C StructuresCracks in building Structural cracks due to incorrect design faulty construction Overloading Non Structural cracks Moisture changes Thermal movement Elastic deformation Creep Chemical reaction Foundation movement and settlement of soil Vegetation Non Structural cracks:Moisture Changes Building materials expand on absorbing moisture and shrink on drying. These are generally reversible. Shrinkage in concrete or mortar depends on a number of factors Cement concrete: Richer the mix greater is the drying shrinkage. Water content: More water in mix induces greater shrinkage Aggregates : Large aggregates with good grading has less shrinkage for same workability as less water is used Curing: 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 for same workability and hence more shrinkage. Temperature: Concrete made in hot weather needs more water for same workability and hence results in more shrinkage. Some Measures for controlling shrinkage Shrinkage in plastering can be reduced by ensuring proper adhesion. The plastered should not be stronger than the back ground. Shrinkage cracks in masonry can be minimized by avoiding use of rich cement mortar and by delaying plastering till masonry has dried after proper curing and has undergone most of its initial shrinkage.Thermal Movement . The cracking of a typical structure due to thermal movement is given in figure below. In case of framed buildings due to thermal movement frames are distorted and cracks may appear as shown in figure below.Prevention of Thermal Cracks To prevent thermal cracks expansion joints, control joints and joints in case of change of shape and direction of wing in a structure are to be provided.General guide lines to provide movement jointsType of StructureMovement of JointsaRC roof slabProvide 20 to 25 mm wide, joint at 10 to 20 M apart bSupports for RC slabs 4 to 6M lengthProvide slip joint between slab and bearing wall.cRC framed structure, other load and bearing structureProvide 25 to 40 mm wide expansion Joints at 30 to 45 M intervaldJunction between old and new structureProvide vertical slip joints.e Compound wallsExpansion joint 5 to 8mm wide at 5to 8M interval and change of direction. fConcrete pavementProvide 20 to 25mm wide joints at 25m to 40m interval with control joints at 5 to 8m. In cross direction control joints have to be provided at 3 to 5 m intervals.hRC RailingProvide expansion joints 5 to 8mm wide at 6 to 9m interval.Elastic DeformationCreep Building items such as concrete and brick work when subjected to a sustained load not only undergo elastic strain but also develop gradual and slow time dependent deformation 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 deformation and hence has to be fully care fully considered. General measures for avoidance reduction of cracks due to elastic strain, creep and shrinkage Water cement ratio is to be controlled. Reasonable pace of construction adopted. Brick work over load bearing 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 one month or suitable groves provided in junction. Shutting should be allowed stay for a larger period say 30 days or so for cantilevers which are bound to defect appreciably. Movement due to chemical reaction Certain chemical reaction in building materials result is appreciable change in volume of resulting products 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 which occupy larger volume and ends in developing cracks. An example of cracking of a floor due to coming in contact of the sub base made of brick khoa with heavy sulphate content and water can be seen in figure below.Prevention If sulphate content in soil is more that 0.2 % or in ground water more than 300 ppm use rich mix of concrete ant mortar has to be adopted. Avoid 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 Reinforcement Corroded reinforcement expands and cracks the concrete cover. To avoid this phenomenon rich mix of concrete using proper quality of water and adequate cover should adopted.Foundation movement and settlement of soil Building on expansion clays are extremely crack prone. The soil movement in such clay is more appreciable upto a depth of 1.5 to 2M and this cause swelling and shrinkage and results in crack in the structure. The cracks due to settlement are usually diagonal in shape. Crack appearing due to swelling 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 cracking is severe. Repair The repair for cracks may be undertaken after ascertaining the reasons for the appearance of the crack. A few basic principles if followed will be more effective. 1.Rendering of minor crack less that 1m wide may be done after observing the crack for some time and then 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 or polyurethene compound. 3.Where sheer crack are observed shear keys made of RCC concrete with at least 1.5 percent steel vein forcemeat may be provided at 1 to 1.5m intervals. 4.If cracks are due to movement of soil in black cotton once, prevention of moisture penetration in the surrounding a