Tim,
In your Mortar Mix for Flagstone article, is the optional hydrated limestone already hydrated (mixed with water), or dry when mixed with the Portland cement. Also, is there a specific amount of the warm water added, or do I just add slowly until I get the consistency of standard Quikrete concrete?
Thanks,
David Saunders

David,
Yes. It is already hydrated. That term means there is a small amount of water in the chemical structure, but not enough that it is wet by our standards. Hydrated lime is a dry powder. Mix it as I say in the column above. I used the terms gallons above as just a convenient method of measuring anything. You can put water into a one-gallon can as easily as you can fill it with a powder.

Hi,

Do you have experience with laying flagstone on a dry mortar mix and then 'watering it in' to set the base and the grouting? Any comments or directions on how to do this succesfully?

thanks

Hi
I am working on a public commission/art project, making 5 concrete columns--pouring into a 24" sonotube and then adding some coloured eletecrete as a topical application for sculptural effects and drawings. I have done some tests on plywood and it seems to work well. I am also wanting to make some other concrete sculptures (all one of a kind) in portland cement and am desperately searching for a recipe for silica sand with portland (general use type 10) so I can play around--trying to embed thing in the concrete as it is drying to get textures and 3 dimensional shapes. If this works I want to place these inside the sonotube when i do the pour, so it will be embedded into the column. Like an artifact/fossil, if you know what I mean. Can you help me with a good recipe for the concrete--and also some ways to tint the concrete?
Thanks very much
Joyce

Tim,
the grout on my flagstone patio is comming off and some tiles are loose. Shoudl I just use the cement/sand mixture to fix them back in? What shoudl I use to grout?
Thanks,
Amit

Hi Tim...Love your advice...I just want to know..Is lime and OPC mixed as a mortar better than a 'regular' mortar? I used lime and OPC mixed with sand....got burned..(literally)...and scars...and a dissatisfied customer.
The contractor I was working for said lime was a necessity....True or not?
He said it made the mortar stronger.....Im the one that paid the price if hes wrong!

Hi I would like to know cement mixture for 4000 lbs

6000 lbs

7000 lbs

Tim
I live in Michigan & was wondering what type of Portland Cement (N,M,S, O,or K) that I should us for my flagstone side walk & patio? I compacted crushed concrete for a base & installed 4" of concrete.

I have read and understood your section on laying flagstone over a concrete patio, which I am planning. My question is this: how should I prep the existing concrete surface. I don't mean repairs, as the patio is sound and free of cracks. I mean should the surface be etched, acid-washed, left alone...?

Thanks.

John

1) When adding hydrated lime, it’s best to combine all of the dry ingredients together first. This is true when using dry or wet sand. But, if you find you need to add a little more hydrated lime to the “plastic” mix (cementitious stuff + sand + water) then add the lime slowly. This will help minimize the number of dry powder lumps you’ll have to break up and mix in.

2) Never heard of the “watering in” method for installing flagstone (or slate or any of these types of flat stones). I can think of two instances where this “watering in” would be used. First, when pouring a concrete slab (driveway, sidewalk, patio, etc.) that consists entirely of concrete--literally a large, flat chunk of concrete--this homogeneous, active surface needs to have water on it to properly cure and to minimize surface cracking. So, the installer will put a sprinkler on it or dam up the sides and pool water on top of the slab. The cement in the concrete (aka portland cement, aka Ordinary Portland Cement, aka meets the requirements of ASTM C150) is activated by the water through a process known as hydration. The concrete slab therefore needs water to continue this chemical reaction, hence the need for maintaining extra water on top. The “watering in” part is the process whereby the extra water pooled on top is consumed during the curing process.

The second instance in which water is added to the surface of a walkway or patio or driveway etc. involves pavers. Once the pavers are installed, there are still small to medium size gaps between each unit. These gaps need to have sand swept into them. This process is absolutely by design because it helps with the small, differential movements among the pavers. As an example, think of a heavy truck driving across a paver driveway. As this truck’s tires move from one paver to the next, each paver is going to move up or down a tiny bit. Having the sand between the pavers keeps the paver from moving out of position side-to-side, but still allows for vertical movement. (remember these pavers don’t really move very much). In contrast, a solid chunk of concrete (which is what a concrete driveway really is) is designed NOT to move but to transfer the weight of the vehicle across the whole slab. The more massive the chunk the more weight it can hold. [Incidentally, if the concrete chunk isn’t thick enough or supported sufficiently underneath, when the weight is put on this chunk, the point loading of the tires will crack the concrete chunk just like a hammer and chisel.]

Now on to the “watering in” part. To better stabilize the sand between the pavers, this sand (called jointing sand) is often mixed with a binder. The binder does a couple of things. One is that it keeps the joint sand in the joint, where it’s supposed to be. Two, it completes the surface of the patio or driveway, because now the top of the sand is at the same level as the top of the paver (roughly). Three, it helps prevent vegetation from growing between the pavers. These binders are elastomeric (aka flexible, think of the caulking around your tub) which allows the paver to move the tiny bit they’re going to move and yet still provide the solidity needed for a solid joint. These binders are water activated, meaning the sand+binder mix is swept into the joints dry (on dry pavers, obviously) and once all of the excess is removed (excess after the joints are filled) water is run over the entire area to activate the binder. The “watering in” part is the process of activating all of the binder in the paver joints.

Because of the nature of flagstone, I don’t recommend the paver + joint binder method. Flagstones need to be part of a rigid system, otherwise they’ll disintegrate due to cracking.
3) Are the decorations added to the Sonotubes before the concrete is added? If yes, how are the decorations suspended during concrete placement and what ensures the maximum effectiveness of the decorations (why aren’t they fully covered by the concrete)? I think the best, best way would be to use a stiff mix (fully consolidated in the tub of concrete) and then remove the tub and place the decorations in before the concrete is fully set. It’s possible and will take some trial and error to find the right cure time to allow the concrete to stand without slumping and still allow for the embedding of the decorations. Another option might be to pour half of the cylinder at a time. You could make a mold with the cylinder face down, place the decorations and then pour the concrete on top; Styrofoam might help hold the decorations during the concrete pour. Another approach could be to form the half of the cylinder face up then at the right cure time, remove the mold face and place the decorations. Either way, I recommend using a high strength concrete or a grout mix because these products are going to have higher cementitious contents which means a creamier mix and added strength.

4) For the hand full of pieces that are loose, there are a few options depending upon how much work you are able to or are willing to do. It basically has to do with how much of the original mortar/grout you can remove from where the stone used to be. Right, this layer of mortar/grout is the exact amount to provide a level stone once placed. If you add something on top of this mortar/grout you’re going to raise the stone above the rest of the floor.

If next to none of the mortar/grout is removed I recommend using a strong construction adhesive, aka a landscaping or retaining wall block adhesive or mastic. These adhesives are used with the heavy concrete blocks. If you can remove some of the mortar/grout then I recommend replacing it with a thinset mortar, typically found in the glass block section of your hardware/box store. The best option is to remove nearly all of the mortar/grout and replace it with new mortar/grout. Please note this grout refers to the cementitious product used for underlayment and not the product used between the joints. Regarding the joint product, the biggest concern is going to be color matching. The dilemma is whether to match the current faded and dirty color or match the color of the original grout. These two colors are not always the same.

4) Hydrated lime doesn’t make a mortar stronger. Lime basically does 2 things. One, it holds water in the mix, which helps with the cement hydration process. Two, lime makes the mix more creamy, which helps with the placement of the product. Categorically claiming one product type (grout or mortar) better is not appropriate. There are job specific factors to consider such as curing and placement environment, substrate type and condition, exact collection of stone, service life environment, etc. Generally speaking, grout is stronger than mortar, but mortar will usually be more than strong enough. More is not always better. A super strong grout may shrink, but non-absorbent stone & extra water in the mortar may not work well either. Many flagstone manufacturers are now recommending a type S mortar and I believe this is based on the cost-versus-potential-for-failure compromise. For DIY installations a type S mortar is a more forgiving product to work with. Remember, type S mortar was originally designed for putting clay bricks (think absorptive) together. Short term problems are possible with both grout and mortar. My advice for those concerned about long term and have a large area to cover is to consult a professional. Paying for two hours of their consulting time is not going to significantly add to the cost of the project and you’re going to be able to access what they’ve already learned from making mistakes.

6) By 4000lb, 6000lb, 7000lb, I presume you’re referring to the compressive strength of these concrete mixes as measured in accordance with the requirements of ASTM C387. Moreover, by lb I think you are referring to the pounds in psi (pounds per square inch). A 4-5 sack mix with 1.5 inch top size aggregate and a 4 or 5 inch slump concrete delivered in a ready mix drum truck will hit 4000psi at its first month of age. A 6-7 sack mix with the same parameters will hit close to 6000psi. And these “ready mix” mix designs are readily available in the literature and on the net with a little research. Reaching compressive strengths above 6000psi requires more knowledge of concrete properties and the properties of its constituents. Simply adding more cement and cutting back on the water content won’t do it. High strength concretes require the use of admixtures and supplementary cementitious materials and well graded aggregates and specific curing procedures. If you need high strength concrete, consult a professional because other concrete properties, both plastic and hardened, have to be taken into consideration. These are job specific and involve locally available materials. This knowledge is consequently typically proprietary.

7) Portland cement comes in a few different types, but these are not designated by M, S, N, O, K. These letters are used to differentiate types of mortar. Portland cement comes in Type I, Type II, Type III, Type IV flavors.

8) The concrete preparation really is just the obvious stuff. It should be free of oil, loose dirt, fine dust, loose concrete chunks, have no soap residue, etc. This should include any paint drops or spills. I’d also consider removing embedded lumber because it might deteriorate over time leaving a void which may lead to the cracking of the stone. Ordinary concrete could be used in place of the removed lumber. Just give it a couple of days head start on curing before placing the stone.

Another thing to consider when beginning the stone installation is moisture content of the concrete slab. A bone dry concrete slab will act as a gigantic sponge on the mortar/grout substrate; it’ll suck all of the water out. I recommend pouring water from a small cup onto the concrete to observe how fast the water is absorbed. The key will be to reduce the rate of absorption. To do this simply add water to the concrete slab. Standing water on top of the concrete slab is way too much. Saturated Surface Dry is not good either; this is when the surface looks dry (but is usually darker in color) but if you put a dry piece of notebook paper on it, the paper will become wet.

Surface etching is nearly always not needed. Pressure washing is sufficient. Concrete is porous and the bonding happens through the cementitious paste being drawn into the concrete forming an intertwined matrix at the interface.

Good luck to all.

---JET_Fusion