Cause estimation of cracked RC structures

Transcription

1 Cause estimation of cracked RC structures General (1) Cause estimation shall be carried out accurately before evaluation of crack, judgment of the necessity for repair and strengthening and their selection. (2) Cause estimation shall be carried out based on standard investigation and/or detailed investigation. Cause of crack Causes of cracking are listed in Table 3.1. Causes of cracking (Table-3.1) Major classification Sub classification Sub-sub Cause (No) A:(Material) B:(Construction) C:(Environment) D:(Structure and External Force) E:Others Material Concrete Steel Formwork Physical Chemical Load Structural design Support condition Cement Joint Aggregate Bar arrangement Concrete Formwork Mixing Support Transport Cold joint Concreting PC Grout Compaction Temp./R.H. Curing Chemical reaction Long term loading Structural design Short term loading Support condition A1 ~A10 B1 ~B18 C1 ~C8 D1 ~D7

2 Typical crack patterns for different causes A Materials A Materials Used Materials Cement Aggregate False Setting (of Cement) Heat of Hydration (of Cement) Abnormal Expansion of Cement Clay included in Aggregate Low Quality Aggregate Reactive Aggregate (Alkali- Aggregate Reaction)

3 A2:Heat of Hydration (of Cement) A5:Low Quality Aggregate External restraint (Penetrated Crack) Internal restraint (Surface crack) Wall Restraining Member Wall which has restrained at the base (thickness of wall is more than 50 cm) Example: retaining wall, culvert, etc. Member with large cross section, such as a foundation girder Cracks generate like pop-out A6:Reactive Aggregate (Alkali-Aggregate Reaction) Deposition of alkalisilica gel Rim of Reaction Product cracks of a column along the steel in axial direction Reticular cracks

4 A Materials Settlement and Bleeding of Concrete concrete Chloride in Concrete Settlement and Bleeding of Concrete Drying Shrinkage of Concrete Autogeneous Shrinkage of Concrete Restraint by steel bar (Slab) Shrinkage cracks generated on the upper surface of the slab due to the plastic settlement. Drying Shrinkage of Concrete Drying Shrinkage of Concrete Example of crack at handrail of road Example of drying shrinkage crack of external wall Stress concentration within the periphery of windows, embedded material, etc.

5 B Construction Mixing Non-uniform Dispersion of Admixture B Construction concrete Transport Placing Long-Time Mixing Change of Mix Proportion at Pumping Inappropriate Placing Sequence Rapid Placing B Construction Compaction Insufficient Compaction B6:Inappropriate Compaction Segregation due to excessive compaction concrete Curing Loading or Vibration before Hardening Rapid Drying during Initial Curing Construction Joint Early Age Frost Damage Inappropriate Joint Treatment

6 B10:Inappropriate Joint Treatment B Construction Water leakage at construction joint Steel Arrangement of Steel Bad Placement of Reinforcement Lack of Cover Formwork Formwork Deformation of Formwork Water Leakage (from formwork, into subgrade) Early Removal of Formwork Support Settlement of Support B Construction B17:Inappropriate Joint or Discontinuity Others Cold Joint Inappropriate Joint or Discontinuity A cold joint forms. PC Grout Insufficient Grouting RC building wall Concrete lining in the tunnel

7 Examples of cold joint Light Light C Environment Significant Significant C Environment Physical Temperature and Humidity Change of Environmental Temperature and/or Humidity C1:Change of Environmental Temperature and/or Humidity Difference of Temperatures and Humidity between two surfaces of member Repeated Cycles of Freezing and Thawing Fire Surface Heating

8 C1: Change of Environmental Temperature and/or Humidity Chemical Chemical Reaction Chemical Reaction of Acid and/or Salt Corrosion of Embedded Steel due to Carbonation Corrosion of Embedded Steel due to Chloride Attack C6:Chemical Reaction of Acid and/or Salt The concrete surface is eroded due to a chemical reaction. Most of the cracks occur along the steel bars, and some parts of the concrete may peel off. C7:Corrosion of Embedded Steel due to Carbonation *B12:Lack of Cover: Cracks occur along the steel bar

9 C8:Corrosion of Embedded Steel due to Chloride Attack C8. Corrosion of rebar due to chloride attack A7. Chloride in concrete Rust spills out from the cracks and it often stains the concrete surface. In the case of severe corrosion of steel bars, peeling of the concrete may occur. The corrosion of exposed steel is always severe. D Structure and External Force D Structure and External Force Load Long-Term Load Long-Term Load within Design load Structural Design Insufficient Cross Section or Quality of Steel Short-Term Load Long-Term Load over Design Load Short-Term Load within Design Load Support Condition Differential Settlement of Structure Freezing Heave Short-Term Load over Design Load

10 D6:Differential Settlement of Structure settlement displacement support settlement displacement Example of cracks in a rigid frame pier due to differential settlement. The systematical procedure of Cause Estimation based on the results of standard investigation Major Classifications of Causes Materials (A), Construction (B), Service Environment (C), Structure and External Force (D) Crack Generation Period, Generating Period, Regularity, Extent (reticular, surface layer, penetration) Deformation of Concrete (drying shrinkage, expansion, settlement, bending, and shearing), Limit of Consideration the Mixture Proportion and Weather Condition (Tables 3.3 and 3.4) Mixture Proportion (rich and poor), Weather Conditions during Placing Concrete Detection of Common Cause Crack Generation Period,

11 The systematical procedure of Cause Estimation based on the results of standard investigation Major Classifications of Causes Materials (A), Construction (B), Service Environment (C), Structure and External Force (D) Crack Generation Period, Generating Period, Regularity, Extent (reticular, surface layer, penetration) Deformation of Concrete (drying shrinkage, expansion, settlement, bending, and shearing), Limit of Consideration the Mixture Proportion and Weather Condition (Tables 3.3 and 3.4) Mixture Proportion (rich and poor), Weather Conditions during Placing Concrete Detection of Common Cause The systematical procedure of Cause Estimation based on the results of standard investigation Major Classifications of Causes Materials (A), Construction (B), Service Environment (C), Structure and External Force (D) Crack Generation Period, Generating Period, Regularity, Extent (reticular, surface layer, penetration) Deformation of Concrete (drying shrinkage, expansion, settlement, bending, and shearing), Limit of Consideration the Mixture Proportion and Weather Condition (Tables 3.3 and 3.4) Mixture Proportion (rich and poor), Weather Conditions during Placing Concrete Detection of Common Cause

12 the Mixture Proportion(Table 3.3) the Weather Condition (Tables 3.4) The systematical procedure of Cause Estimation based on the results of standard investigation Major Classifications of Causes Materials (A), Construction (B), Service Environment (C), Structure and External Force (D) Crack Generation Period, Generating Period, Regularity, Extent (reticular, surface layer, penetration) Deformation of Concrete (drying shrinkage, expansion, settlement, bending, and shearing), Limit of Consideration the Mixture Proportion and Weather Condition (Tables 3.3 and 3.4) Mixture Proportion (rich and poor), Weather Conditions during Placing Concrete Main cause of cracks Detection of Common Cause Example: Cracks generated in the box type RC rigid frame Investigation Results: >the crack spacing: 2 to 5 m >The crack width: from 0.2 to 1 mm Cracks >The finding of cracks: approximately 2 weeks after placing of concrete >cement content per unit volume of concrete: 350 kg/m3 temperature at casting: 33 Joints The systematical procedure of Cause Estimation based on the results of standard investigation Major Classifications of Causes Materials (A), Construction (B), Service Environment (C), Structure and External Force (D) Generation Period, Generating Period, Regularity, Extent (reticular, surface layer, penetration) Deformation of Concrete (shrinkage, expansion, settlement, bending, and shearing), Limit of Consideration the Mixture Proportion and Weather Condition (Tables 3.3 and 3.4) Mixture Proportion (rich and poor), Weather Conditions during Placing Concrete Detection of Common Cause

13 Major Classifications of Causes 1) A Materials, B Construction, D Structure and External Force The systematical procedure of Cause Estimation based on the results of standard investigation Major Classifications of Causes Materials (A), Construction (B), Service Environment (C), Structure and External Force (D) Generation Period, Generating Period, Regularity, Extent (reticular, surface layer, penetration) Deformation of Concrete (shrinkage, expansion, settlement, bending, and shearing), Limit of Consideration the Mixture Proportion and Weather Condition (Tables 3.3 and 3.4) Mixture Proportion (rich and poor), Weather Conditions during Placing Concrete Detection of Common Cause Generation Period, Major Classifications of Causes>>> Table 3.1 1) A Materials, B Construction, D Structure and External Force Generation Period, Regularity, and Extent of Cracks>>>Table 3.1 2) A2,A8,A10,B2,B3,B4

14 The systematical procedure of Cause Estimation based on the results of standard investigation Major Classifications of Causes Materials (A), Construction (B), Service Environment (C), Structure and External Force (D) Generation Period, Generating Period, Regularity, Extent (reticular, surface layer, penetration) Deformation of Concrete (shrinkage, expansion, settlement, bending, and shearing), Limit of Consideration the Mixture Proportion and Weather Condition (Tables 3.3 and 3.4) Mixture Proportion (rich and poor), Weather Conditions during Placing Concrete Detection of Common Cause Major Classifications of Causes>>> Table 3.1 1) A Materials, B Construction, D Structure and External Force Generation Period, Regularity, and Extent of Cracks>>>Table 3.1 2) A2,A8,A10,B2,B3,B4 Deformation and Limit of Consideration >>> Table 3.2 3) A2,A9,A10,B2,B3,B8 The systematical procedure of Cause Estimation based on the results of standard investigation Major Classifications of Causes Materials (A), Construction (B), Service Environment (C), Structure and External Force (D) Generation Period, Generating Period, Regularity, Extent (reticular, surface layer, penetration) Deformation of Concrete (shrinkage, expansion, settlement, bending, and shearing), Limit of Consideration the Mixture Proportion and Weather Condition (Tables 3.3 and 3.4) Mixture Proportion (rich and poor), Weather Conditions during Placing Concrete Detection of Common Cause

15 the Mixture Proportion(Table 3.3) Major Classifications of Causes>>> Table 3.1 1) A Materials, B Construction, D Structure and External Force the Weather Condition (Tables 3.4) Generation Period, Regularity, and Extent of Cracks>>>Table 3.1 2) A2,A8,A10,B2,B3,B4 Deformation and Limit of Consideration >>> Table 3.2 3) A2,A9,A10,B2,B3,B8 the Mixture Proportion and Weather Condition >>>Tables 3.3 and 3.4 4) A2,A6,A9,A10,B2,B8 Major Classifications of Causes>>> Table 3.1 1) A Materials, B Construction, D Structure and External Force Generation Period, Regularity, and Extent of Cracks>>>Table 3.1 2) A2,A8,A10,B2,B3,B4 Deformation and Limit of Consideration >>> Table 3.2 3) A2,A9,A10,B2,B3,B8 the Mixture Proportion and Weather Condition >>>Tables 3.4 and 3.5 4) A2,A6,A9,A10,B2,B8 Major Classifications of Causes>>> Table 3.1 1) A Materials, B Construction, D Structure and External Force Generation Period, Regularity, and Extent of Cracks>>>Table 3.1 2) A2,A8,A10,B2,B3,B4 Deformation and Limit of Consideration >>> Table 3.2 3) A2,A9,A10,B2,B3,B8 the Mixture Proportion and Weather Condition >>>Tables 3.4 and 3.5 4) A2,A6,A9,A10,B2,B8 B2 (Long-time mixing): the concrete transporting distance from plant to construction site was not long (about 12 km) >>>A2 (Heat of Hydration of Cement), >>>A10 (Autogeneous Shrinkage of Concrete)

16 Exercise 1: Explain the cause of cracks Reinforced concrete structure built in 1972 Location: Osaka prefecture Peeling and crack along with re-bar of beam Cracks became obvious from 1990 to 2000 Design strength of concrete is 18MPa Exercise 2: Explain the cause of cracks Exercise 3: Explain the cause of cracks 5 stories RC structure built in 1965 Span of trabecular direction is 20m and ridge direction is 100m Location: Inner part of Kanto region Pushed out peeling can be seen at the top of wall. Asphalt at joint between water proofing cinder concretes (thickness 100mm) is also pushed out. Design strength of concrete is 21MPa Exposed concrete building Exercise 4: Explain the cause of cracks and explain why cracks occur partially in the member Span of trabecular direction Cinder Joint concrete Bricks Water proofing Slab of top floor Pushed out peeling Insulation Interior Pushed out peeling Pushed out asphalt Parapet of top floor