Removing laitance from the top of footing for CMU retaining wall

[JoeDirtSoCA]

I'm brand new to this site and kind of new to special inspection. I have a lot of questions!

I was called out to do a special inspection for a CMU retaining wall. The project plans gave the contractor the option to wet set the first course of block into fresh concrete after the footing had been poured,OR they would have to construct a keyway. I was called onsite by the owner to begin inspection after the footing had been poured but before they started to lay up the wall. I immediately noticed that they had not wet set the first course AND there was no keyway. In addition, masonry code and ACI specify that laitance be removed and the footing be cleaned before putting down the first mortar bed. I notified the contractor of this and he seemed perplexed. He didn't know what "laitance" was. The owner called the guy who designed the wall and was told that the contractor needed to grind out a keyway and remove the laitance. After chatting with the contractor as the job progressed I learned that he hadn't done much masonry work in the past (just concrete work) and that this project was a costly learning experience.

On another project I had the same experience with bewildering the contractor when pointing out laitance removal. Is this something that is looked over by other inspectors? Am I being too strict in this regard? Any tips are appreciated. Thanks!

 

[BUGGAR]

You are correct. It is unfortunate how little some contractors know. Others will post in support of this.

 

[XR250]

Honestly, I have never heard of laitance before -good to know. I have also never seen a keyway on a CMU retaining wall. Couldn't extra rebar be aded to resist sliding?

 

[Ron]

Yes, laitance should be removed. It results from too much bleed water and has a very high water-cement ratio and will scale away from the base concrete if left in place, jeopardizing the bond of anything put on top of it.

Removing laitance is not difficult. It can be done mechanically or chemically. I prefer mechanically. Use a bush hammer or even a weld de-slagging tool (air powered). Comes right off and then you can bond to it.

As for a keyway and wet setting the first course of block.....I think neither is a good idea. Use rebar. Craftsmanship in construction has declined to the point the finess construction is no longer viable!

Yes....it is deplorable how little knowledge there is in the construction industry.

 

[XR250]

So when does laitance need to be removed? For normal foundation construction or only for retaining walls?

 

[JoeDirtSoCA]

Without the code books in front of me, I'm pretty sure it needs to be removed whenever there is a cold joint--might be wrong on that though. I think it is to ensure that the fresh concrete bonds to the concrete already is place. In my pretty limited experiences I've seen contractors use a bush hammer (like Ron said) to remove the laitance, and the other time they used a machine they called a 'scabber'(?).

 

[BUGGAR]

I think it a scabbler but I'll let you look it up. Keyways are rare on for CMU walls.
Another term is green cutting, which is sandblasting or otherwise removing laitance while the concrete is still green or not fully cured.

 

[sandman21]

ACI 6.4.1 "Surface of concrete construction joints shall be cleaned and laitance removed"

6.4.2 "Immediately before new concrete is placed, all concretion joints shall be wetted standing water removed"

11.6.9 "... If µ is assumed equal to 1.0 lambda, interface shall be roughened to a full amplitude of approximately 1/4""

ACI 530 Specification 3.2 page S-52 "Prior to placing masonry, remove laitance, lose aggregate, and anything else that would prevent mortar from bonding to the foundation"

Keys are required at grout pour joints ACI 530 3.5 S-68, the codes intent was not to include the foundation as needing a key.

Wet setting anything is bad practice, a key in the base of the wall is unnecessary for typical applications but are "easy" to form and place correctly.

 

[JoeDirtSoCA]

Thanks all for the posts! BUGGAR is right...they used a 'scabbler'. The contractor had to rent one and spent a day removing laitance from the retaining wall footings. To me, ACI 318 stated pretty plainly that this was required. But in my limited instances in dealing with contractors they made me feel like I was being too difficult regarding this. The company I work for is mostly involved on smaller projects so the contractors are always the smaller companies. Is this a problem on bigger projects with contractors with more wide ranging experience?

 

[BUGGAR]

I've recently been discussing substation design/construction on an adjacent forum.
The contractors that worked for the utility, SDG&E, knew the utility's "system" and were very cooperative with field staff since they always wanted that next job.
Other contractors can be crooks. I've been threatened with chainsaws and airless paint sprayers.