A Grade Beam is composed of concrete and steel. They are designed and constructed for the purpose of acting as horizontal ties between footings. The concrete grade beams are constructed utilizing the earth as the formwork. The construction of Grade Beams is a four (4) step process.
Step 1: Constructing the Grade Beam Formwork
Once the house is staked out, and the location of the walls are identified, the trenches for the grade beams are dug out of the earth. Using a backhoe, the trenches are dug to the dimensions as designated on the Construction Drawings. The use of the backhoe will allow for smooth and level elevations on the side and bottom of the trench. Should a certain area require further excavation, hand digging is usually involved.
Should the grade beam extend above the ground plane, formwork is installed to the appropriate height.
Once the trenches are dug, insulation board is placed in the trench, on the interior side of the grade beam. The insulation board is designed to provide a thermal break from the exterior to the underside of the slab-on-grade. It is important to not break or fragmentize the insulation board during the installation and when pouring the grade beam.
Step 2: Installation of Steel Reinforcing
Once the trenches are complete with the insulation and any formwork, the next step is the installation of the reinforcing steel, commonly known as “rebar”. Per the drawings steel rebar cages are constructed. If required, shutters are utilized on the bottom and sides of the trench for the rebar.
Rebar is installed under the premise of binding the concrete and steel together in resisting force. While concrete acts best in compression, it falters in tension. The opposite is true of reinforcing steel.
The rebar also bonds well with the concrete. The grooves, or deformation, of the steel rods allow for greater bonding strength. The rebar comes in many sizes and lengths. The rebar are noted on the drawings and should be installed per the specifications.
Once the rebar cages are installed, along with vertical steel rods to tie the slab-on-grade to the grade beam, it is vital to shutter the cage from the sides. . If required, shutters are utilized on the bottom and sides of the trench for the rebar. Shuttering, as in soil blocks and haunches, are utilized to verify that concrete can flow between the cage rebar and ground walls, for proper coverage.
Extra reinforcements, lab length and placement, hooks, spacers and clear reinforcement cover shall be verified after the steel cage is completed. To complete the trenches, pipe sleeves are installed allowing for electrical and drain line penetrations.
Step 3: Pouring the Concrete Grade Beam
For the typical house it is best to have the concrete mix delivered by an on-site machine mixed concrete provider. Delivered by a mixing truck, chutes are attached which allow the concrete to flow to all areas of formwork.
When the concrete delivery truck arrived at the site, three (3) items need to occur. First, it is of special notice that the concrete delivery should not be at the end of the day. If so, then reject the concrete and ask for another delivery the next morning. The reason for rejection is that the delivery truck is on their last pour of the day, and the concrete has been in the mixer for quite a while, thus reducing its strength. Second, take a coffee can and place some concrete in it for testing purposes. While one might not want to pay the costs for testing, it is important to have a sample available is a problematic issue arises. Finally, make sure the delivery truck does not clean his chute, or truck, on the property.
As the concrete flows into the formwork, it is maneuvered so as proper coverage is provided, and the grade beam is completely filled. Once in place, the concrete is vibrated to allow air entrained in the concrete to raise out of the concrete. A rental of a concrete vibrator is typically not required for a simple installation. In the past, I have utilized a hand vibrator and a steel rod is adequate enough. Finally, the concrete is topped out at the form bords for a finished and level installation. After topping out, anchor bolts can be easily installed into the concrete.
After the grade beams are poured and topped out, they can be covered from the weather. They should also be left to harden to their strength before constructing the exterior walls.