Grade beam is a part of a substructure of a building. It is normally built with reinforced cement concrete.
A grade beam connects two or more short columns. Grade beam can be directly built on ground or above the ground.
Grade beam sometimes support the ground floor slab and sometimes don’t. It will be shown in the structural drawing. We will not go to the design related issues. We will just discuss the construction related issues.
In this article, I will discuss the basic construction process of grade beam.
Basic construction process of grade beam
Before constructing grade beam, you need to complete the construction of footing and short column.
Construction process of grade beam involves the following steps –
- Filling sand
- Placing reinforcement of grade beam
- Making formwork for grade beam
- Pouring concrete
1. Filling sand
Sand filling depends on the level of grade beam. In which level do you want to make grade beam? This level is shown in the drawing.
Suppose, our grade beam level is shown in the drawing as +4′-0″. That means, the top of grade beam is 4′ above from the road level. Road level is considered as +0′-0″ level.
If the cross sectional dimension of grade beam is 10″ x 12″, then the height of grade is 12″ (1 foot).
As we know, our grade beam top level is +4′-0″ and grade beam height is 1′, so we have to fill sand up to 4′-1’= 3′ level.
As I told before, if you want to build your grade beam directly on the ground then you have to fill the sand up to this level. After filling the sand, you should compact the filled sand or soil.
If you want to build your grade beam above the ground then you don’t have to fill sand. You can just proceed to make formwork.
In both case, you will find instruction in the drawing.
2. Placing reinforcement of grade beam
Before placing reinforcement, you need to build the bottom of the grade beam.
If you want to build grade beam directly on the ground then you have to make a brick flat soling on the compacted soil. The brick flat soling should be built along the grade beam.
And, if you want to make the grade beam above ground then you have to make the bottom shutter of the grade beam. I will discuss details about this in the 3rd step of this post.
Okay, our grade beam bottom is ready for placing reinforcement. Now we’ll place the reinforcement.
You’ll find the reinforcement details of the grade beam in the structural drawing. For example, we’ll place the reinforcement of the grade beam as shown in the image below.
From the above image, we found 2-20mm top bar and 2-20mm bottom bar are used in the grade beam.
So we need to place those 4 numbers of bar in the grade beam.
First, we need to cut the bar as required length and then we need to provide L-bend in the both ends of the bar. We also need to make the required quantity of stirrups for our beam.
In this stage, I want you to read the following articles for better understanding the cutting length of bar and placing reinforcement of the beam.
If you read the above articles, you now know that how to place the reinforcement for the grade beam. Follow the instruction provided in the above articles to place the grade beam reinforcement.
3. Making formwork for grade beam
To make the formwork of grade beam, you need to make one shutter for grade beam’s bottom and two shutter for grade beam’s sides.
But, when you build the grade beam directly on the ground then you don’t need to make the bottom shutter for the beam. In that case, you just need to make a brick flat soling along the grade beam.
When you build the grade beam above the ground then you need to make a bottom shutter for the beam.
Beam bottom shutter is fixed before placing the beam reinforcement. After placing beam reinforcement, you can fix the shutter for the beam’s sides. But, you can fix the side shutters before placing reinforcement. But, the previous method is convenient.
After completing the form work of the grade beam, make sure enough cover block is provided to maintain the concrete cover for reinforcement.
4. Pouring concrete for grade beam
You can cast grade beam with readymix concrete or on-site machine-mix concrete. I would prefer to cast grade beam with on-site machine mix concrete. But, if the concrete volume is large then I would suggest to cast grade beam with readymix concrete.
If you want to cast grade beam with readymix concrete then find out the details about concrete strength from the structural drawing.
The strength of concrete is specified in the drawing as “psi” or as “ratio” of the concrete ingredients.
When the concrete strength is specified as “psi” then it will be written as “3000 psi” or something like this.
When the concrete strength is specified as “ratio” of the concrete ingredients, it will be shown as “1:2:4” or something like this.
Sometimes, both “psi” and “ratio” is shown in the drawing.
If you order concrete from ready mix supplier then they will ask you about the concrete strength. Tell them which concrete strength you want for the grade beam.
If you decided to cast the grade beam with on-site machine mix concrete then you will use the “ratio” of the concrete ingredients as specified in the drawing. Suppose, it is specified in the drawing as 1:2:4. Then, you will mix 1 unit cement, 2 unit sand and 4 unit chips.
Follow the instruction I wrote in the following posts for mixing concrete with mixture machine.
However, pour concrete in the grade beam and vibrate properly then finish the top of grade beam.
You can remove the side-shutter of grade beam next day after pouring concrete. But, for removing bottom shutter of the grade beam, you have to wait till the concrete gains its strength. This time duration for stripping bottom shutter depends on the clear span of the beam. You’ll find notes about this in the structural drawing.
Do you have any better ideas about the construction process of a grade beam? Please share in comments…