Remove Wood from Concrete

Wood and concrete may not seem like two materials that can go together; but they actually do. Concrete is poured into a wood mold, called a “form,” it can have wood expansion joints in it, and it’s not uncommon to find wood embedded into it, either in the form of fence posts or roof support posts on a patio. Even a wood-framed house is built onto a concrete foundation or slab, requiring an interface between the two.

But just because wood and concrete are used together, doesn’t mean they always have a great relationship. The concrete can damage wood that is used for forms and wood fence posts or structural posts that are set into concrete can still rot (usually right above the concrete) and when they do, it can be a real problem to deal with. But deal with it we must, as it only gets worse when left alone.

Wood Forms with Concrete

Whenever concrete is poured, some sort of mold or form is used. In some cases, such as the building of bridges where a decorative surface is desired, the forms are made of steel. But in most cases, concrete forms are made of some type of wood; either dimensional lumber or plywood products.

A special plywood product has been developed specifically for use in making concrete forms, called Plyform. Another excellent plywood product for forms is phenolic plywood. But in most cases, especially with smaller contractors and do-it-yourselfers, standard construction-grade softwood plywood is used. The use of B-C grade softwood plywood provides one smooth surface, to be placed against the concrete, so as to facilitate a smooth concrete surface. If a C or D grade surface is put up against the concrete, the wet concrete will run into the knot and check in the face veneer, molding those into the surface of the concrete and making it harder to remove the sheet once the concrete is set.

Any plywood product or dimensional lumber which is used for concrete forms needs to be thoroughly oiled, before the concrete is poured. While concrete is a highly viscous liquid, the water leaching out of it, which contains some of the lime, is not much more viscous than water. As such, it can soak into the surface of the wood, permanently bonding the wood to the concrete when the water evaporates. Since oil and water don’t mix, the oil seals off the wood from the concrete soaking into it, making it possible to remove the wood forms once the concrete has set enough.

That raises the question of when the concrete has set enough for the wood forms to be removed. Contrary to common belief, concrete doesn’t dry, but rather sets by a chemical reaction. Water is used as a catalyst to start this chemical reaction; but the water itself isn’t used up. While it does dry eventually, how fast it dries has nothing to do with the reaction itself, once that reaction has started.

In fact, it is necessary to keep freshly poured concrete wet for the first 28 days after pouring, long after the initial water would have evaporated. This helps to keep the reaction going, so that the concrete will reach its maximum possible strength. It will have set enough to remove the forms after 24 to 48 hours and the concrete will have reached its maximum strength after 7 days.

Wood as Expansion Joints in Concrete

Most materials expand when they are heated and contract when they are cooled. Concrete is no exception to this rule; therefore it is necessary to make provision for it whenever large amounts of concrete are used. If expansion joints are not used in concrete slabs that are poured outdoors, there is a chance of the concrete cracking from expansion and contraction. The rule of thumb is that expansion joints should not be any farther apart than two to three times (in feet) the width of the concrete (in inches). So, a 30” wide sidewalk should have expansion joints every 60 to 90 feet.

A variety of different materials have been used through the years for these expansion joints, including wood. As the concrete is much more durable than the wood, there can come a time when the wood has rotted, while the concrete is still good. When that happens, the expansion joint should be replaced.

Flexible vinyl expansion joint filler is usually used as a replacement for the old wood expansion joint. It is readily available and fairly easy to install. The challenging part of the project isn’t installing the new expansion joint, but rather removing the old one. This is essentially a chiseling operation, but a chisel is not used. Any contact the chisel would make with the concrete would be damaging to the chisel, so bent pry bars, the kind that look like a bent screwdriver are used. Since we’re dealing with rotted wood, a sharp edge is not needed; simply pound the edge into the wood expansion joint and then pry it, breaking strips of wood out of the seam.

Another tool that is used is called a “hookaroon,” a tool that comes from the logging industry. In appearance it looks like something a firefighter might use as a breaching tool, consisting of a cast hook on an axe handle. It can be slid down the seam until it runs into the wood, where it can then be used to pry upwards on the wood, breaking off chunks of it.

Once the wood has been removed from the expansion joint and the seam cleaned of debris, the new flexible vinyl expansion joint can be placed in the joint and hammered down flush with a rubber mallet. If the ends are exposed, it is recommended to backfill with sand or gravel.

Wood Fence Posts or Columns

A bigger challenge is dealing with rotted wood fence posts or patio roof support columns, set into concrete. While concrete does provide an effective moist barrier, helping to keep moisture in the ground from getting into the wood from below, it can’t do a thing to protect the wood above the concrete itself. The most likely place for wood fence posts to rot is the few inches just above the concrete anchor, necessitating a replacement of the fence post. Even pressure-treated lumber will eventually rot; the chemicals injected into the wood can only protect it for so long.

There are a couple of different ways of going about this repair, depending on the degree of rot, how big and heavy the concrete base is. Ultimately it depends more on the decision of the person doing the work. All of the methods require considerable physical exertion, but in some cases one might seem less than the other.

Removing the Post and Base

One possibility is to remove the post and concrete base together, replacing them both. This is an ideal method to use when the post is still in good shape and there isn’t a wide concrete base. As concrete weighs 150 pounds per cubic foot, it doesn’t take a lot of concrete to make the base extremely heavy.

A typical post hole, dug by hand, will be six inches in diameter. Since two feet of fence post is normally buried underground, that hole will have roughly 33 pounds of concrete in it. That’s not a lot to remove and can be removed fairly easily. But if the hole was originally dug using a power auger, the hole might be considerably larger, making for a much heavier chunk of concrete to remove.

The key to removing the lighter, smaller concrete base is leverage. As long as the fence post is still intact, it can be attached to, allowing the use of a lever. If however, the fence post has rotted through, this method is impossible. In that case, if the concrete base still must be removed, it will have to be dug out.

Obviously the first step is to remove any backfill that was placed in the hole, covering the concrete. This will allow an opportunity to explore the size of the hole and just how wide the concrete base is. From there, the approximate weight of that base can be calculated by:

  1. Divide the diameter of the concrete in half to get the radius. 
  2. Multiply the radius by itself, and then multiply it by 3.14, otherwise known as pi.
  3. Multiply that number by 24 to account for the depth of the hole. This should give the total cubic inches of concrete and wood. 
  4. Subtract 294 from that number if the post is 4”x 4” or 726 if the post is 6”x 6”. This will give you the approximate number of cubic inches of concrete in the hole. 
  5. Divide that number by 1,728, the total number of cubic inches in a one foot cube. The resulting number will probably be a decimal, less than 1. 
  6. Multiply that number by 150, the weight of the concrete, to find the weight, in pounds, of the concrete base. 

To gain the necessary leverage for pulling out the concrete base, a lever will need to be attached to the base of the column, as close to the ground as reasonably possible. This can be done by nailing 2”x 4” cleats to the sides of the column, but the best way is to drill through a long 2”x 4” (at least 8’ long) and use a long lag screw or bolt to go through the 2”x 4” into the fence post or even all the way through it. Ideally, if two long 2”x 4”s are available, one can be attached to each side.

Place a sizable stone, cinder block or stack of wood less than a foot in front of the post to act as a fulcrum for the lever. Then lean on the lever, applying pressure at the end farthest from the post. The farther that you are and the more weight applied to the lever, the easier it will be to pull the concrete out of the ground. Once removed, a new post can be set in place with fresh concrete poured to fill in the hole.

wooden concrete forms
Wooden concrete forms, David Cedrone

Removing the Fence Post from the Concrete

Any time the post is set into a large amount of concrete, it will make more sense to remove the wood from the concrete in order to replace it. This might be in the case of a supporting column set into a concrete patio or a fence post that is set into the edge of a driveway slab. Too much concrete would have to be removed in these cases and the resulting patching of the concrete would be obvious.

Before doing anything, remove any soil overburden, exposing the top of the concrete. This method is obviously easier if the wood post is rotted and has already broken off. Posts will usually break off close to the ground and the closer it breaks the better. If it isn’t close to the ground, then it might be worthwhile cutting the post off close to the ground with a reciprocating saw, before proceeding. The more than can be cut off with the saw, the less has to be broken and chipped out of the hole.

The tool to use for this is a digging tool that looks like a long pole with a cold chisel attached at one end. While a sharp chisel might cut through the wood easier, if that chisel makes contact with the concrete, it will damage the chisel. So, if it turns out that a chisel is needed, it might make more sense to buy a cheap chisel for the project, with the idea that it can be thrown away, if it becomes damaged.

The digging tool can be driven into the exposed top of the rotted fence post, by weight alone; lifting the tool and dropping the chisel end onto the wood. As this is done repeatedly, it should break up the wood, allowing chunks and chips to be removed. bending the tool from side to side, once driven into the wood, should help with the breakup process.

Please note that this is a time-consuming process, which will require considerable patience, as the wood comes out in chips. But it is possible. Once the wood is out, a new post should fit in the hole, although it might require a bit of trimming to fit. Fill any extra space around the new post and above it, up to ground level, with concrete to seal the beam in place and keep water from getting into the gap between the concrete and the post.

A Cheat that Works

There are various types of post bases which are used in cases where a homeowner doesn’t want to drill a hole in the ground for a wood post or where it would be extremely difficult to make that hole. One such case would be if there is a pre-existing concrete slab and the homeowner wants to put an awning over that slab to use it as a shade patio. In that case, rather than digging a hole in the ground for the post to be set into concrete, one of these bases would be mounted to the surface of the concrete. It’s not as secure; but functional.

These same bases can be mounted to the concrete anchor for a post, once the old post has been cut off flush with the concrete. The problem with this though is that it isn’t as stable or strong as sinking the bottom of that post two feet into the ground and surrounding it with concrete. So while it would work fine for an awning, it would not be sturdy enough for a fence post.

One option would be to provide additional support for the fence post, such as a 2”x 4” running off at an angle to the ground. This is not attractive and runs the risk of being a trip hazard, but might be a possibility in a hidden area where nobody goes. Another option is to use the type of metal base that has a tube running down from it, intended to be driven into the ground. A hole can be bored in the part of the fence post that is set into the concrete and the tube from this type of base runs into it. As long as the wood set into the concrete is not badly decayed, it will provide adequate support. That support can be improved by pouring a concrete apron around the top, up to slightly above ground level.

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