Concrete vs Grout in a Retaining Wall 1

[PrescottEngineer]

The contractor asked me if he could use an equivalent strength "grout" instead of "concrete" for a concrete retaining wall I designed. What he is asking is if I will approve the use 3/8" aggregate instead of 3/4" aggregate. He wants to pour using a 2"Ø hose with a grout pump instead of using a 4"Ø hose with a rock pump or a boom pump. I called the local concrete suppliers and the only downside they mentioned when using smaller aggregate is that there tends to be more shrinkage of the material. I would like to know what others think about this and what additional downsides there may be. Thanks.

 

[dcarr82775]

Shrinkage is my main concern. In this area for lightweight concrete you can only get 3/8" coarse aggregate. Since it is a retaining wall I would stick with the 3/4" aggregate.

 

[JAE]

More shrinkage. Typically your retaining walls would have control joints at 25 ft. maximum spacing and perhaps expansion joints at 100 ft.

With the smaller aggregate you would possibly get some additional vertical cracks down the wall closer than the 25 ft. The response to the 3/8" mix for me would be to use closer control joint spacing and perhaps up my horizontal temperature/shrinkage steel in the wall.

It's a trade off.

 

[dicksewerrat]

He probably owns a grout pump and would have to rent the concrete pump. I would also be there when he is pumping.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[grantstructure]

Make sure you get a separate mix design submittal for the 3/8" mix--you need concrete with 3/8" agg that meets your specs for A/E and such, not whatever masonry grout mix the local plant is turning out.

Also, I'd make sure he's not going to backcharge the project for the difference--with the additional paste, I would think the smaller aggregate mix would be more expensive.

 

[Ron]

Agree with others..shrinkage issue. When the aggregate size is reduced, the surface area of the aggregate goes up, so more cement paste is required to cover the aggregate surfaces. More cement>>>more shrinkage. Strength is not the issue as that can be obtained with either.

Stay with the 3/4" aggregate...lots more cracking otherwise.

 

[TXStructural]

Our concrete guy says to not do it. It will not perform well. If you have no choice, be sure it is crushed limestone aggregate rather than pea gravel, but expect excessive shrinkage, and possible durability issues. (I get this from a guy with 30+ years in the business who has designed mixes for Trump and Calatrava, as well as for everyday uses.)

Also, when you specify 3/8", you are getting materials that passes a 3/8" sieve, so only a few percent is actually 3/8, with the remainder being smaller. The same goes for other sizes, so it is not like you have a pile of each exact size of rock and they get mixed, unless you specify a very expensive gap-graded mix.

 

[dcarr82775]

TXS, I assume your concrete guy designs only the most terrific amazing luxurious highest possible quality mixes for Trump

 

[PrescottEngineer]

Thanks for the responses everyone. I appreciate the help. I am planning updating this and future project specs to require a tighter spacing for the control joints when 3/8" aggregate is used.

 

[TXStructural]

dcarr, I expected him to say "well, you can do this and that, and get it to work." Instead, I got a resounding "um, no, that's not really gonna work, at all." Jack is a really good problem-solver with a history of making poor situations and mixes work well enough, but his response was basically to not try. He said "tell the guy to get a bigger pump and do it right, because doing otherwise won't really save money."