CMU Grout Mixing Time

[southard2]

I was performing a threshold inspection on a project of mine and caught the mason pouring grout into a short section of wall using a grout mix that had been in the truck between 2 and 2.5 hours. The maximum allowed mixing time per the masonry spec is 1.5 hours. Though you can extend that pour time for a little bit if the slump is still within a certain range.

Basically we had a morning pour. They finished one side of the building and had to move the hose and pumps to the other side of the building. The mixer truck pulled into the washout area where I assumed he was washing out. I go into the trailer and watch from the window as they move everything. Finally the new truck shows up 40 minutes to an hour later. They pour. I go to grab the ticket only to find its the same truck that had pulled into the washout area whom apparently never washed out.

The testing company at this point was long gone. Does anyone know what kind of impact this extended mixing could have had on the grout?

The slump was fine throughout the pour. How knows if they added water or not. For comparison I went back to the other side where the pour took place within the normal time range. It was still soft at the top and actually cold. The over the time poured grout basically felt the same at the top of wall. >:-<

John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[concretemasonry]

First, grout is not like concrete and the requirements and use are totally different.

Are you referring to mixing time or agitation (continuous or intermitent)?

Is this a large job with only grout in the reinforced cells or a small section the is specified to be dumped full everywhere?

Water is not an enemy in grout where the slump must be between 8" and 11" and there is no large coarse aggregate. The purpose of grout is to provide a flowable mix the will fill the block cells that is dictated by the grouting cell to separate into a "sand" grout or a "sand-pea rock" grout.

Grout consolidates and cures over time as the excess moisture get absorbed by the masonry units and really not a structural component except to transfer the loads from the masonry units to the reinforcement. Usually grout must be topped off after a period as absorption creates shrinkage vertically. If this is a multi-lift grout job it is important that the newer lifts are vibrated and consolidated in the previous lifts that may have been placed and hour ot so earlier.

Notmally, grout strength is not a problem since good engineers will even place a maximum strength od grout in the specs.

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[southard2]

Well the pour height and lift was only 4 feet. I assume the mixer truck was turning the drum the entire time but I'm not sure. The slump appeared fine so I don't think pockets forming due to water absorption is going to be problem. The mix design was for 3000 psi and I don't think you need more than 2000psi to achieve f'm of 1500 psi.

My worry is what the extra long placement time might have with the mix curing properly. The masonry code limits the time that water is introduced into the mix to placement to 1.5 hours. For a small section the grout wasn't poured till abotu 2.5 hours.

This could be no big deal but I'm not the one who wrote the spec and I don't know the ramifications. Perhaps if the heat of hydration passes while still not being poured we have a problem. Or perhaps the heat of hydration period doesn't start until the agitation stops. I just don't know.

John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[concretemasonry]

Is the f'm the assumed minimal strength usually used is masonry design? That is for a minimal strength block prism for a guideline for most masonry design.

Since the design of masonry is controlled by the masonry units and not the grout that is in line with unreinforced masonry. I have seen ungrouted masonry prisms with an f'm of 4800 psi.

The purpose of grout is to transfer the loads (mainly flexural or shear) from the masonry units to any reinforcement if needed. - It sounds like a "belt and suspenders" boiler plate design for a 4' high wall unless that is just for a unnecessary low lift grouting requirement.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.