Pouring a concrete ramp at the waters edge.

[check]

Hi, folks, I am planning on pouring a sloped concrete ramp behind my boat house, stretching down to the water so that I can take my boat in and out of the lake. (The boathouse is open at the back.)

The water level varies quite a bit over the year, and right now it is about as low as it's going to get for this year, over 3 feet lower than the summer maximum.

The temps are quite low right now, maybe 10 degrees F above freezing at night. So, I would need to order special concrete.

One question is, how thick should the pad be, and do I need rebar?

The soil is rocky-sandy, and I would of course lay down gravel first (probably 3/4" clear, because any fines would simply wash away).

Or, I could do it in the Spring when the water drops down again in March, but of course the ground would still be frozen.

Any advice from my fellow TBNers would be much appreciated!

James

 

[Gary Fowler]

1. As long as you are above freezing, you don't need anything special on the concrete.
2. You really shouldn't try to pour over frozen ground so the best time to do it is now.
3. Yes you likely need at least some rebar or minimum of 4x4x 1/8" reinforcement mesh wire. I would put a #3 rebar 3'x3' grid down and then lay the mesh wire on it. Be sure to put some concrete blocks under the rebar to hold it up at least 1 1/2"-2" off the ground. You want it pretty much centered on your slab, which leads to the next issue.
4. A 4" slab is pretty much the minimum for driving vehicles over. A good rock foundation for the slab would be preferable especially in water areas like boat ramps. Also you want to really thicken the end and edges to prevent wash out. I would dig down at least 12"deep x12"wide on the perimeter and preferably 18". This will help prevent the water from washing the dirt and gravel out from under the slab as the water level rises and falls.
5. Don't worry if a little water seeps into your forms when you dig down at the water level. The concrete will force the water back out, Just place it from the upper end toward the water and the concrete will flow under the water and lift it right out.

 

[ajmaxham]

I have had good results with rebar 2' x 2' grid, 6'' thick, 5000 psi concrete and pour as stiff as you can. I poured a 12' x 12' slab 4'' thick with wire mesh and it has cracked and the cracks have opened up.

 

[Sprinklerman]

concrete will harden under water

 

[Wagtail]

concrete will harden under water

Yep. Specifically 'Roman concrete'. 'Portland' concrete will give you about a 50 year lifespan but the Roman formula will give you centuries!

"The secret to Roman concrete lies in its unique mineral formulation and production technique. As the researchers explain in a press release outlining their findings, “The Romans made concrete by mixing lime and volcanic rock. For underwater structures, lime and volcanic ash were mixed to form mortar, and this mortar and volcanic tuff were packed into wooden forms. The seawater instantly triggered a hot chemical reaction. The lime was hydrated—incorporating water molecules into its structure—and reacted with the ash to cement the whole mixture together.”
The Portland cement formula crucially lacks the lyme and volcanic ash mixture. As a result, it doesn’t bind quite as well when compared with the Roman concrete, researchers found. It is this inferior binding property that explains why structures made of Portland cement tend to weaken and crack after a few decades of use,"

 

[Streetcar]

How heavy is your boat? This determines slab thickness
Are there regulations for working in the water
Do not place concrete on frozen ground
Keep concrete above freezing point for seven days after pouring and you will be ok
Rebar in the middle of the slab adds very little strength, but will hold cracks together. For strength place rebar near the bottom of the slab but with minimum of 2" cover
Will trailer need to go off concrete into lake or will concrete extend into lake

 

[check]

How heavy is your boat? This determines slab thickness
Are there regulations for working in the water
Do not place concrete on frozen ground
Keep concrete above freezing point for seven days after pouring and you will be ok
Rebar in the middle of the slab adds very little strength, but will hold cracks together. For strength place rebar near the bottom of the slab but with minimum of 2" cover
Will trailer need to go off concrete into lake or will concrete extend into lake

Boat and Motor are about 3500 lbs. And I have access to a thick tarp to keep the frost off, whilst the ground will of course still have enough heat to keep the underside from freezing. Also, if I pour now, the lake will be up 2-3' when I put the boat in or take it out, and there will be a fairly solid bottom beyond the end of the ramp in any case.

 

[check]

I have had good results with rebar 2' x 2' grid, 6'' thick, 5000 psi concrete and pour as stiff as you can. I poured a 12' x 12' slab 4'' thick with wire mesh and it has cracked and the cracks have opened up.

I agree. I DON'T like wire mesh. Rebar all the way!

 

[check]

1. As long as you are above freezing, you don't need anything special on the concrete.
2. You really shouldn't try to pour over frozen ground so the best time to do it is now.
3. Yes you likely need at least some rebar or minimum of 4x4x 1/8" reinforcement mesh wire. I would put a #3 rebar 3'x3' grid down and then lay the mesh wire on it. Be sure to put some concrete blocks under the rebar to hold it up at least 1 1/2"-2" off the ground. You want it pretty much centered on your slab, which leads to the next issue.
4. A 4" slab is pretty much the minimum for driving vehicles over. A good rock foundation for the slab would be preferable especially in water areas like boat ramps. Also you want to really thicken the end and edges to prevent wash out. I would dig down at least 12"deep x12"wide on the perimeter and preferably 18". This will help prevent the water from washing the dirt and gravel out from under the slab as the water level rises and falls.
5. Don't worry if a little water seeps into your forms when you dig down at the water level. The concrete will force the water back out, Just place it from the upper end toward the water and the concrete will flow under the water and lift it right out.

Good point #4, Gary. I will in fact be piling rocks on either side to prevent wash out.

 

[EddieWalker]

The way I've seen it done, is they pour the slab, then push it into the water as far as the can after it cures, then they pour another slab next to that one. This was with a full sized 4x4 backhoe. I don't know what it weighed or how much power it takes to push a slab down hill into the water, but it sure looked pretty easy when I saw it being done.

 

[Renze]

My friend in Finland uses hot water and quick curing compound to pour concrete in the winter. And boy it does freeze over there !

 

[Streetcar]

The way I've seen it done, is they pour the slab, then push it into the water as far as the can after it cures, then they pour another slab next to that one. This was with a full sized 4x4 backhoe. I don't know what it weighed or how much power it takes to push a slab down hill into the water, but it sure looked pretty easy when I saw it being done.

You would need a heavily reinforced slab because you are not likely to have good bearing from soil on slab.
What kind of soil was at the lake and how big was the slab?

 

[Streetcar]

I forgot to ask how much wave action at the lake.
I would pour 7" slab with reinforcement 12" on center

 

[dstig1]

You would need a heavily reinforced slab because you are not likely to have good bearing from soil on slab.
What kind of soil was at the lake and how big was the slab?

Yeah - I agree with this. Steel is cheap for that size slab. Go overboard given the questionable bearing soils in a lake (plus wave erosion). 1/2" rebar at 12" centers or more. It is best to keep it below the center line of the slab as steel in a slab works in tension, really, so you want it on the tensile side of the loading (the bottom of the slab). But it needs to be fully embedded in cement too. That is hard to do in a 4" slab. If you went thicker, you could do it more easily, but of course that costs more. Mostly it is just material costs as the placing and finishing work do not change much from 4" to 6", for example. Good idea to put rock around the edges for erosion.

 

[oosik]

The only concrete boat ramps I've seen around here were constructed by DNR(Dept of Natural Resources). They are not a "pour in place" operation. They first grade the ramp area and lay down a layer ( looked like 6" to 8") of crushed basaltic lava. The lava rock was what I would call 2" screened. Then here comes the precast concrete ramp pieces. They were 6' x 20' x 8". They were lifted from a large semi truck and set using a rather large crawler backhoe. Each piece had cast in place tie lugs. So as each piece was set, it was linked to the previous one with a large bolt thru the tie lugs. There were three tie lugs on each side of each piece.

The result looked like a large grey track off a crawler tractor with a 6" gap between each piece. The supervisor said one of the advantages is if one of the pieces became damaged - it was simple to pull the bolts and replace that piece. The other neat thing was the crawler backhoe just walked out the ramp to set the next piece out in the water. The limit to this operation was - how far out in the water the crawler backhoe could deposit and level the initial crushed basaltic lava base.

 

[ajmaxham]

In a 6'' slab should the rebar be at the center line. The force of frost will come from the bottom side of the slab. The weight of truck and boat will come from the top. Should the rebar that will be under water for part of the year be a corrosion resistant epoxy coated rebar?

 

[check]

I forgot to ask how much wave action at the lake. I would pour 7" slab with reinforcement 12" on center

Not much wave action at all. The ramp will be at the "back door" of a 50 foot long boathouse.

 

[check]

The way I've seen it done, is they pour the slab, then push it into the water as far as the can after it cures, then they pour another slab next to that one. This was with a full sized 4x4 backhoe. I don't know what it weighed or how much power it takes to push a slab down hill into the water, but it sure looked pretty easy when I saw it being done.

Interesting idea, but it won't work in my application. As I mentioned earlier, the lake rises and falls to even as much is 4 feet over the course of the season. Right now the lake is down 31" from the average Summer level. So, if I poured now, the slab will be under plenty of water when I put the boat in and take it out. The slope of the slab will be a drop of about maybe 2 feet over a total length of 30 feet. And then it will be Rocky/Sandy at the end of the ramp. Should be no problem.

 

[deere36]

We poured one like EddieWalker said here in Mississippi 20'x12' slab wire & rebar pushed in with mini-excavator, then poured a second slab to match. Left about 3 feet wire & rebar out of first slab to connect second slab for stability, this was in 2007 & have launched full size bass boats with ease.

 

[Streetcar]

In a 6'' slab should the rebar be at the center line. The force of frost will come from the bottom side of the slab. The weight of truck and boat will come from the top. Should the rebar that will be under water for part of the year be a corrosion resistant epoxy coated rebar?

Rebar in the center of the slab will only help to hold the cracked concrete from spreading. It adds very little to the strength of the slab.
For fresh water I would not worry about epoxy rebar. Almost every structure I have built in fresh water used black steel. In brackish or salt water use epoxy steel