How To Build Concrete Wall Forms With Plywood

Introduction

Building concrete walls with plywood forms is a great way to create durable, long-lasting structures. With the right planning and materials, you can create professional-looking concrete walls for garages, basements, foundations, retaining walls, and more.

This guide will walk through the entire process of building concrete wall forms with plywood, from constructing the form panels to pouring the concrete and stripping the forms when done. We’ll cover how to determine the right size and layout for your wall project, select suitable plywood and other supplies, assemble sturdy formwork, properly install rebar reinforcement, apply release agents, and correctly handle the concrete pour.

Following the steps in this guide will allow you to safely construct high-quality concrete walls that live up to building code standards. Once your forms are built and the concrete has cured, you’ll be left with a super-strong concrete wall perfect for both structural and decorative purposes. With careful attention to detail and proper techniques, even first-time DIYers can achieve professional-looking results. Read on to learn how to efficiently build safe, long-lasting plywood concrete forms.

Materials Needed

To build concrete forms from plywood, you’ll need the following materials:

  • Plywood sheets – Use new, high-quality plywood that is at least 5/8″ thick. Thicker 3/4″ plywood is preferable for sturdier forms. Ensure the plywood does not have knots, splits, warping or other defects.
  • Lumber – 2×4 lumber is commonly used to frame the plywood panels. Use straight, defect-free boards.
  • Nails or screws – Galvanized nails or exterior wood screws are best. The length will depend on the plywood thickness, but 1-1/2″ to 3″ lengths are typical.
  • Wall ties – Metal snap ties specially designed for concrete wall forms are required to hold the two sides of the form together. The ties must be adjustable.
  • Release agent – A release agent or form oil is brushed onto the plywood so the cured concrete does not stick to the wood. Commonly used options include diesel fuel, petroleum-based oils, or commercial release agents.
  • Concrete and reinforcing steel – Obviously concrete and rebar are needed too. We’ll cover those details later.

The specific materials required depends on the wall size and design requirements. But having high quality plywood, lumber, hardware, ties and release agent is essential.

Sizing and Cutting the Plywood

Once you have determined the dimensions of the concrete walls you will be building forms for, you can start sizing and cutting the plywood sheets. Plywood for concrete forms is typically sold in 4×8 sheets in various thickness such as 3/4 inch.

First, measure the wall length and height to determine how many full sheets you will need. Mark the measurements on the plywood sheets and use a circular saw to cut them to size. Make sure to cut both the inside and outside form panels to the same dimensions.

For full wall coverage, you may need multiple plywood sheets. Butt joint the edges, keeping the inside smooth. Secure adjoining sheets together with screws or nails to prevent concrete leaks. Use plywood splice plates between joints for added strength.

Cut any openings in the plywood, such as for windows or doors. These can be cut with a jigsaw. Make sure edges are smooth.

Once all plywood pieces are cut to size, label each panel to identify their location. This will make assembly quicker.

Assembling the Form Panels

To assemble the form panels, start by laying the plywood sheets flat on a level surface. Position the sheets side by side with about 1/4 inch gaps between them to account for wood expansion.

Cut 2×4 lumber braces to length so they extend across at least two sheets of plywood. Place these horizontally across the joint between sheets about 2 feet apart. Use exterior wood screws, at least 2 inches long, to fasten the braces firmly to the plywood.

It’s important that the braces are perfectly straight and level so your wall forms end up square and plumb. Use a carpenter’s level frequently when attaching the braces.

Continue joining plywood sheets together into panels in this way until you have the desired length for your forms. Make the panels in easy-to-manage sizes, such as 4 feet wide by 8 feet long. Leaving the braces long past the edges of the plywood will allow you to connect panels together later on.

When assembled properly, these plywood form panels will create a sturdy temporary structure to build against when you pour concrete walls. The braces hold the sheeting firmly together and resist bowing pressure from wet concrete.

Installing Wall Ties

Wall ties are essential for connecting the two sides of the concrete form and holding them together. They counteract the pressure exerted by the wet concrete so the forms don’t bow outward.

There are several types of ties available, but the most common are metal snap ties. These have a metal rod that’s bent in a zig zag shape in the center. The ends are flattened to create a small panel on each side.

To install snap ties:

  • Space the ties evenly apart, usually 12-24 inches depending on the thickness of the wall. Consult your plans for the recommended spacing.
  • Make sure the ties line up vertically on each side.
  • Hammer the ties into the plywood so the metal panels are embedded in the wood. Make sure they are snug and won’t pull out.
  • Bend any protruding ends over so they lie flat.

If using an alternative style of tie, follow the manufacturer’s instructions for proper installation. The main goal is to securely connect both sides of the form.

Once the ties are installed, double check their placement and spacing. They need to withstand a lot of outward pressure when the wet concrete is poured, so the forms must be tightly tied together.

Setting Up the Forms

Once the plywood panels are assembled, it’s time to set them up to create the form for the concrete wall. Proper setup of the forms is critical for getting a smooth, straight wall.

First, stand up the form panels and position them where the wall will be located. Make sure the panels are plumb and aligned correctly. Use braces on the exterior side of the panels to hold them in place. The braces should be strong 2×4 or 2×6 boards, cut to size and nailed securely to the panels. They keep the panels from bowing outward when the concrete is poured.

In addition to the exterior bracing, you’ll also need to place studs vertically between the panels to prevent them from pushing inwards. Cut 2×4 or 2×6 studs and toe-nail them into the panels every 2-3 feet. They should span the entire height of the wall form.

Check that all braces and studs are secure by pushing against the panels before pouring concrete. The form needs to be completely stable to withstand the pressure. Make any necessary adjustments to ensure the panels are plumb.

Finally, double check that the tops of the panels are level with each other. Use a long level or straight board to compare heights. Shim up any low spots as needed so the concrete will be poured to a consistent elevation. The forms are now ready for the rebar and concrete!

Inserting Reinforcing Bars

Reinforcing bars, or rebar, add strength and reinforce the concrete in a wall. Rebar helps resist tensile stress and prevent cracking as the concrete cures and shrinks.

Before pouring concrete into the forms, rebar needs to be inserted. The size and spacing of rebar is specified by building codes and engineering requirements. Typically #4 or #5 rebar is used spaced 12-24 inches apart horizontally and vertically.

To insert rebar into the forms:

  • Measure and cut rebar to the correct lengths with cinches or wire cutters. Cut rebar so there is 3-6 inches extra extending out of the tops of the forms. This will tie into the next section of wall.
  • Use rebar ties, clamps or wire to attach rebar to interior form ties. This helps hold the rebar in place suspended in the middle of the wall cavity.
  • Use rebar supports or dobies as needed to prevent sagging, especially for horizontal lengths over 5 feet long.
  • Follow the rebar grid spacing required. Use zip ties or wire to attach overlap joints where vertical and horizontal rebar cross.
  • For openings, frame and tie rebar around openings as well as above and below. Add extra rebar around openings to reinforce.
  • Double check rebar spacing and cover requirements before pouring. Rebar needs to be centered in the wall with even cover on both sides.

Properly inserting rebar according to the engineered plans is crucial for structural integrity. Taking the time to correctly place rebar helps strengthen the concrete and control cracking as it cures. The rebar reinforcing system ties the entire wall together.

Applying Release Agent

Before pouring the concrete, it’s crucial to coat the inside of the forms with a release agent. This will prevent the concrete from bonding to the plywood forms. There are a few options for release agents:

  • Form Oil – A lightweight oil that is brushed or sprayed directly onto the forms. Form oil provides good concrete release and is inexpensive. However, it can leave an oily residue on the finished concrete surface that needs cleaning.
  • Form Coating – A thick liquid that is roller-applied to the forms. Form coatings provide excellent release and leave no residue. They can be more expensive than oils but are cost-effective for frequent use.
  • Form Release Film – A thin plastic sheet applied to the form faces. The film prevents bonding and tears away after the forms are removed. It provides clean, easy release but can be more costly than other options.

When applying a release agent, fully coat all form faces that will touch the concrete. Be sure to reach inside corners and detailing. Apply an even layer and avoid puddles or thin spots. Let the release agent dry per manufacturer instructions before placing concrete. Too much release agent can cause blemishing on the final surface. Test on scrap wood first if uncertain about application. Proper form release is cheap insurance against stuck forms and costly repairs later on.

Pouring and Finishing Concrete

When the forms are completely set up and the reinforcement bars are in place, you are ready to pour the concrete. This is one of the most important steps, as proper pouring and finishing will ensure a smooth, durable concrete wall.

  • Pour the concrete slowly and steadily into the forms to avoid air pockets and voids. Use a concrete bucket or wheelbarrow to carefully distribute the concrete evenly.
  • As you pour, use a concrete vibrator to settle the concrete and remove any trapped air bubbles. Insert the vibrator head into the concrete in a series of verticals spots about a foot apart. Do not over-vibrate or move the vibrator side to side as this can cause segregation.
  • After vibration, insert a screed board on top of the forms and drag it back and forth to level and smooth the surface. Use a magnesium float to hand float the concrete and create a uniform finish. Smooth any ridges or imperfections.
  • Apply a final finish texture with a broom or trowel depending on the desired appearance. A broomed finish will create a slip resistant, decorative texture. A steel trowel finish creates an ultra smooth surface.
  • Avoid walking on the finished surface to prevent damage. Let concrete cure undisturbed for at least 24-48 hours before stripping forms. Proper curing is critical for strength and durability.

Stripping the Forms

After the concrete has cured sufficiently, it’s time to strip the forms to reveal the finished concrete wall. This is a crucial stage, as you need to remove the forms without damaging the wall.

The concrete should be properly cured before removing the forms. As a general rule, forms can usually be removed after 1-2 days for standard concrete mixes. Check the concrete manufacturer’s recommendations to be sure. Trying to strip the forms too early can damage the surface of the green concrete.

When ready to strip, start by removing any wedges, braces, or clamps. Then begin removing the panels slowly and carefully. Pry against the studs or joists, not the concrete itself. Tap forms lightly to loosen them rather than forcing or prying aggressively.

As you take off panels, check the concrete for any damage such as chips or spalling. Minor issues can often be repaired with patching compounds. For larger defects, consult a concrete professional for the best repair method.

Finally, inspect the concrete thoroughly once all forms are stripped. Look for any honeycombing, cold joints, voids, or other flaws that will need addressing. Leaving forms on for longer allows the concrete to gain strength and reduces form stripping damage. But it pays to take care when removing forms to reveal a smooth finished concrete wall.

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