Concrete Ledge Construction

In 2003, our city installed the first prefabricated modular skatepark in San Antonio. At the time, we were not very familiar with this equipment, but within six months the modular ledge ended up looking like this…

The ledge basically crumbled due to heavy skating, and the vendor did not honor the warranty to repair the equipment. I later found out this is very common with modular companies because there is a fine line between defects and “wear and tear”.  Since this ledge was on the way out we decided to approach the city about building some concrete ones.

When we approached the city we already had raised the money, we already had a design and we had already lined up a skatepark contractor to complete the work. After the city reviewed the proposal they gave us the go ahead.

Once we had the permission to build we began to collect materials…

Materials:
¾” plywood
2x4s
2” and 3” deck screws
2 ½ Tapcon screws
Masonry bit for Tapcons and rebar
Pilot hole drill bit for drilling holes
1” or 1 ½” self tapping metal screws
3/8” Rebar
Rebar tie wire
Fill dirt or rock
Concrete (We ordered a truck)
2” angle iron or 2” HSS square tubing
Abrasive grinding wheel
Portland cement
Concrete bonding agent

These were the tools we used…

Tools:
Welding rig (for welding rebar and angle iron)
Drill driver or impact driver (for driving screws for form work)
Hammer Drill (for drilling holes in concrete for Tapcons)
Circular saw (for cutting form work or cutting angle iron and rebar with abrasive blade)
Angle grinder (for cutting angle iron and rebar with abrasive grinding wheel)
Magnesium float
Margin Trowel
Pool trowel
Wheelbarrow
Shovels

Step 1
The first step involved building the form work. Stake out the area where you want to build the ledge, and trace on the existing concrete where you want the ledge to be. Next you are going to start on your base forms. Lay down your 2x4s around the traced area (as shown in picture). Once the 2x4s are lined up to your liking use your pilot hole bit (approx. ¼”) to drill holes for the Tapcon screw. After you have drilled through the 2×4, then use a masonry bit (many of your Tapcon screw packets come with a masonry bit, if not the package will indicate which masonry bit will work) to drill into the slab. Drill the holes deep enough so the Tapcon screw will sink into the 2×4 and the slab. Make sure you sink a Tapcon screw every foot around your base, and that each screw is offset from the vertical 2x4s you are about to install. (Note: If the ledge is in a climate that will be subject to a lot of moisture or a “freeze/thaw cycle” like in the climates in many of the northern states and Canada. Then the ledge needs to be placed using a footing that extends into the ground to reach the frost line. The local building codes will dictate the depth.)

Next measure two to three inches inside the base and use a 3/8” masonry bit to drill holes for your 3/8” rebar to run vertically. Drill the holes about 5” to 6” in the slab. Make sure the holes are aligned with each other on both side of the base, so you can run your horizontal rebar pieces later on. Cut your rebar to the appropriate length, and then use a hammer to tap it into the pre-drilled holes. We bent the top of the rebar L shaped to connect with the rebar cage we were about to build.

After you have installed your base, then begin to install your ¾” plywood. Make sure the plywood is cut to the height of your ledge, and it fits within your base. Use your 2” deck screws to tack the plywood into place. Once you have installed your plywood around the base then begin to frame out the form. Notice how the form below is framed with side supports to prevent any movement when the concrete is placed.

When you cut your 2x4s make sure they are to the right height, and take into account the 1 ½” for the top plate 2×4. Notice in the picture how the plywood sticks up 2” above the 2×4 framework. This is so you can properly place your angle iron with self tapping screws once the framework is finished.

After you have cut your 2x4s place them vertically every foot around the base. It is important that you do not cover up your Tapcon screws with your vertical beams, so you can remove the forms easily. Once your vertical beams are in place then add your top beam for added support. Make sure you use the 3” screws to frame the 2x4s together. Next use your 2” screws to drill the plywood into the 2x4s. Drill from the inside of the form through the plywood and into the 2×4.

This should give you a strong enough form, so it does not move when you pour in the concrete. You do not want the form to move or bend when the concrete is poured in place.

Step 2

Once your form has been built it is time to fill the form, tie in rebar and install the angle iron. For fill you can use compacted soil or rock. We filled trash bags with soil and rock, and packed them in the middle of the form. Keep the fill below the tops of the vertical rebar, and make sure you pack the fill in tightly. You do not want loose fill.

After the fill is in place begin running rebar the width of the ledge connecting all the vertical rebar. Next run the rebar across the length of the ledge. Make sure the rebar is tied securely together with rebar tie wire. Last but not least, run rebar down the length of the mid section.

After you have tied together your rebar cage, it is time to install the angle iron. We welded rebar hooks to the inside of our angle iron, and hooked it into the existing rebar cage. As shown in step 1 we left a 2” section of plywood sticking out of the form. Line up the angle iron to this 2” section. Use self tapping screws to drill through the plywood and into the angle iron. Before screwing into the angle iron make sure it is flush with the top of the form. At the top of the form a self tapping screw needs to be drilled through the plywood into the angle iron. Place a screw about every foot. This is very important because you do not want the concrete to bulge out under the angle iron and ruin the ledge. It is easier to drill a small pilot hole before running the self tapping screw. I would suggest running the angle iron around the top of the entire ledge. When installed the angle iron needs to be welded together. As an alternative to angle iron I would also suggest using HSS square tubing. The fillet edges are not as sharp as angle iron.

Also this is a great time to paint the underside of the angle iron with some non-rusting paint.

Step 3

After your form work is sound and the rebar is tied in place, it is time for the concrete. Depending on the size of the ledge you can either mix it yourself or call in a truck. If you are going to mix it yourself check out the concrete calculator and figure out how many bags you need. When using sack crete throw in a shovel of Portland cement with every bag of concrete. This adds more cream to the mix and makes it easier to work with. If you are going to call in a truck make sure you use a 3/8” aggregate mix.

When filling the forms with concrete make sure that all of the voids are filled. Use a shovel to insure the concrete is reaching all the remote areas of the form. We began with a wet mix then ended with a dry mix to fill the form. Use a margin trowel to make sure the concrete is reaching the areas around the coping. Once the form is full tap the sides with a hammer to insure there are no voids or hollow spots in the concrete.

The next step is running a screed across the top of the ledge to remove any excess concrete. A straight 2×4 can be used to screed the concrete. Simply run it across the width of the ledge using the angle iron as your guide. Keep this excess concrete nearby in case you need to fill any voids while floating the concrete.

After a screed has been ran across the concrete, then begin to float the concrete. This is the process of getting the concrete aggregate down.

After screeding the concrete, it is then time to float the concrete pushing the aggregate away from the surface. We used a magnesium float which is commonly used in skatepark construction. After floating the surface you must wait for the concrete to cure to a point where you can work it with a steel trowel. The goal here is to burn it with a smooth concrete finish. Sometimes this process can take hours depending on the weather. On hot days the concrete will go off really quick and on cold days it will take hours before you can achieve a smooth finish. This process is where you need someone with experience in working concrete.

The final step is using a steel trowel to create a smooth finish.

Since we used angle iron and not HSS steel tubing there was no edging involved. If you are using steel tubing make sure you run an eighth inch edge on the concrete. This is on top where the steel and concrete meet.

After you are finished with the steel trowel you want it to look like this.

Step 4

After the concrete has been finished let the ledge sit for 24hrs, then it is time to remove the forms. When removing the forms it is not uncommon to find voids on the sides of your ledge. To take care of these voids use a cement, sand and bonding agent mixture. Before applying this mixture paint the void with bonding agent, and then apply the mixture using a pool trowel. Do your best to work this mixture into the ledge and cover the trouble spots.

Using a patch mix to touch up the sides of the ledge.

Once the sides of the ledge have been patched, then give it a few more hours to cure before people skate it. Take this time to paint the angle iron coping again.

The finished product! Give it some time to cure when the concrete is this green.

Conclusion

In the end, we were able to build two five foot wide and ten foot long ledges at different heights. After materials were paid for and a crew was paid the project costs $3000 dollars. When we did a price comparison for two modular ledges built out of steel with similar dimensions the cheapest we could find was approximately $5000 dollars. Concrete is always deemed as a more expensive option for constructing a skatepark, but in reality the cost per square footage is much lower on small projects like this when compared to prefabricated modular equipment.

Build it to last. Time to shred!!!!!!!!