Deterioration by Construction Errors
- 1. Repair& Rehabilitation of Structures RamanuJ Jaldhari Asst. Prof. KITE, Jaipur
- 2. Voids in concrete RustStain PlasticShrinkage scaling DryingShrinkage Causes of Distress(Damage) in Concrete
- 4. Chemical Reactions • Acid attack • Aggressive water attack • Alkali carbonate reaction • Alkali silica reaction • Sulphate attack Design Errors • Inadequate structural design • Poor Design Details •Plastic •Drying Shrinkage • Internally Generated • Externally Generated • Fire Temperature Changes Accidental Loadings Construction Errors Freezing and thawing Settlement and Movement Weathering Common Causes for Deterioration of Concrete
- 5. Plastic Shrinkage • During the period between placing and setting, most concrete will exhibit bleeding to some degree. Bleeding is the appearance of moisture on the surface of the concrete. • Usually the bleed water evaporates slowly from the concrete surface. If environmental conditions are such that evaporation is occurring faster than water is being supplied to the surface by bleeding, so high tensile stresses can develop. • The stresses lead to development of cracks on the concrete surface. Generally these cracks are wide and shallow. • The primary cause of plastic shrinkage cracks is rapid evaporation of water from the surface of concrete. • These cracks occur within few hours after placing concrete. • These cracks occur on horizontal surfaces spaced 0.3m to 1.0m apart • They can be deep and width varying from 0.1mm to 3mm.
- 6. Preventive Measures for Plastic Shrinkage Cracks • Moisten the subgrade and formworks. • Erect temporary wind-breakers to reduce wind velocity. • Erect temporary roof to protect green concrete from hot sun. • Reduce the time between placing and finishing. If there is delay use Polythene sheets, • Minimise evaporation by covering concrete with burlap and curing compounds.
- 7. Drying Shrinkage • It is the long term change in volume of concrete caused by the loss of moisture. • If this shrinkage take place without any restraint there would be no damage to the concrete. • Generally concrete structure is subjected to some degree of restraint by foundation , by another part of structure, or by the difference in shrinkage between the concrete at the surface and that in the interior of a member. • The combination of shrinkage and restraints causes tensile stresses that can lead to cracking. Symptoms: • Drying shrinkage is caused by physical loss(evaporation) and chemical loss (hydration)of water during the hardening process. • These cracks appear at 7-10 days after concreting and 80% of drying shrinkage takes place in about a year.
- 8. Preventive Measures of Drying Shrinkage • Use of minimum water content • Use of highest possible aggregate content • Providing adequate and early curing • Eliminate the external restraints as much as possible.
- 9. Freezing & Thawing • The most severe climatic attack on concrete occurs, when concrete containing moisture is subjected to cycle of freezing and thawing. • The capillary pores in the cement paste are of such a size that water in them will freeze, when the ambient temperature is below 0oC. • The freezing water contained in the pore structure expands as it is converted into ice. The expansion causes localized tension forces and leads to fracture of concrete. • The repeated cycles of freezing and thawing have a cumulative effect. • Freeze and thaw deterioration generally occurs on horizontal surfaces that are exposed to water. Preventive measures: • Use of low water cement ratio. • Adequate air entrainment has been found effective to control the freezing damage. • Use of durable aggregate • Designing the structure to minimize the exposure to moisture i.e. providing positive drainage rather than flat surfaces.
- 10. Accidential Loading • These may be characterized as short duration, one time events. • These loadings can generate stresses higher than the strength of the concrete resulting in localized or general failure. • Visual examination will usually show spalling or cracking of concrete. Preventive measures: • Accidental loadings by their nature cannot be prevented. Minimizing the effects of some occurrences by following proper design procedures like earth quake resistant design and proper attention to detailing. Will reduce the impacts of accidental loadings.
- 11. Construction Errors • Failure to follow specified procedures and good practice or outright carelessness may lead to number of conditions that may be grouped together as construction errors. • These errors do not lead directly to failure or deterioration of concrete but they enhance the adverse impacts of other mechanisms. 1. Adding water to concrete 2. Improper alignment of formwork 3. Improper consolidation 4. Improper curing 5. Improper location of reinforcing steel 6. Setting of the concrete 7. Vibration of freshly placed concrete 8. Improper finishing of flat work.
- 12. Settlement & Movement • There is two types of mechanism in settlement and movement in concrete structural members: Differential and Subsidence • Various elements of a structure are moving with respect to one another are caused by differential movements. If it increases the member subjected to overstressed condition and cracks will appear. • Single element of a structure such as monolith is moving with respect to the remainder of the structure are caused by subsidence. • In case of subsidence the concerns are not over cracking or spalling but rather stability against overturning or sliding.
- 13. Temperature Change • Changes in temperature cause a corresponding change in the volume of the concrete. Basically there are 3 temperature change phenomena that may cause damage to concrete. 1. First there are temperature changes generated internally by the heat of hydration of cement. 2. Second there are temperature changes generated by variations in climatic conditions. 3. Third there are temperature changes generated from external source-fire damage.
- 14. Internally Generated Temperature Differences: Mechanism •The hydration of Portland cement is an exothermic chemical reaction. •In large volume placements significant amount of heat may be generated. The temperature rise is not uniform through out the mass of the concrete and temperature gradients developed. •These temperature gradients give rise to a situation that outer portions of concrete loosing heat while inner portions are gaining. •If this difference is more cracks may occur. This is known as internal restraint condition. • Due to that internal restraint condition, as the concrete mass begins to cool a reduction in volume takes place. • If that reduction is prevented by external conditions the concrete is externally restrained and cracking occurs
- 15. Symptoms & Preventive Measures Symptoms: • Cracks from internal restraint will be shallow and isolated. • Cracks from external restraints will extend to full section. • Temperature induced cracks are perpendicular to large dimensions of concrete. Preventive measures: • Using low heat cement • Placing concrete at minimum practical temperatures • Selecting aggregate should have low modulus of elasticity and low coefficient of thermal expansion.
- 16. Any Query???