Admixtures in unit masonry mortar/grout mix submittals 1

[StructEngineer90]

My company's specs say to not use admixtures, accelerators, retarders, etc. in the mortar and grout mixes. I've asked around and no one seems to be sure why we state that. One of the senior engineers told me to just ignore what the spec said and allow them to use the retarder as long as the required strength, etc. were met which made me question why we even had that requirement in our specs to begin with.

Anyone have any opinions on this matter?

 

[STrctPono]

IMO, there is no reason why you should state anything like that. I have seen and added to our notes that chemical admixtures shall not be added on-site and shall be added only at the plant. This is logical as the last thing I want is a mason trying to do some quick math and add some high-range water reducer on site to try and improve the slump. I really can't see any reasons why the contractor would add a retarder to the grout or mortar anyway. Most mortar is mixed on site and is typically nothing more than portland cement and lime (or masons cement) and sand. I've never seen a mason try and add anything additional to the mix but it's possible they may try to add an accelerator if they are laying block in the winter time to prevent freezing. The grout is typically delivered by a ready mix truck. You are concerned with strength, durability, and slump. I see many benefits to allowing chemical admixtures to this. In fact, I would in some cases require certain admixtures. Typically, SRA's, Plasticizers, and Accelerators (for cold weather climates) may be added to the mix. With that being said, you need to know what you are doing before specifying admixtures. But I would not specifically restrict their use. No admixtures should contain chlorides.

 

[GC_Hopi]

Admixture to mortar would be rare, of course a pre-mixed mortar may already have admixtures, I haven't checked. The use of admixture in grout mix is pretty common for a ready mix delivery situation. I also suspect it is put in the pre-mixed grout but again I haven't checked.

I would add that based on your comment it sounds like the no admixture is a relic in the spec from the day when mortar and grout where mixed on-site. Now its all pre-mixed or ready mix but I have seen some on-site mixing for mortar so I guess it lives on.

 

[jsharkbait]

We usually ask for statements from the admixture supplier saying that the admixtures are suitable for the mix and also have tests of the mixes done (they've usually got these from previous projects or they are standard mixes).

 

[masonrygeek]

Blanket statements about restricting admixtures for masonry mortar and grout is wrong. Delete that sentence from your specs. You should also review your masonry notes for blanket requirements such as "#5 bars @ 48" o.c." or "place control joints @ 20' o.c." These blanket statements may be in conflict with the project design requirements.

As for admixtures in mortar, the biggest no-no is admixtures containing chlorides since they can cause the metal ties to rust. But some admixtures are fine such as integral water repellents (IWR). Split face block that are used in single wythe applications often contain IWR in the block itself and the IWR is also required to be in the mortar. During cold weather construction you may also want to use accelerators as noted above (but they shouldn't contain calcium chloride). If you want to limit the types of admixtures, and put some performance requirements on them, specify ASTM C1384 Specification for Admixtures in Mortar. They require mortars to maintain certain strengths when using an admixture.

As for grout, you often WANT to use an admixture, especially those that are plasticizers. Since masonry grout has to flow around rebar in very small spaces having a plasticizer is very important. As noted above, most of the grout is delivered in a ready-mix truck so the admixtures are added at the batch plant. For mortar, its best if the additive comes in the bag of cement (think pigment), but some could be added onsite. If you have trained craftworkers they should know what they are doing.

When you look around at industry publications from BIA and NCMA you will see that they generally don't recommend admixtures, but there are circumstances where they are helpful.