Forming a Cast in Place Frame and Grate Concrete Trench Drain
Concrete trench drains provide an effective way to manage surface water and minimize erosion. Although poured in place trench drains are less expensive, hand-formed concrete trench drains are flexible in design and can be customized to meet specific requirements. In this article, we will discuss the basics of how to form a concrete trench drain.
It All Starts With Design
To begin with, the design of the concrete trench drain must be determined. The frame and grate must be selected based on the design criteria. Failure to do this before forming the drain can result in difficulty finding a grate that fits the trench dimensions. It can also affect the required load rating because of an improperly sized trench seat. An embedded frame is recommended for a hand-formed concrete trench drain to provide a proper ledge for the grate and support the upper edge of the trench drain.
In order to form a concrete trench drain, it is important to understand the design criteria, required materials and the necessary steps for constructing the drain. While some may choose to create a poured in place concrete trench drain to save costs, it should be noted that this method is labor intensive and requires a great deal of skill to be done properly. A hand formed concrete trench drain is more flexible in terms of design, allowing for easy adjustments in width, depth, slope and features. However, it is also labor intensive and requires significant skill to get right.
Choose The Right Dimensions
The first step to forming a concrete trench drain is to select the frame and grate based on design criteria before forming the drain. This ensures that the grate will fit properly and is not too narrow or too large for the drain opening. It is also important to determine the width of the drain, which is typically the width of the grate plus 1/4 inch, to ensure that the grate has a proper ledge to sit on.
Once the frame and grate have been selected, the width of the trench drain must be determined. The width of the grate seat should be the grate width plus 1/4″, and this should govern the width of the trench drain form work. The inside face of the trench drain frame should be 1/4″ wider than the grate. An opening that is too narrow will make it difficult to remove the trench grates for cleaning, while an opening that is too large will cause the grates to slide, look unsightly or fall into the opening.
To hand-form a concrete trench drain, begin by excavating the ditch to the depth of the drain plus the thickness of the concrete. Install any required rebar, including U-shaped rebar for a poured in place concrete trench drain to hold the trench drain walls and footer together. Next, set the elevation of the top of the footer concrete, which will become the flow line of the concrete channel drain. The footer of a hand-formed concrete channel drain should be 6″ wider on each side than the total width of the trench walls to allow the trench wall forms to be installed on top of the concrete footer.
When constructing a hand formed concrete trench drain, the ditch should be excavated to the depth of the drain plus the thickness of the concrete. The required rebar should then be installed, with U-shaped rebar being recommended for poured in place concrete trench drains. The elevation of the top of the footer concrete should be set, which will be the flow line of the concrete channel drain. It is important to note that the footer of a hand formed concrete channel drain is typically 6 inches wider on each side than the total width of the trench walls.
After pouring the concrete trench drain footer, you may need to install a water stop if the trench drain is deep or inside a building. Wet setting PVC water stop into the concrete before it cures is possible, while a swell stop can be installed after the concrete firms. The inside of the concrete trench drain needs to be troweled smooth to improve flow and reduce debris, while the outside should be flat enough to place forms on but does not have to be perfectly smooth.
Next, form the inside of the concrete trench drain by setting two forms with the smooth side out toward the rebar cage. Cross brace these forms from one to the other across the inside of the trench drain to prevent any racking of the forms during the concrete placement. This is a critical step because any racking of the form work will make the trench drain grating not seat properly. Finally, set the top of the trench drain frames at the finish elevation by nailing a strip of wood to the forms at the prescribed elevation.
Once the trench drain footer has been poured, the inside portion of the concrete should be troweled smooth, as it will be the flow line of the hand formed concrete trench drain. The outside should be flat enough to place forms on, but does not need to be perfectly smooth. The inside of the concrete trench drain should then be formed with two forms set with the smooth side facing outwards. These forms should be nailed or screwed to the bottom of the trench drain footer and then braced from one to the other across the inside of the trench drain. The top of the trench drain frames should then be set at the finish elevation using a strip of wood the thickness of the grate and the width of the frame, which should be nailed to the forms at the prescribed elevation.
Step by Step:
A concrete trench drain is a popular choice for managing water flow on construction sites and in residential areas. In this guide, we will provide you with the detailed steps to form a concrete trench drain, which can be a time-intensive process but offers great flexibility in terms of design.
- Determine the Design Criteria Before you start forming the drain, you need to select the frame and trench drain grate based on your design criteria. This is critical because if the dimensions of the drain are not compatible with the grate size, you may need to pay for a custom-fabricated trench drain grate later, which will be costly. Additionally, it may be difficult to reach the required load rating if the trench seat size is improperly sized.
- Choose the Frame A hand-formed concrete trench drain can have a formed concrete grate seat or an embedded frame, and we recommend an embedded frame. Without a frame, the grates sit directly on the concrete, which can cause excessive stress on the concrete, leading to concrete ledge shear over time. Moreover, the upper lip next to the grate is unsupported, which leaves the upper edge of the trench drain susceptible to spalling and shear failures. A good trench embed frame will provide the grates with a proper ledge to sit on, ensuring the hand-formed concrete trench drain has a long life.
- Determine the Width After selecting the frame and grate, you must determine the width to form the drain. The grate seat is the ledge where the grate sits to prevent it from falling into the trench. This ledge must be the correct width and co-planar (meaning at the same elevation). The width of the grate seat is usually the grate width plus 1/4". This should govern the width of your trench drain form work. The forms will be set such that the inside face of the trench drain frame is 1/4" wider than the grate. If the opening is too narrow, the trench grates will not fit in the opening and will be challenging to remove for cleaning. Conversely, if the opening is too large, the grates will slide, look unsightly, or fall into the opening.
- Excavate the Trench Once you have determined the design criteria, frame, grate, and width, the next step is to excavate the ditch to the depth of the drain plus the thickness of the concrete.
- Install Rebar The next step is to install any required rebar. For a poured-in-place concrete trench drain, you should have U-shaped rebar to hold the trench drain walls and footer together.
- Set the Elevation The elevation of the top of the footer concrete needs to be set next. This will end up being the flow line of the concrete channel drain. This can be done with a laser on a slope or forming lumber can be placed outside the rebar cage and set on the correct slope to show you at what height to pour the concrete. Note that the footer of a hand-formed concrete channel drain is typically 6" wider on each side than the total width of the trench walls. This is to allow the trench wall forms to be installed on top of the concrete footer.
- Pour the Footer Once the elevation has been set, the next step is to pour the concrete trench drain footer. If the trench drain is deep or inside a building, you will need to install a water stop. If you choose to use a PVC water stop, you can wet set it into the concrete before it cures. If you are using a swell stop, it can be installed after the concrete firms. The inside portion of the concrete needs to be troweled smooth because the inside will be the flow line of the hand-formed concrete trench drain and should be very smooth to improve flow and reduce debris.
Forming a concrete trench drain involves a number of important steps to ensure that the drain is constructed properly and is able to perform effectively. Careful attention to design criteria, materials selection and proper construction techniques can help to ensure that a hand formed concrete trench drain provides a long and effective service life.
A hand-formed concrete trench drains require skill and time to get it right but offer flexibility in design. The above steps provide a basic understanding of how to form a concrete trench drain, but it is recommended that those without the necessary experience should consider using precast trench drains or trench drain forming systems.