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Sampling and Testing Concrete Mixes

Sampling and Testing Concrete Mixes. MATERIAL TESTING LABORATORY. Purpose Testing on freshly and hardened concrete is necessary to ensure that specific properties are achieved It is indispensable part of the construction process imparting quality control and acceptance criteria.

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Sampling and Testing Concrete Mixes

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  1. Sampling and Testing Concrete Mixes MATERIAL TESTING LABORATORY

  2. Purpose • Testing on freshly and hardened concrete is necessary to ensure that specific properties are achieved • It is indispensable part of the construction process imparting quality control and acceptance criteria

  3. Sapling Freshly Mixed Concrete • ASTM C 172 and ACI 318 (Latest Edition) • Perform by qualified field testing technicians • Frequency- once a day, or once every 150 cu yd of concrete, or once for each 5,000 sq ft of surface for slabs or walls Not required for less than 50 cu yd with evidence of satisfactory strength

  4. Sapling Freshly Mixed Concrete (Cont.) Sampling • Sample from middle part of the batch discharge • Sample from the mixer delivery point unless other locations are required by specifications

  5. Sapling Freshly Mixed Concrete (Cont.) • Elapsed time- no more than 15 minutes between first and last portions. • Size of Sample - minimum of 1 cu ft. • Transport samples to area where tests or molding will be performed • Combine and remix to ensure uniformity

  6. Testing Freshly Mixed Concrete Consistency - ASTM C 143 (Slump Test) • Standard cone 100 mm (4 in.) (top), 200 mm (8 in.) (bottom) diameters, and 12 in. tall • Fill mold in 3 equal volume layers • 70 mm (2 ⅝ in.) • 160 mm (6 ⅛ in.) • Top

  7. Testing Freshly Mixed Concrete (Cont.) • Tamp each layer 25 times with round steelrod 16 mm (5/8 in.) diameter and 600 mm (24 in.) long • Level top of the cone

  8. Testing Freshly Mixed Concrete (Cont.) • Lift up cone 300 mm (12 in.) in a single movement in 5 ± 2 sec. • Measure vertical difference between top of mold and specimen original center • Within 2.5 minutes

  9. Testing Freshly Mixed Concrete (Cont.) • Temperature - ASTM C 1064 • Temperature limits are usually set for concrete mixes • Thermometer accurate to ± 0.5 °F • Wait for 2 minutes or until reading stabilizes • 76 mm (3 in.) minimum should surround the sensing portion • Complete reading within 5 minutes after obtaining sample

  10. Testing Freshly Mixed Concrete (Cont.) • Unit Weight and Yield - ASTM C 138 • To Determine: • Unit weight- Weight / Volume • Yield– Ratio of measured unit weight of concrete by sum of weight of individual components • Air content- percentage of voids • Cement content

  11. Testing Freshly Mixed Concrete (Cont.) • Procedures • Deposit concrete in 3 equallayers • Compact each layerwith a standard rod with the following frequency: • 25 strokes for volumes less than 0.5 ft3 • 50 strokes for 1 ft3 • One per 3 in.2 of surface for larger volumes • Tapthe exterior with a mallet10 to 15 times • Strike-off top surface andfinish it • Measure weighof sample and container

  12. Testing Freshly Mixed Concrete (Cont.) • Computations • Unit weight: • D = (Wt – Wm) / Vm • Yield: • Y (ft3) = Mix weight / (D x 27) • Cement Content: • C = Wcement / Y • Air Content: • A = [(Dtheo – D) / Dtheo] x 100

  13. Testing Hardened Concrete • Preparing and Curing Specimens from the field - ASTM C 31 • Specimens of cylindrical shape are prepared to determine compressive (ASTM C 39) and split-tensile strengths (ASTM C 496) • Molds (length-to-diameter ratio  2): • 150 mm by 300 mm (6 in. by 12 in.) for aggregates less than 50 mm (2 in.) • 100 mm by 200 mm (4 in. by 8 in) for aggregates less than 25 mm (1 in.)

  14. Testing Hardened Concrete (Cont.) • Procedures • Place molds inlevel, rigid, horizontal surface • Deposit concrete in3 equal layers • Compact each layerapplying 25 strokes with a standard rod, and tapping the exterior of the mold with a mallet 10 to 15 times

  15. Testing Hardened Concrete (Cont.) • Finish top surface • Cover with nonabsorbent, nonreactive plate or sheet of impervious plastic • Initial curing (up to 48 hours) - away from direct sun rays and radiant heating devices at temperatures between 60 to 80°F • Standard curing - remove from mold and store in humid condition at a temperature of 73.4 ± 3 °F until time of testing

  16. Compressive Strength of Concrete (Cont.) • ACI 318 • Strength - average of two cylinders from the same sample tested at 28 days • Acceptance criteria • Average of three consecutive tests is equal or exceed design strength • No individual strength test falls below design strength by 500 psi

  17. Compressive Strength of Concrete • ASTM C 39 • Testing Machine - Enough capacity and appropriate loading speed • Displacement - 0.05 in/min • Pressure - 20 to 50 psi/sec. (565 to 1,414 lb/sec. for 6” cyl.) • Strength - Maximum load / Average cross- sectional area • Length correction factors - length-to- diameter ratios (L/D) smaller than 1.8 • L/D: 1.75 1.50 1.25 1.00 • Factor:0.98 0.96 0.93 0.87

  18. Concrete Behavior

  19. EVALUATION OF HARDENED CONCRETE • Conventional Methods • Compression Test • Split-Tension Test • Flexural Strength Test • Non-Conventional Methods • Rebound Hammer Test (ASTM C805) • Penetration Resistance Test (ASTM C803) • Ultrasonic Pulse Velocity Test (ASTM C597) • Maturity Test

  20. Rebound Hammer Test (ASTM C 805) (Schmidt or Swiss Hammer Test)

  21. Some Details • Concrete 4 in. thick and fixed to the structure • Avoid areas with honeycomb, scaling, or high porosity • Prepare at least 6 in. area • Smooth surface with abrasive stone • Use readings from the same hammer for comparisons • Minimum of 10 readings, 1 in. or more apart (discard reading lower than 7 units from the average. Only 2 are allowed)

  22. Limitations • Sensitive to local conditions • Hard aggregates produce high rebound number • Air voids produce low rebound number • Do not compare grounded with ungrounded surfaces • Interior condition of concrete is not evaluated

  23. Penetration Resistance Test (ASTM C 803) (Winsor Probe)

  24. Some Details • Concrete should have certain resistance to penetration • Probes should be firmly embedded • Probes have to be at least 7 in. apart • Minimum of 3 probes to be considered as a test. Discard probes that exceed the average established by ASTM and make another reading • Test is insensitive to surface conditions

  25. Limitations • Aggregate type affect penetration for the same concrete strength

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