Are there any problems using lightweight concrete in a retaining wall?

[cliff234]

A contractor on one of our projects accidently poured a cantilevered site retaining wall (retaining 5’ of soil) using 5,000 psi lightweight concrete (115 pcf)– and now we need to tell them whether or not it needs to be ripped out and rebuilt with the proper concrete. I never heard of lightweight concrete being used in foundation walls, and my gut tells me that it should not be used in retaining walls because of its increased permeability and reduced durability (reduced freeze/thaw resistance). I can’t find any ACI recommendations specifically prohibiting or cautioning against using lightweight concrete in concrete exposed to soil or weather. Can anyone point me to an ACI document cautioning against using lightweight concrete in foundation walls.

 

[JAE]

Lightweight concrete will alter your development lengths and splice lengths.

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[RickyTickyTavi]

Lightweight concrete has a tendency to be more porous. My first intuition would be not to cast it against earth because of its porosity. This may lead to long term problems for the retaining wall such has water leaking, embedded metal corrosion, etc... It seems you came to a similar conclusion as well.

Theres two ways I would approach this. Does the mix design meet the exposure class requirements--- do they even know? Did they splice their rebar in accordance with lightweight concrete or normal weight concrete? I'm skeptical they did. If either of those instances are afoul of ACI 318, in my opinion you have justification. (moreso than you already do).

 

[PEinc]

Using lightweight concrete will also reduce the resistance to overturning and sliding, resulting in lower safety factors.

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[BridgeSmith]

With only 5' of retained soil, there may not be any laps, or they may be adequate with the reduction for As required / As provided.

If the external stability was designed close to the limit, there may be an issue with the decreased weight not providing enough resistance to overturning or sliding. It's unlikely that small change would make it unstable, but it's something to look at.

As others have said, the main concerns would be freeze/thaw durability and inadequate protection of the reinforcement. Both of those are rather long-term durability concerns. Depending on the expected useful life of the wall (how long is the wall anticipated to be needed?), you could consider a compromise where the contractor gets paid less, due to the anticipated shortened lifespan of the wall.

 

[structSU10]

Was the concrete air entrained?

ACI 318-14 19.3.3.1 normalweight and lightweight concrete subject to freezing-and-thawing...

Appears to imply it is OK for that exposure, as long as it meets the criteria laid out in table 19.3.2.1. If the original mix didn't meet these requirements, it may be a counterpoint by the contractor.

 

[cliff234]

Thank you everyone for the feedback. The wall works structurally. The issue that we had was that they used the wrong concrete and we are concerned about durability. I don't know the exact mix design, but in any case, I've never used lightweight concrete for retaining walls. It's interesting how ACI 318-14 Section 19.3.3.1 specifically says, "Normalweight and lightweight concrete subject to freezing and thawing...". Thanks again!

 

[abusementpark]

You could make them add a protective coating on the wall with no additional cost. Or Mr. Gorbachev, tear down this wall.

 

[TLHS]

How critical is it? Does it hold up structure or is it just landscaping?

Basically, what are the consequences of durability problems. If it starts deteriorating, what are the problems?

If you're just worried about durability, it might be possible to make the contractor give some sort of financially backed warranty, in lieu of replacing it. That's into lawyer realm, though.

 

[chicopee]

My first thought is from abusementpark "You could make them add a protective coating on the wall with no additional cost..."