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Accelerated Mortar Bar Testing[edit]

Accelerated Mortar Bar Testing (AMBT) is detailed in ASTM C1260 "Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method)" and ASTM C1567 "Standard Test Method for Determining the Potential Alkali-Silica Reactivity of Combinations of Cementitious Materials and Aggregate (Accelerated Mortar-Bar Method)". These methods allow rapid determination of potential for aggregate reactivity (ASTM C1260) and mitigation effects of SCM replacement of portland cement to control expansion when reactive aggregates must be used (ASTM C1567).

Download the AMBT worksheets designed to facilitate this process here.


  • Safety glasses are required at all times in Benedict lab and M&M 715.
  • Gloves (nitrile are sufficient) should be worn while mixing mortar.
  • Cement and SCM's can be weighed out without creating a cloud of dust. Take care not to aeresol fine powder by taking your time and minimizing the distance between the source and scale.
  • The mortar mixer will happily incorporate into the mortar flesh and other parts of hands placed into the mix bowl while the paddle is rotating. Keep hands out of the bowl when the paddle is moving.
  • A rubber apron is required when working with soaking mortar bars. 1N sodium hydroxide at 80 degrees C is unpleasant a best, tissue removing at worst.

Preparation prior to mixing[edit]

Mortar mixing and casting should be completed on Tuesdays as this minimizes the potential for weekend work. On the Monday before mixing, prepare the molds and weigh out aggregate and cementitious materials. Four inch cement cylinder molds labeled with a letter followed by -C or -A (cement and aggregate, respectively) are available in the Mortar Mix room for storing weighed out components.

  1. Aggregate must be sieved into -4+8, -8+16, -16+30, -30+50 and -50+100 fractions, washed to remove fines and dried at 100 degrees C for 24 hours prior to weighing out. Store aggregate by size fraction in clearly labeled, sealable containers. Mark the containers with the name of the contents, size fraction and your MTU email address.
  2. Molds for the 11" x 1" x 1" mortar bars are located in the Mortar Mix room in Benedict lab. There are a total of 15 double molds, accommodating a total of 10 batches of mortar. Assemble the molds noting that the screws holding the stud blocks into the mold ends are specially machined and cost more than your average screw. They can be spotted easily as they are an odd length and the threaded end terminates with a very cleanly machined surface. Also take note that the molds are numbered sequentially so that multiple differing mix designs can be completed and uniquely identified.
    1. Molds should be stored clean and assembled. Do not attempt to cast bars in dirty molds! Thoroughly clean and assemble the molds when you are done.
    2. Insure that the screws securing the stud blocks are tight and the stud blocks don't wiggle.
    3. Insure that the measuring studs are screwed fully into the stud blocks.
    4. Dampen a clean rag with WD40 and wipe the entire surface of the mold such that a thin layer of lubricant is left.
  3. Weigh out the cementitious material and record the actual weight on the mortar mix sheet. Store the cementitious material in the appropriate container (i.e. A-C).
  4. Weigh out aggregate per the recipe defined on the mortar mix sheet and record the actual weight of each fraction on the mortar mix sheet. Store the aggregate in the appropriate container (i.e. A-A).

Mixing mortar and casting mortar bars[edit]

  1. The mortar mix procedure is summarized on the mortar mix sheet and must be followed to comply with ASTM requirements. The procedure is expanded upon below. Obtain a timer (available in the lab) to keep track of mix and rest times.
  2. Clear the timer.
  3. With the mixer assembled, plugged in but NOT turned on, rotate the paddle by hand until it is as far away from the front of the bowl as it can be.
  4. Set speed lever to position 1.
  5. With the bowl in the lowered position, add all of the water.
  6. While the bowl is in the lowered position, add all of the cementitious.
  7. Raise the bowl and insure that the bowl position handle sits in the detent.
  8. Turn on the mixer and start the timer.
  9. After mixing 30 seconds, add the aggregate over a 30 second period.
  10. After mixing for a total of one minute, turn the mixer off and wait for the agitator to come to a complete stop.
  11. Set the speed lever to position 2.
  12. Start the mixer noting the elapsed time on the timer.
  13. After mixing at medium speed for 30 seconds, turn off the mixer.
  14. Within 15 seconds of turning off the mixer, scrape down the paddle and sides of the bowl.
  15. Restart the mixer after resting a total of 90 seconds. The timer should show a total elapsed time of three minutes plus a couple seconds.
  16. Turn off the mixer after mixing for another 60 seconds. Timer should show a total elapsed time of four minutes plus a couple seconds.
  17. Lower the bowl, remove the bowl and paddle.
  18. Wash the paddle in water.
  19. Cast beams in the prepared molds noting the number of the molds on the mortar mix sheet. Fill molds in two lifts taking care to tamp mortar around the studs and tamp after each lift.
  20. Strike off with a straight edge.
  21. Wipe off excess mortar with fingers and return to mix bowl.
  22. Wipe off remaining mortar on the sides of the molds with a damp rag - this makes cleaning up the molds a lot easier.
  23. Place filled molds in a suitable container with water in containers to provide moisture while the mortar sets. The light green fiberglass totes work well for this purpose and can hold all 15 molds with small spacer boards between layers. Place another of the totes over the top, upside-down, and tape the edge to seal.
  24. Clean scrapers, tamps, scoops and straight edges in water after every mix.
  25. Deposit excess mortar in one of the plastic tubs in the concrete mix lab.
  26. Wash interior and exterior of the mixing bowl after every mix.
  27. Partially dry the interior of the mixing bowl with a paper towel.
  28. Wipe off any mortar that may have splashed or spilled on the mixer body.
  29. The mortar must set for 24 hours at room temperature and high humidity before demolding.

Demolding mortar bars and initial measurement[edit]

  1. The mortar bars must be demolded after setting for 24 hours at room temperature and at high humidity. They are very fragile and should be treated carefully.
  2. Remove all of the screws from the mold separating the stud block screws from the mold end screws.
  3. Remove the mold ends.
  4. Slide the bars and mold sides to one side of the mold to loosen them from the base.
  5. Move the mortar bars and mold sides all the way to the edge of the base plate such that one of the sides is just off the edge of the plate. Gently tap the mold side with a small mallet to loosen it from the mortar bar. Repeat with the opposite side.
  6. Stand the two mortar bars and center divider up on the edge of the base plate such that the center divider and one of the mortar bars is off the base plate. Strike gently with the small mallet to loosen the center divider from one of the bars. Support the suspended mortar bar - don't let it drop.
  7. Loosen the center divider from the remaining mortar bar by placing the end of the mortar bar on the edge of the base plate with the center divider off the edge and gently tap the center divider.
  8. Remove the stud blocks by carefully turning them counter clockwise. Use a large adjustable wrench if they are tight, but be careful not to break the mortar bar.
  9. Once all of the mortar bars from the same batch of mortar are extracted from their molds, label them (use a letter between A and J as the mortar bars will then match the labels in the soaking containers) and take an initial comparator reading. Use the Comparator Reading worksheet.t
  10. Place the mortar bars in 80 degree C water and return to the oven for 24 hours. Containers are located in M&M 715 and marked with an S followed by a letter (i.e. S-A indicating that it is a setting container for mortar batch A).
  11. Mix solutions for soaking the mortar bars in and place in appropriate containers in the 80 degree C oven.