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ASTM C 305-06: Standard Practice for Mechanical Mixing of Hydraulic Cement Pastes and Mortars of Plastic Consistency[edit]

This practice is intended for use in mechanical mixing of pastes and mortars for the testing of hydraulic cements.

Also included: ASTM C 185 08: Test for air content in hydraulic cement mortar


Apparatus

1. Mixer

  • electrical epicyclic type
  • 1st speed: paddle 140 ± 5 r/min, planetary 62 r/min
  • 2nd speed: paddle 285 ± 10 r/min, planetary 125 r/min
  • electric motor 124 W (1/6 hp)
  • adjustment screw or clearance adjustment bracket for clearance of > 0.8 mm and < 2.5 mm


2. Paddle

  • removable; stainless steel
  • clearance from bowl ~ 4. 0 mm but not < 0.8 mm


3. Mixing Bowl

  • nominal capacity of 4.73 L
  • stainless steel
  • lid of nonabsorbing material not attacked by cement


4. Scraper

  • semirigid rubber blade 75 mm long, 50 mm wide, edge 2 mm thick
  • handle 150 mm


5. Others

  • balance, weight, straightedge


Temperature and Humidity

1. RT and dry materials: 20 – 27.5 oC

2. mixing water 23 ± 1.7 oC

3. relative humidity cannot be < 50%


Materials, Proportioning and Consistency

  • relative to particular method



Procedure for Mixing Pastes[edit]

1. Place dry paddle and bowl in mixer

2. Pour in all mixing water into the bowl

3. Add cement, wait 30 sec

4. Mix at slow speed for 30 sec

5. Stop for 15 sec; scrape pastes from the side of bowl

6. Mix at medium speed for 60 sec


Procedure for Mixing Mortars[edit]

1. Place dry paddle and bowl in mixer

2. Pour in all mixing water into the bowl

3. Add cement, mix at slow speed for 30 sec

4. Add all sand (as required by method) over 30 sec while still mixing at slow speed

5. Stop mixer; mix at medium speed for 30 sec

6. Stop mixer and wait 90 sec

  • in 1st 15 sec scrape any side mortar, then cover with the lid

7. mix at medium speed for 60 sec


  • In cases of remixing intervals, scrape any stuck mortar



ASTM C 185 08: Test for Air Content in Hydraulic Cement Mortar[edit]

  • Flow determination

1. Place flow mold in centre of flow table

2. Using spoon, place 25 mm thickness of mortar in mold, tamp 20 times

3. Overfill mold with ~20 mm, then tamp again 20 times

4. Cut off the mortar in a plane surface using straightedge

5. Lift mold away after 1 minute

6. Immediately drop table 10 times


  • Mass per 400 mL of mortar
whole process must be done within 1.5 minutes

1. When water needed for 87.5% flow has been determined, immediately determine mass of 400 mL of mortar remaining in mixing bowl (CANNOT reuse the one used for flow)

2. Using spoon, place mortar in 400 mL measure in 3 equal layers, tamp each layer 20 times in a complete revolution

3. Overfill final layer by ~20 mm

4. After filling and tamping, tamp the sides of the measure 5 times in different locations using stick of tamper

5. Cut off mortar using straightedge using two strikes, perpendicular to each other

6. Determine the mass of the mortar in grams


  • Calculation

Air content, volume% = 100 – W[(182.7 + P) / (2000 + 4P)]

W = mass of 400 mL of mortar, g P = percentage of mixing water, based on mass of cement used