#78 Aggregate in Concrete Mix
#78 Aggregate in Concrete Mix
(OP)
I have a project where I have approved mix designs for all of my cast-in-place concrete. Each design has properly graded aggregate.
Now, the GC has asked if he can use a different mix on the elevator pit walls. He tells me that he's poured his footings, doesnt want to take the concrete truck over the footings, doesn't want to bring out a concrete pump to get the properly graded concrete to the pit walls, but, can have his mason use his grout pump to pump concrete with #78 stone (pea gravel), that is normally used to grout CMU voids in reinforced masonry, to the elevator pit, in order to pour the walls.
I am apprehensive about using this #78 mix, as it is not as well graded. I do use this mix in reinforced CMU due to the limited space available for rebar and concrete, but, I have never used it as the concrete for a formed wall. I have been submitted a concrete mix design, and the compressive strengths are fine.....but for some reason, in my mind, this does not seem like a good idea.
Any suggestions or comments?
Now, the GC has asked if he can use a different mix on the elevator pit walls. He tells me that he's poured his footings, doesnt want to take the concrete truck over the footings, doesn't want to bring out a concrete pump to get the properly graded concrete to the pit walls, but, can have his mason use his grout pump to pump concrete with #78 stone (pea gravel), that is normally used to grout CMU voids in reinforced masonry, to the elevator pit, in order to pour the walls.
I am apprehensive about using this #78 mix, as it is not as well graded. I do use this mix in reinforced CMU due to the limited space available for rebar and concrete, but, I have never used it as the concrete for a formed wall. I have been submitted a concrete mix design, and the compressive strengths are fine.....but for some reason, in my mind, this does not seem like a good idea.
Any suggestions or comments?






RE: #78 Aggregate in Concrete Mix
RE: #78 Aggregate in Concrete Mix
RE: #78 Aggregate in Concrete Mix
RE: #78 Aggregate in Concrete Mix
I would worry about a wet #78 mix that pumps well, but if it is placed in non-absorbent forms, the excess water that may be added for use in some pumps can only harm harm the strength, shrinkage and durability.
The testing/sampling procedure for masonry grout is totally different from concrete cylinders because of the aspect ratio and the requirement for an absorptive forming method.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
RE: #78 Aggregate in Concrete Mix
The mix you are suggesting is typically a high slump, high water-cement ratio mix that is used in a non-exposed application (filled cells), and as a result doesn't have the durability and will have slower strength gain and higher potential for strength variation/failure.
Tell your contractor to place the approved mix design. His unwillingness to supply the job with proper equipment is a big red flag and you're only at the foundation stage. If you don't make your point known and stick to now, you'll be asked to compromise quality throughout the job.