In-Situ FRACTURE TOUGHNESS DETERMINATION

Jim Ryan

Effect of Uniaxial Confinement

• Goal: Develop a methodology for field fracture evaluation by first conducting experiments simulating idealized field conditions in a laboratory environment.

• Fied evaluation can be of upmost importance to quantify effects of Alkali-Aggregate reactions on the safety of concrete dams.

    
  Table 1.1: Summary of apparent fracture toughness data obtained from the cubical specimen tests.
  

  Advantage:

  1. Field properties are more representative of the dam under consideration than the laboratory values obtained from small core samples.
  2. Cost effective.

  Disadvantage: New method, never tested before.

  Number of tests: 12 Specimens (4 each of 0.75", 1.5", 3.0" MSA) same concrete mix as splitting specimens.

  Only determined from the compliance method

  Main Conclusions:

  1. The testing procedures and data reduction techniques developed in the laboratory environment are applicable for a fracture based field test of a gravity dam.
  2. Fracture toughness and effective Young's Moduli appear realistic.
  3. Due to the the uniaxial confining stress, the average fracture toughness values of the cubical specimens is approximately higher than those obtained from the WS specimen tests.
  4. Acoustic emmisions equipment was well suited for detecting in-situ tensile strengths and crack propagation in laboratory specimens.
  5. Method could potentially provide a single parameter reflecting the internal state of concrete on the basis of fracture toughness, tensile strength, and Young's Modulus, i.e

     

     This expression can be used to quantify effect of AAR on concrete with time. Thus, it can be used as a ``damage parameter'' reflecting the internal ''health'' of the structure.

FIELD TESTS

• Goal: Develop a methodology for fracture mechanics based field tests which would provide accurate concrete properties for implementation in a fracture mechanics based finite element safety investigation.
• Difficulties: New method, never tried before.
• Main Conclusions:
  1. Although the results from the field test are very preliminary and approximate, the methodology developed shows great promise for future testing.
  2. AE equipment has applicability in monitoring crack initiation and growth during pressurization.
  3. The crack in the dam resulting from the probe pressurization is very small, and difficult to visually locate, even with the borehole camera.
  4. Due to field conditions,i.e. biaxial, or triaxial confining stresses, the in-situ values for the fracture toughness appear to be three times the unconfined laboratory test values.
  5. The field testing procedure was made possible through the sequential unconfined, and confined specimen testing in the laboratory.

Effect of Confinement on Fracture Properties