Garage slab

[ralph_ricketts]

I want to build a garage. My site is about four feet lower than the driveway. I will build stem walls on footings to start that will reach the elevation of the existing drive and will totally enclose the new slab. I read on this site that pouring a slab on top of 5/8 crushed rock with no additional compaction of the crushed rock is an acceptable method of slab construction. I would like to spend the extra money on the crushed rock and then not have to worry about meeting any other compaction requirements if possible. I would like verification that this is an acceptable method of slab construction. Thank you for any help or other ideas or suggestions about my project. Regards, Ralph

[Steve Barker]

are you talking about using crushed rock for the entire four feet?

[Independent old cuss]

It all depends upon your soil. Sand, clay, etc. If you have a good clay base you've got half the battle won. 3 sack or 5 sack is a consideration as well. Hopefully the more astute on this subject will chime in with more help than I can offer. My expertise comes from having a daddy-in-law that is a contractor and tuning him out most of the time.

[ralph_ricketts]

Thanks for the reply. The area is older fill and native soil. So how does the clay soil help?

[Independent old cuss]

As I understand it, it makes for a more stable base for everything else to rest upon. I know that where we're located (south Texas) we can dig down about 6 or 8 inches and hit clay, then we put sand and gravel in and build the frames, rebar it and pour. My garage, we poured a 4 inch slab using 5 sacks but some might say that was a bit of overkill. Have you searched on google for "how to pour a slab" or maybe checked out the DIY web page? You might find what you need there.

[Steve Barker]

what is this "3sack / 5 sack" stuff?

[Independent old cuss]

I believe it relates to the psi capacity of the concrete or something. I just remember my dearly departed father-in-law always telling his procurement guy to order 5 sack instead of 3 sack. I always deferred to him because he was in the business for over 55 years and was the GC on Greenway Plaza's pour with no cold joints which took place over a period of 96 hours. If he was still alive I could ask him what he meant but if he said 5 sack was best, then I believed him.

[Steve Barker]

ok, thanks. Maybe it had to do with the old school measurement of portland in a yard of mix, or something like that.

[Tom Cular]

Perhaps this will help a bit. Suggested Concrete Mix Ratios The strength of concrete varies with the amount of cement and water in each yard. Trailered Ready-mix concrete is not rated by pounds per square inch (PSI) strength, but by the amount of cement sacks in each yard. The following information represents a rule of thumb for trailered ready-mix strengths and suggested uses.

Fence Posts

4-sack mix (four 94# bags of cement per yard): This mixture has a PSI of between 200011 and 250011 and is recommended for things like setting fence posts.

Foot Traffic

5-sack mix (five 9411 bags): PSI of between 2500# and 3000# and is recommended for things like walking surfaces such as sidewalks.

Auto Traffic

6-sack mix (six 94# bags): PSI of between 30011 and 35011 and is recommended for things like driving surfaces such as driveways.

Cold Climate Tip In areas of multiple freeze/thaw cycles, there should be 1 sack of cement added to each of these recommendations and add an air entraining agent.

And keep the water content as low as possible, 3-4 " slump MAX before any additives, add a plasticizer (Super P) if necessary for workability. More water means more shrinkage and more CRACKS! Super P will allow you to pour (Pea Soup) without harming the final product and it's effects dissapear in about 30 min. depending on the brand.

[jloomis]

Good list of sack mix...... john

Perhaps this will help a bit. Suggested Concrete Mix Ratios The strength of concrete varies with the amount of cement and water in each yard. Trailered Ready-mix concrete is not rated by pounds per square inch (PSI) strength, but by the amount of cement sacks in each yard. The following information represents a rule of thumb for trailered ready-mix strengths and suggested uses.

Fence Posts

4-sack mix (four 94# bags of cement per yard): This mixture has a PSI of between 200011 and 250011 and is recommended for things like setting fence posts.

Foot Traffic

5-sack mix (five 9411 bags): PSI of between 2500# and 3000# and is recommended for things like walking surfaces such as sidewalks.

Auto Traffic

6-sack mix (six 94# bags): PSI of between 30011 and 35011 and is recommended for things like driving surfaces such as driveways.

Cold Climate Tip In areas of multiple freeze/thaw cycles, there should be 1 sack of cement added to each of these recommendations and add an air entraining agent.

And keep the water content as low as possible, 3-4 " slump MAX before any additives, add a plasticizer (Super P) if necessary for workability. More water means more shrinkage and more CRACKS! Super P will allow you to pour (Pea Soup) without harming the final product and it's effects dissapear in about 30 min. depending on the brand.

[Steve Barker]

thanks for the info.

[HerHusband]

Hi Ralph,

I did something similar when I built our garage, though I only had a foot to build up, not four. We formed and poured the footings and stem walls ourselves, then hired out for the actual pour.

It is my understanding that fill of any kind should be compacted in layers. I believe it's 6 inches max for each layer. Filling the space in one lump is likely to settle over time. I rented a plate compactor to compact my gravel base, quick and easy.

I don't know what size your garage is going to be, but my 24'x28' garage worked out to roughly a single dump truck load for a 6" layer (12.5 yards). If you're going four feet deep, that's about eight dump truck loads. Dump, spread, compact. Dump, spread, compact. Repeat... :) If you want to save some labor, have the gravel delivered by conveyor trucks that can shoot the gravel wherever you need it. Or rent a bobcat and have fun driving it around! :)

Be sure to lay down a layer of 6 mil plastic sheeting before the concrete pour too to keep moisture from coming up through the floor.

I also recommend adding fibermesh to the concrete for the floor slab. It really helps minimize cracking. My slab is about 12 years old now and I only have a single small hairline crack in one of the doorways. I wouldn't even know it was there other than it's discolored from water running down in it.

Good luck!

Anthony

[jloomis]

I do not see a problem using crushed rock. I also would be careful as to "when" the rock is put in the concrete interior wall. The concrete should be allowed to set a bit before putting full load on the wall is my concern. Also if the rock has fines in it, that will need to settle also. If the rock is clean it may not need that. If it has fines I would use water to settle the fines......in layers..... Rake out the rock as it is dumped in, and then wet it..... I know you may not want to do this since I am not sure where the water will go. I usually have very porous soils here, and water is gone. Now about a vapor barrier..... Maybe one is not needed? If a vapor barrier is used, the rock can or will perforate it too? Always my concern. I also usually put sand on my vapor barrier to allow the water in the concrete to migrate throughout the pour. In this case, the sand may leave in the perforations and make voids.... I have used fill sand with a 99% compaction for backfilling stem walls also. In this case the vapor barrier does not get perforated, and then the sand I put on top of the barrier does not migrate either. We then flood the sand with water to settle it also. Sometimes we have brought trucks in with a sand mix only to flow the sand out into the stem wall area. It just makes putting sand in much easier. The water does leave, and I do compact the sand with a vibratory plate sometimes. So many details..... John

wrote in message news: [e-mail]...

I want to build a garage. My site is about four feet lower than the driveway. I will build stem walls on footings to start that will reach the elevation of the existing drive and will totally enclose the new slab. I read on this site that pouring a slab on top of 5/8 crushed rock with no additional compaction of the crushed rock is an acceptable method of slab construction. I would like to spend the extra money on the crushed rock and then not have to worry about meeting any other compaction requirements if possible. I would like verification that this is an acceptable method of slab construction. Thank you for any help or other ideas or suggestions about my project. Regards, Ralph

[Tom Cular]

Snip.

Anthony is correct in stating that fill should be placed in 6-8" lifts and compacted, the type of compaction equip. depends on the soil type. If it's sandy or granular, a vibratory plate tamper will do fine, if it's a clayey, fill an impact tamper (jumping jack) will do better. The poly moisture barrier not only retards moisture from rising to the surface, but also holds the moisture in the concrete through the curing process and will help the concrete CURE not DRY.

[Tom Cular]

[phildcrow]

Ralph, What area of the country are you in? This will have a large effect on the = foundation required for your garage. In southern Louisiana, we have to be = very careful about the settling of foundations (mostly because the soil is = very much akin to the gumbo we love to eat). It may be worth spending a fe= w bucks (about $2K around here) on an engineered design, particularly given= that you want to fill 4 feet.

As far as the fill and slab, I agree with other posters here. For most are= as of the country, the most readily available and cost effective structural= fill is some sort of sand/gravel mixture. Recycled concrete is another po= pular option if you're a greenie (not a pejorative term, BTW). Deposit in = layers and compact, put down a vapor barrier and pour your slab. Layer thi= ckness and type of material can be specified by your engineer if you choose= to go that route.

Good luck and let us know what you decide. Phil