FRACTURE PROPERTIES OF LARGE SPECIMENS, STATIC

• Goal: Determine fracture parameters from ``large'' concrete specimens with aggregate sizes ranging from 0.75" to 3" MSA under controlled laboratory conditions.

  

  Specimen Type Aggregate MAXIMUM AGGREGATE SIZE MIX

  Shape 19-mm (0.75-in.) 38-mm (1.5-in.) 76-mm (3.0-in.)

  ``Small'' 1 ft. rounded 3 3 -

  ``Medium'' 3 ft. rounded 3 3 3

  ``Large'' 3 ft. rounded - 3 3

  ``Medium'' 3 ft. subangular - 3 -

  ``Large'' (cold-joint) 5 ft. rounded - 3 3

• Note:
  1. Largest fracture specimens ever tested at that time (size and MSA).
  2. Cold-Joint tests are the first attempt to characterize the fracture properties of lift joints.

  Type 19-mm 38-mm 76-mm Summary

  ``Small'' Mean 1.05 (953) 0.86 (783) - 0.93 (843)

  5.9 13.4 - 13.9

  ``Medium'' Mean 1.01 (923) 0.99 (909) 1.09 (991) 1.03 (941)

  20.3 12.7 15.1 16.0

  ``Large'' Mean - 1.23 (1,120) 0.98 (893) 1.16 (1,060)

  - 6.3 12.7 12.8

  Summary Mean 1.03 (941) 1.02 (929) 1.05 (957) 1.03 (941)

  13.5 16.9 15.3 15.2

  ``Medium'' Mean - 1.34 (1,220) - -

  (subangular) - 12.3 - -

  MSA A B C

  38-mm Mean 0.64 (582) 0.50 (457) 0.72 (656)

  (1.5-in.) 9.8 3.3 12.1

  76-mm Mean 0.71 (643) 0.52 (476) 0.62 (567)

  (3.0-in.) 4.0 2.2 2.7

  Type 19-mm (0.75-in.) 38-mm (1.5-in.) 76-mm (3.0-in.) Summary

  ``Small'' Mean 206 () 238 () - 226 (

  9.0 11.2 - 12.2

  ``Medium'' Mean 173 () 223 () 226 () 211 ()

  29.1 7.5 15.1 15.5

  ``Large'' Mean - 259 ( 237 () 254 ()

  - 18.1 - 15.7

  Summary Mean 189 () 240 () 230 () 226 ()

  19.1 13.5 6.5 15.7

  ``Medium'' Mean - 277 () - -

  (subangular) - 13.8 - -

  MSA A B C

  38-mm 99.7 80.6 138.4

  (1.5-in.) () () ()

  76-mm 133.0 85.8 98.1

  (3.0-in.) () () ()

• Main Conclusions:
  1. A fracture toughness, practically independent of specimen and aggregate size, was determined.
  2. For the concrete mixes tested (obtained from two separate ready-mix local suppliers) fracture toughness is a material constant of .
  3. The roughness of the cracked surfaces resulting from bond failures around the smooth faces of the rocks in the large aggregate mixes increases with the specific fracture energy, .
  4. The fracture toughness and fracture energy appear to be dependent on the nature (i.e. shape, geographical origin, quality) of the aggregate rather than the size.
  5. Conclusions are supported by a fractal analysis of the cracked surfaces.
  6. Cold-jointed specimens exhibited approximately reduction in peak load, , and , along with a greater variation in results.
  7. Variation in CJ specimens much greater than in normal concrete, thus high sensitivity on bond surfaces.
• Size effect law was scrutinize on the basis of these results
  1. Same , different d, OK

     

  2. Same d, different , NG
  3. Fracture toughness prediction:

     

     For = 1570 mm = 1.54 (1.40 ), 51% higher than = 1.02 (0.93 ) obtained using the compliance method (CM)