Although most concrete is porous, I would like to make a mixture as impermeable as possible. What ingredients (e.g. cement, sand, other additives?) and quantities do I use in the mixture?
Hot mopped asphalt with a membrane embedded in it will usually prevent and water from moving through the concrete. Better yet is to contrive a system to keep it from getting wet in the first place. Untreated, moisture will move through it relentlessly if slowly, from the side with more moisture to the side with less moisture, but you will not stop it entirely without a kind of surface application of some kind, from a clear sealer to a hot-mopped tar. The barrier is best applied to the wet side.
What is this hypothetical "concrete" anyway?
To lower the permeability of poured-in-place concrete you can reduce the water/cement ratio and/or use fly ash, silica fume, ground blast furnace slag, etc. in the mix. Your concrete mix plant will offer a mix that will suit your situation but don't count on it to keep your basement dry; the main intent of low permeability concrete is to reduce rebar corrosion.
What is the use of the concrete?
The problem is that all Portland cement based products wick moisture.
It can be reduced but not eliminated.
A membrane of some type (tar, plastic, synthetic rubber compound, etc.) can make it close to impervious to water initially, but maintenance becomes an issue.
The exterior of basement walls are not normally left accessible, and protecting from the interior has its own problems.
Thanks all for the responses. This is an unusual application that is difficult to describe, but I'll try: There is a concrete pad that abuts the house, about 4 feet above the basement floor where there is an unfinished closet. Nearby, there are slate stones placed on the ground against the wall (18 x 36 inch stones) to make a 36-inch wide path. At the point where the walls meet the concrete the grout has separated from the wall and is decaying, and rainwater seeps into basement closet. The same problem happens where the wall meets the slate. These voids are at lease several inches down, and probably deeper in some places, and 1/2 to 1.0 inches wide. My initial solution is to try to fill in these voids with solid material such as concrete, well tamped and top it off either with the same material or regular mortar/sand mix.
One idea is to pour in a loose-type cement product like anchoring cement (e.g. Rockite) that could be mixed loose enough to pour and go all the way down the holes. My concern here is that some of these products could expand and damage something that I cannot see below ground.
Although the concrete slab is solid and not likely to move, the slate stones are not on a concrete base and can shift slightly. They just sit on the soil and are pretty stable, but I am shure they shift over time.
I would appreciate any ideas.
You need something flexible. One recommendation is Butyl caulk for concrete. Google that and see what else you can come up with.
Don't fill with concrete, it will just break out again.
"The same problem happens where the wall meets the slate."
The first thing to do is add slope to the exterior slates away from the foundation, then use caulk to seal the joint of slate to wall.
It will be a continuing maintenance item wince the caulk WILL start leaking from relative movement and exposure.
One method I have used numerous times is to put a layer of EPDM buried a few inches and sealed and flashed on the vertical wall. then place stones in a good sand bed with some mortar added to help fix it in place.
The EPDM is also on a sand bed with some grade to help it drain to the far edge.
The counter flashing can be above grade (I use copper usually for this so it looks good) or just below grade so it does not show much (again copper for longevity).
One customer had me pint it the same color as the foundation to make it be less apparent.
I followed up on Hebicus's suggestion and looked for products on the internet. Came up with the following: Fill any voids that go down deep with a non-shrink construction grout (e.g., Quikrete Non-Shrink General Purpose Grout). This grout can be mixed stiff or thin enough to be poured. Although when mixed thinner, it has less strength, it will be strong enough. Fill the void till it comes up to about 1 to 1-1/2 inches from the surface. After this cures, place a closed cell backer rod over the grout, which would bring it up about Ã¯Â¿Â½ inch from surface, then top that with polyurethane self-leveling sealant. This is the same type of sealant used over expansion joints on concrete sidewalks and driveways and stays flexible. I stayed away from butyl rubber, for fear that it would not last as long as polyurethane (donÃ¯Â¿Â½t remember the source of this, so I might be mistaken).
Brickee's solution is brilliant, but too much work. (I know, I should do it right, but...). If The stones weren't in place already I would do this. However, they are already properly sloped and after looking at it again, most of the water is coming from the part with the slab. So I will use the sealant between the stones and the wall, as well as between some of the stones.
My biggest concern is that the gray color may look like plastic, and not match the grout. I may sprinkle some gray bluestone powder over it before it dries. I probably wonÃ¯Â¿Â½t mind the look around the edges, but it may look awful between the stones. I will experiment.
Well off find materials in next day or two and need to carefully watch weather here, since these materials cannot be used under 40 deg. F for sealant and 40-50 deg. for grout, and need at least a couple of days without dipping below 32.
Try some polyurethane caulk.
It has very little shrinkage as it cures, good gap filling, and for a wide but shallow repair can be tooled.
It hardens up and looks very much like mortar if you get he correct color, and does not form a plastic looking surface (common in the 'swimming pool' type caulks).
Thanks to you all, I have a plan that sounds logical. I will now do my shopping and, weather permiting, do the job. Hopefully will be able to report back in a few days.