Will powerhammer crack slab?

[R Funk]

Ice Czar makes a very good point. The subgrade under the slab is almost as key as the slab or foundation itself.

Unfortunately there is no concrete answer to whether a slab will crack under a power hammer (no pun intended).

There are too many variables. These variables include:

*Type of soil (clay, sand, high organic matter (muck) etc
*Mositure content of soil both at concrete pour and current mositure level
*Compaction of soil prior to concrete pour.
*Strenght of concrete in psi (typically 3000 to 4000psi)
*Thickness of Concrete
*Reinforcement if any
*Hammer characteristics (Total weight, hammer weight, speed etc, wood subbase etc)
and many other variables.

You could go to an engineer and ask them to design a slab to withstand the hammer operation. However the first question they will ask is what is energy and characteristics of the hammer impacts. I would not know where to go for this information. Plus the cost of engineering a slab is not likely to be money well spent on engineering may be better spent on more concrete and reinforcement.

To be safe an independent foundation would be recommended. However as you can tell by the many comments that people have had sucess and failure placing a power hammer on a slab.

If you are pouring a new slab make certain that the subgrade is undisturbed low organic matter soil. If the soil has been disturbed it must be compacted in 4" to 6" lifts of soil. Place 4"-6" of soil compact, place another layer of soil, compact and repeat. Use a plate compactor or other vibaratory compactor. A bulldozer or similar equipment is a very poor compactor and not acceptable due low ground pressure of the equipment. A 4" or 5" slab would not be adequate in most applications

When pouring concrete get at least 4000 psi concrete. DO NOT ADD WATER to make the concrete easier to work as this will decrease the strength of concrete.

Do not allow concrete to set up in direct sun if possible. Wet the slab periodically with a light spray of water as it cures over the next several days.

Add reinforcement as reinforcement is relatively cheap in the overall project cost. Use "chairs" under the reinforcement to hold the reinforcement off the ground. It does not work to lift the reinforcement as you pour as it invariably ends up on the ground at bottom of the slab. I have demoed a lot of slabs and found the reinforcement on the ground at the bottom of the slab.

If slab is more than 15' to 20' in a linear dimension, joints should be cut to reduce slab size as soon as it cuts with an abrasive wheel without damage. (3 to 6 hours after the pour). One rule with concrete is if you pour it it will crack. As concrete sets up it shrinks. This shrinkage will cause cracks in larger slabs. It is better to cut expansion joints so the cracks will be uniform and straight and not random spider webs.

Clays and high organic matter soils move more due to the impact of hammer operation. Clays are bad for transitting vibration to your neighbors. A foundation will reduce this vibration transmission

Adding a wood subbase decreases the likehood of slab cracking as the wood cushions the impact from the hammer on the floor. Likewise rubber reduces the impact as well.

Good luck in what ever you decide.

[Valentin]

my power hammer sits on wood 25x25 mm thick and the floor has no crack ... and my normal anvil has cracked the floor from the sledge hammer

I fogot to mention the hammer is in use about 10 hour/day 2-3 days/week
cioc1 - Blacksmith Picture Gallery

[Peter HIrst]

At the shop where I have been helping out, we have a 25# Little Giant sitting idle because we are wary of crqacking the slab, and cant cut through it to build a pad. We are in a historic, town-owned building, and are very careful guests. Is there some mount we can use that has a reasonable expectation of working on a conventional 4" garage floor type slab?

[Mills]

Peter, the most reasonable expectation is some solid wood some thin rubber then the hammer. The rubber allows a little movement so that the lag screws don't come out as fast. I have done this with a 50 LG and it walks around so I have also lagged it into the slab. it is better.

No heavier than a 25 is, a friend has simply caulked the floor of his shop and set the hammer (same as yours) directly to the slab. It doesn't walk or bounce or crack. It is also well tuned and stays that way.

[R Funk]

This is a question with no definitive answer.

I guess I would go back and reexamine the premise that you can't put in a seperate foundation. Is it an issue of cost? Are there feelings that a separate foundation will jeopardize the future utility of the building?

As you can see many from the postings some smiths have good luck not having a slab crack. Others cracked almost imedeately.

Factors effecting why slabs crack or don't crack include:

What is the subgrade, the dirt under the floor (sandy, clay, high organic matter) Sand is best, clay and high organic matter worse

Is the subgrade compacted?, Is it tight against the floor. Subgrade can subside leaving the slab in poor contact with the subgrade.

What is the concrete specs and condition?

What reinforcement is present if any?

I doubt if you have any above information other than the general condition of the concrete floor.

Thus it is pretty much the luck of the draw....

Strategies to minimize the potential for cracking include:
1) A Larger base. Construct a base larger than the hammer to spread out the stress more.

2) Resiliant material. Wood, rubber, and similar items will absorb the vibration. Incorporate these into the base as they will reduce the shock transmission to the floor

3) I admit this one is way out in left field. Springs, air bags and similar items can be very effective at dampening out vibration and shock if properly selected. Some are 98% plus effective which is what you need. These present 2 problems: 1)Cost, you can spend hundreds of dollars and more on these. 2) Movement of the power hammer during operation. (it may go up and down as the isolators flex)

Google "Punch Press Vibration Isolators" or "Vibration Isolators"

Let us know how you procede.

Remember there are no garuanties (sp)

[Kallsme]

under my Beche`500kg hammer its 270cm of concrete and lots of
reinforcement to hold the concrete, so it not will crack.

And also 10cm of oak wood.

My little 150kg Beche