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.