Pea rock pump mix for concrete beams.

[AJ817]

Hey everyone,

We are having a discussion in the office about the use of pea rock pump mix for concrete beams designed to span openings, not just for use as tie beams with full support below. Concern being cracking in the pump mix.

What are some thoughts on this? Do you all have any limitations to the pea rock mix?

Thanks for any input!

 

[SwinnyGG]

We typically avoid pea gravel mixes unless a design contains a large amount of closely spaced bar, in which case smaller aggregate is required to avoid voids, and pea gravel in particular results in a higher slump mix which will consolidate more reliably.

Unless you have a beam with a lot of closely spaced bar, all pea gravel does is make the concrete more expensive. There is no performance benefit that I'm aware of.

 

[GC_Hopi]

In general agreement with SwinnyGG here, cost typically steers contractors away from using a 3/8" mix if they can avoid it. I have used 3/8" mixes where congestion was a concern at beams or columns and in thin sections ie overlays. Cracking was not an issue with the finished product. Curing and dimensions are the critical factors with cracking.

Also my experience is that local market conditions for aggregates sometimes make a 3/8" mixes more common.

 

[dauwerda]

Maybe I'm missing the topic, but it sounds to me like the (potential) contractor is asking the OP if they can use a pea gravel mix because it pumps much better, so it's not about cost or spacing of bars, its more of a means and methods thing. The OP is wondering if there are any negative side effects to allowing a pea mix rather than typical (larger) crushed aggregate.

Ron has a great response regarding aggregate size in this thread:

https://www.eng-tips.com/viewthread.cfm?qid=447708

In short, with smaller aggregate, more cement is required (because there is more aggregate surface area to coat) in order to maintain the same concrete strength, more cement also means more water (to maintain the same water/cement ratio), more water means more shrinkage. Will the increase in shrinkage significantly impact the observed cracking in a reinforced concrete beam? I wouldn't think so, but I also don't have much experience with this. I would think it would have a bigger impact on things like slabs on grades.

Local market definitely determines typical mixes and and cost. Denver, you are getting crushed granite all day long. Western and central Nebraska you are getting 3/8" pea gravel pumped from the local gravel pit - cost goes way up when you want crushed aggregate and it is typically limestone imported from Eastern Nebraska and Western Iowa

 

[AJ817]

Thank you everyone for your replies.

You are right dauwerda. The gc wants to use pea rock but there is an argument being made that the pea rock may not be structurally sound enough for some beams that we have spanning openings. Thought is to limit the pea rock to tie beams that are fully supported by walls below and use a larger aggregate for beams spanning.

 

[SwinnyGG]

'not structurally sound enough'?

That's a function of the entire mix design, not just the aggregate. If you specify a 4500 psi mix, and the contractor orders a 4500 psi mix, and the concrete company delivers a 4500 psi mix, and it's placed without adding 300 gallons of water, you're going to get the performance of a 4500 psi mix.

If you've designed things with large amounts of bar, bar that's very tightly spaced, or both, such that a pea gravel mix is required to place the concrete correctly... well, then you should probably approve the request to use a pea gravel mix.

If the contractor is just being lazy and doesn't want to pump a normal 3/4" crushed aggregate mix (which is done every day all day without major problems, assuming the right equipment and competent crews) than you should reject it if you aren't comfortable. There are other ways to increase the 'pumpability' of a particular mix design without just changing the aggregate.

 

[Greenalleycat]

@SwinnnyGG, it's not necessarily true that a 4500psi mix is a 4500psi mix is a 4500psi mix
Under our code at least you get reductions in shear strength if the aggregate drops below 13mm (~1/2")
So for a shear-dominated beam the reduction in aggregate could be quite critical

Edit: went back to fact check myself as I had a niggling feeling that 13mm was not the correct number
Apparently the relevant factor on concrete shear strength for beams is 1.0 for 19mm aggregate (~3/4") and 0.85 for 10mm of less (~3/8") with linear interpolation to be used between these values
It's the same for columns but I can't find anything for walls, so I will assume the same constraint applies
The Code commentary also notes that both larger and stiffer aggregates reduce creep, so that is something worth considering too for a beam design

 

[Lomarandil]

Agreed, while ACI isn't as nuanced as NZS in that way, there is a real-world correlation between aggregate size and shear strength. It's up to the engineer to decide whether that is critical in their case. Remember, the code is the minimum legal standard.