#78 Aggregate in Concrete Mix

[marinaman]

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?

 

[TheBlacksmith]

Concrete is not a strong area for me, but in general, if you have designed the project and specified a certain material, convenience or cost avoidance by the GC doesn't make a substitution right. Are you the engineer of record on this project? Have you worked with this GC before and if so, has he made sound recommendations? Follow your instinct.

 

[VTPE]

Typically, you want a well graded mix to reduce the amount of cement and to provide larger stones for better strength. I would suggest that is you do allow the #78 stone, that you limit the amount of cement (say to 540 #/CY) so that you do not get excessive cracking, especially in an elevator pit that may be subject to more infiltration than say a frost wall.

 

[ron9876]

Nothing wrong with a pea rock mix but masonry grout is not appropriate.

 

[concretemasonry]

The masonry grout, if made properly has a slump of 8-11" because it is intended to flow well and fill all voids and the masonry units will absorb the excess water to great a good bond and is normally placed in multiple lifts and then is topped off after all consolidation. Since masonry grout may not have minimum strengths and many engineers place an upper limit on the grout strength.

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.

 

[Ron]

No. Elevator pit walls must have high density and good strength for durability and to decrease the potential for water intrusion into the pit.

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.