"floor" for garage

[patrick_g]

Eddie, I'm not going to write a 20 page thesis on concrete. Be assured I understand about water in concrete and how some of it goes into chemical reactions and is no longer available as water and the excess (added to make handling the concrete easier and to control cure speed, cracking, etc) humidifies the space above the pour during cure.

The chart we were given the link to agrees with all my previous learning. More or less standard mixes reach about 90% of the final strength in about 28 days. In theory concrete never finishes curing but instead asymptotically approaches a final value (also visible in the referenced chart.)

The test I described (vapor barrier taped to the concrete) can be used as you describe to judge the rate at which concrete is losing water during initial cure but it can also be used on 50 or 100 year old concrete too and if the test shows too much water you don't but certain kinds of floor coverings down. Among those that aren't suggested for wet locations is porcelain tile because it is like glass and doesn't pass the water, allowing the vapor pressure to loosen the tile. There are other coverings that don't "like" water as well.

Excess water indicated by the afore mentioned test applied to old slabs is NOT left over from the initial pour but is transported through the slab. Uncovered floors may not appear wet because the water may evaporate as fast as supplied. Your shirt sleeve may not appear wet on a low RH day even when you are working but that isn't because you are not perspiring at all it is because it is evaporating fast enough to not feel wet.

Now for your supporting cast who are hung up on water going uphill. Can you say capillarity? stick a rag in a glass of water and, over time, the rag gets wet way above the water level. Can you find a wick for an oil lanp or Tiki torch? Put one end in water and support the other end STRAIGHT UP INTO THE AIR. Water will make it STRAIGHT UPHILL to the top of the wick. Concrete does this too, just not as good. Now I know we have all heard the term heart wood but surely no one thinks a tree has a heart that pumps water up to the top of the tree. How do you suppose it gets there? (See also capillary action.)

I'm sorry we ended up on opposite sides of this discussion as you have quite a following on this site and I am not getting any points for taking up a contrary position but I feel the need to support the side of science and engineering on this one.

Pat

 

[gemini5362]

This is good a nice topic with two different sides to the discussion.

1. Eddie I use laminate floor quite a bit. I have started reflooring all my rental houses in laminate to get away from carpeting. The instructions for laminate flooring tell you to put a vapor barrier down. All the brands I have used tell you tape the vapor barrier with a cloth tape with agressive adhesion capbilities. I interpret that to mean duct tape. I have not seen any that advocate just masking tape. I know some people do that but it is contrary to the instructions and in my opinion negates the vapor barrier. The instructions in the laminate flooring I have been using tell you to do a test to see if moisture is coming through the concrete. Prior to installing the laminate

2. A friend of mine and I were in OKC and went to visit patrick g. I have worked with engineers for years. I have been a field engineer and have worked a lot with design engineers. I can tell you beyond any shadow of a doubt that Pats house just screams out " I was designed by an engineer. " His house has so much overkill that I cannot imagine him not knowing or at least researching every facet of the material that went into the building of it. I would include the concrete floors in that. I agree with him 100 per cent about the wicking action of concrete.

 

[woodchuckie]

Is there a moisture barrier I can apply on top of the concrete in the floor of my shop. It use to be the third bay of a three car carport till I walled it up and insulated it. Anything not cromed or painted will rust in no time. When the temp and humity is right, water pools inside the shop just like outside on the carport.

 

[Red55]

JUST a thought could you both be right? Personally I would never pour a slab without a vapor barrier. . . But something has to be said about field conditions. (ie hot dry Texas vs damp & wet north?) My understanding of vapor barriers down south are on the exterior vs on the interior up north, with regards to wall insulation. And homes are conditioned spaces either heat or A/C which will remove the moisture out of the air and dry out the concrete slab ED

 

[Builder]

Is there a moisture barrier I can apply on top of the concrete in the floor of my shop. It use to be the third bay of a three car carport till I walled it up and insulated it. Anything not cromed or painted will rust in no time. When the temp and humity is right, water pools inside the shop just like outside on the carport.

I have seen this happen in other buildings.

Try ventilating the garage with a ridge vent in the roof or gable end wall vents at the peaks or just crack a window or two if possible. If none of those are possible, run a fan to help dry out the air.

You can check out the garage floor paints that are made to be applied over concrete garage floors. Read the label first to make sure it will work in your conditions.

 

[EddieWalker]

I live in East Texas. We get around four feet of rain per year and it's extremly lush and almost tropical with the extreme humidity and moisture. Saying that it's drying in Texas might be accurate, but not so for East Texas. Imagine Louisiana if you're not familiar with this part of Texas.

Yes, I understand capilary action and realize that to some degree, this is possible. What I fail to grasp is why gravity doesn't have the same effect on concrete, and why capilary action only happens inside a shop, not the living area of a house?

If it works in one area, it should be a universal occurance all throughout a slab foundation. Why isn't there moisture on the concrete in the areas of a home that have AC and Heat? I'd think that if this was true, then around all the plumbing drain lines, there should be wet areas. Has anybody ever come across this? Capilary action would sure work allot better with a seam then through solid concrete.

I'm not aware of aged concrete not being suitable for floorings. I've never come across this, nor heard of it happening. I don't know enough to say it's not true, just that it must be very rare and one of those exceptions to the rule type of situations.

I've also never heard of concrete being too wet for tile. Once concrete has cured, it will hold tile. I've never heard of it being tested before putting down tile, nor heard of it failing because of wet conditions. Swimming pools, tubs, fountains and patios all hold tile without any problems when properly installed. What could be wetter then a swimming pool?

Laminate flooring is the main one that I'm aware of that you have to be concerned about with moisture content in the slab. Once you can leave the plastic down for 24 hours and not get any wet areas, then it's safe to install. If capilary action, wicking, or any other method for moisture to work it's way up through solid concrete was in effect, then it should be reasonable to expect that plastic test to fail forever in some areas. For those of you who live in what you consider wet locations, does this happen? Here, with the worse raines of the last fifty years, concrete dried without any problems and floors passed the test to install laminate flooring. With all that rain and moisture, this shouldn't happen under those conditions if I'm wrong.

I'm not trying to disregard science or the experts here. These are just my observations and opinions based on what I consider to be common sense. Swimming pools never gain water when empty from the ground, and water doesn't leave a pool in the oposite manner. I don't know if concrete is water proof, but it sure does a great job of keeping water in a tank for livestock, or a septic, or just about anything it's used to store water. It's not different concrete that's used, so how come it lets moisture travel one way through it, against gravity on slabs in shops, but not the other direction with gravity, or in any other application that it's used?

Eddie

 

[Lynkage]

Lynkage - not sure if they have the recycled asphalt around here. How tiny do they grind it?

It is usually about 1/2" to 3/4". It is also best if you can do it on a hot day with a plate compactor. It will sort of melt together again. You should be able to find it where you can find crushed concrete (road base). Most of the ready mix companies will have a sand and gravel pit.

 

[jimmysisson]

As someone posted this may a ground-conditions difference. Here in the northeast, our soils are fairly damp. Vehicles parked over grass rust. Cars and pickups parked in dirt-floor garages rust, if the doors are closed. Open sheds, cars don't rust. Tractors are mostly painted underneath (at least when new) and rust less.
We typically put 1" of foam under garage slabs and 2" under house slabs, this with a full frost wall (4' below grade per code). In the garage, that lets the slab maintain temp closer to atmospheric, and reduces sweating, which is wicked in early - mid summer. Insulated slabs barely sweat. In a house, the 2" breaks the capillary action, and keeps the slab closer to house temp.

Per the OP, around Mass you'd scrape off the topsoil maybe 12" and replace with pervious material - whatever's best/cheapest in your area, over landscape fabric to keep the under-soil from pumping, if it's wet or clay. Recycled asphalt must have something else for binder or it won't pack, in my experience. But, many materials will pack and not wick. If you later want concrete, scrape out enough to get your slab where you want it, plus 1" foam. You won't regret it.

 

[Builder]

I use crushed concrete. If it gets damp and compacted, it makes an amazingly tight, compact surface.

 

[daTeacha]

After reading all the various opinions and positions on the moisture topic, may I offer a simple suggestion?

Check with your local building code. If local conditions are such that water vapor from the ground is a problem there will be a requirement for a vapor barrier beneath occupied structures. If the conditions do cause such a problem, there will not likely be such a requirement.

This, of course, presupposes that you have a local code rather than adhering to national standards or perhaps like some counties around here, have none at all.

Water can travel in all directions through soil, up, down, laterally, or whatever combination of the above you wish. Same for concrete. Whether it shows up as condensation above the floor will depend on the temperature of the object in question, the temperature of the air, the relative humidity, and the dew point.

Regarding trees, Pat, water moves up through a combination of factors such as capillarity, transpiration pull, and root pressure. The latter is essentially caused by osmotic pressure as water moves through the semi-permeable membranes of the root hair cells from the relatively low salt/nutrient concentrations of the soil water into the higher solute (and thus lower water) concentrations of the interior of the cells. The cells also undertake active transport of the water, expending energy to bring it into the cells and increasing the pressure inside the root. This is why a stump will "bleed" when a tree is cut down.

Eddie, regarding the movement of water in concrete, the direction of net motion is dependent on simple osmosis. Concrete is slightly permeable to water. Given time, water will move through it in whatever direction the balance of forces acting on it direct. Like soil, it can be up, down, sideways, or whatever combination works for you. Like soil, whether you see the result of that movement will depend on the conditions extant at the end of the journey. Quite often it will enter the air as vapor, which can then condense on cold objects like tractor steel, the inside walls of a building, etc. Sometimes, it will simply stay in the air as a vapor (not fog, an invisible gaseous form of water) and leave the room with a change of air.

The whole business of whether or not you will get condensation in a building of a particular design is highly dependent on local conditions, which include soil as well as air.