Foundation Masonry Layers

May 10, 2012
Day 20: Foundation Masonry layers

Last post talked about components that make up foundation of the house. This post focuses mostly on masonry work. First, it may be obvious, but just to be explicit, house construction requires a LOT of water. Till the super-structure is completed, almost every activity, starting from excavation, making cement, watering foundation and walls, compacting earth metal, and many more that I will only realize in due course. On an average, expect to use up 20,000 litres each month i.e. about Rs 2,000 on an average.

Masonry work is done with a set of cement concrete blocks that are "glued" together with cement mortar. You might recall that excavation was about 900mm deep. Each block is about 150-175mm tall and including mortar height of each block would roughly be 200mm. Foundation of a house is typically up till plinth which generally is 450mm above the ground level. This implies several layers of foundation masonry.

In our house, ground at the rear-end of the house is 300mm lower than that at the entrance-end.

Hence, plinth-level will be 900 + 450 + 300 = 1650mm from the excavation level. Considering each layer of masonry is 200mm, total number of masonry layer at rear-end is about 8 and at the entrance-end is about 7.

 

The section view of cement blocks is sketched here. As is evident each layer of cement block works its way towards the center. Space on the outer periphery is filled with earth metal whereas the space between cement blocks is filled with cement mortar. I was surprised at the speed with which this work got done.. It took workers just a little over one week to do about 4 layers across most of the house.

Columns: There are 4 columns to bear the load of the first floor balcony and the family room areas. Excavation for columns is about 1500mm. Base of the column is filled with mortar. Then a steel column of about 3000mm is erected as shown in sketch here. Rings are tied approx 150mm from each other to hold the column together. Finally, they'd pour cement concrete around it to create the actual column structure.

Center-line marking pillars: Towards 4-minute mark in the video is description of center-line marking pillars. Center-line marking is to mark center-line for all the edges. Picture shows marking of center-line for window. Blue squares are center-lines that were already marked for main edges of the room. Those were the basis for excavation to occur.

Additionally, a new center-line is now marked (shown with red rectangle) for where the window edge on ground floor would be established. The center-line is marked with pillars (refer to the video) that will remain placed until ground floor actually comes up. These center-lines are marked throughout the house wherever a nook changes the direction of the primary edge of the room.

Finally, this video shows the actual work to build that column described above. 

May 4, 2012
Day 14: Foundation plan 

Excavation is more or less complete for load bearing walls now. Now that we know quality of the ground, I asked Ravi to explain plan to reach the plinth level. Plinth level is the base level above which ground floor walls come up.

At a high-level there are four main areas:

1. Foundation masonry - Cement building blocks that are about 38cm tall, 15cm wide and 15cm deep are the foundation of the building. They are "stuck" together with cement concrete masonry. It is recommended that anti-termite treatment be done twice - once before placing any cement blocks and next after placing all layers of cement blocks. As Ravi says, they make this mixture with 1 part of cement, 4 part of sand, and 8 part of aggregate. They do a number of "layers" of such building blocks which together provide load bearing support. In our case, ground level at living room side is about 6 to 12 inches higher than that on the guest bedroom side. Hence, they'd do more layers at latter so plinth beam will be at even level. 
2. Rain water tank - Bangalore outskirts have water shortages.. As you see from our plan we have decent size garden and we didn't want to feel guilty of over-burdening already acute supply of water. Hence, we decided to go with what seems like a good size rainwater (RWH) tank - 60,000 litres. My calculation for 60,000 was as follows:
• Our monthly usage for a family of four is about 25,000 litres each month (~200 litres per person per day). Out of this, my guess is about 10,000 litres is used for gardening and 5,000 for bathroom usage. Those are the obvious low-hanging applications for RWH water. 
• Bangalore gets about 1000mm rainfall each year with raingods showering in for all except four months (December to April) within a year. I wanted to have sufficient water to live through that period. Hence, we got the tank size to be about 60,000. For those interested, amount of rain that'd be accumulated is roughly calculated as - Accumulation area x Amount of rain. Our roof area is 180 square meter and September rainfall is 200mm. So, accumulation in September would be 180x200 = 36,000 litres. Chances are with this size RWH tank it may not overflow. But, in case, it does, there would be a recharge well to let overflown water seep back into the ground.
3. Columns - Four columns need a bit of special work to get set up. They would essentially setup iron columns in those areas and put concrete around them. Once columns come up to the plinth level, they would be connected to the rest of the house via plinth beam. 
4. Plinth beam - Foundation around load bearing walls will be done first followed by columns and finally by RWH tank structure. Once each component is built they will be joined together through plinth beam. Our expectation is plinth beam will be about 2.5 feet above the natural ground level. This so as to keep house safe from flooding as well as provide appropriate elevation. 

General expectation is foundation level will be reached within 6-8 weeks from start. However, this doesn't consider construction of RWH tank. With that, we should expect 10-12 weeks, first fortnight of July, to complete plinth level and start the ground floor construction.