Why are walls important?
Walls are very important parts of buildings, they have several useful functions:
* Protect us from dangerous animals and people
* Provide shelter from elements such as wind and rain
* Makes a ‘secure’ home
* Bears the loads of the roofs, floors and furniture
Standard blocks are 6 times larger than standard bricks, this means that bricklayers can build the wall more quickly, therefore taking less time and reducing labouring costs.
For the walls, the external walls will be cavity walls made from aerated concrete blocks and the internal walls will be made from dense concrete blocks. The reasons for these choices will be explained further on.
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What are and Aerated Concrete blocks and why I have chosen
Duncan Marshall (2008) says that; Aerated blocks consist of water, cement, pulverised fuel ash, sand and lime. Aerated concrete blocks are very versatile as they can be used in many different parts of the building, such as party and solid walls, cavity walls (internal and external leaves) and foundations.
We will be using Celcon aerated concrete blocks, which are BBA certified (British Board of Agrément, which is the UK’s major authority offering approval of construction products). Sizes (in mm):
Reasons for using Aerated blocks for the inner and outer leaf:
* Good thermal insulation (Thermal conductivity is 0.15W/mK, Thermal conductivity is the ability of a material to transmit heat, the lower the value the better)
* Easy to handle for labourers/bricklayers, as they’re relatively lightweight and can be cut and chased
* Closed cell structure means they have good water penetration resistance
* Good fire protection – Class O surface spread of flame (the best rating)
* Non combustible
* Relatively strong (Strength 3.6 N/mm²)
What are Dense Concrete blocks and Why I have chosen them?
Dense concrete blocks consist of cement, aggregate (fine and coarse). We will be using Celcon Hi (7) Strength blocks. Sizes (in mm):
Reasons for using dense concrete blocks for the internal walls (party/solid walls):
* Very strong (7.3N/ mm²)
* They provide a good surface for most renders and plasters
* Good sound protection
This is an illustration of a building regulation (part A) for the compressive strength needed of walls. For the external walls it will be a cavity wall comprising of aerated concrete blocks (7.2N/mm2), and the inner walls will be a solid wall of dense concrete blocks (7.3N/mm2).
Height of wall
Not exceeding 3.5m
Up to 12m
190mm for whole height
Exceeding 3.5m but not 9m
Not exceeding 9m
190mm for whole height
9m – 12m
290mm from base for 1 storey and 190mm for remaining height
Exceeding 9m but not 12m
Not exceeding 9m
290mm from base for height of 1 storey and 190mm for rest of height
Exceeding 9m but not 12m
290mm from base for the height of 2 storeys and 190mm for rest of height
This is a table from the building regulations (Part A) about the thickness of walls. For our design, the internal and external walls both exceed 290mm, therefore the walls will have the correct thickness.
Mortar is needed to bond the blocks together to provide stability for the wall, therefore building. It also seals gaps between the blocks to prevent water getting through the gaps when it rains.
Blocks (aerated and dense) – Duncan Marshall (2008) recommends our buildings will have a mortar ratio of 1:1:6 (cement: lime: sand). The reason for having a low amount of cement (cement provides the strength) compared to the sand (sand gives volume), is that making the mortar not that strong is so that it doesn’t limit movement and will cause cracks in the joints rather than the concrete blocks. This mixture is recommended by a professional concrete company, therefore it won’t be too weak, as then the mortar will fail.
bucket_handle.gifAll of the buildings will have the bucket handle design. This is because Duncan Marshall (2008) says it helps seal the joint, also it compresses the mortar slightly and the shape improves resistance to water penetration.
Walls – Quoins
Out of these three different types of bonding block work we will use 5 (reveal quoin), because it’s the most simple bond, therefore easier for bricklayers to build the wall, and reducing the likelihood of errors occurring. Also 3 bonding will be used if needed to keep the whole wall to the correct dimension.
For all our cavity walls in our buildings, they will be partial fill cavity walls. Kingspan Insulation says; one of its main advantages is that it prevents rain getting to the inner leaf, which would cause dampness in the walls, therefore weakening the walls.
One disadvantage with partial fill cavity walls happens in the physical construction of the wall takes place, if the walls aren’t sealed completely, airflow in the wall leads to an enormous amount of heat loss within the wall (shown by red areas in 2). This can be caused by dirty ties and mortar snots, which are pieces of mortar which have dropped into the cavity, and cause moisture.
125 75 20 125 20
Block Cavity Insulation Plasterboard
365mm F .
Cavity Walls – Wall ties
Duncan Marshall (2008) says; as we are having a partial fill cavity we have to use this special tie with plastic retainer. In the 2004 Regulations (Approved Document A) there is a requirement to use stainless steel ties.
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Building regulations says the tie should be bedded at least 50mm in each leaf. In terms of the tie angle, it should be slightly sloped toward the outside leaf; otherwise the other leaf will get damp.
Cavity Walls – Movement joints
The materials that make up a wall will expand and contract because of reasons such as temperature and moisture content. Because concrete blocks are made of cement, they will shrink slightly. Therefore, the big sections of wall need to be shortened to accommodate for the movement, if movement joints are placed correctly, this will prevent the wall from cracking.
Duncan Marshall (2008) says that the movement joints of internal concrete block walls should have movement joints approx every 6 metres, but the 1st joint should only be 3 metres away from a corner or bend in the wall. We will incorporate this into our designs.
Cavity Walls- Jambs
The blue line is the window
Damp proof course prevents water penetration from the jamb of the window frame
Cavity Walls – Lintels
Duncan Marshall (2008) say that the lintels carry the distributed load of brickwork across the top of the window frame. Lintels can be up to lengths of 4500mm and when constructed should have at least 150mm bearing on the wall either side.
Lintel with insulation to prevent cold bridging, cold bridging happens when heat is lost from the inside of the building to the outside because the material (this case the lintel) is the ‘bridge’ for the heat loss. When the insulation is there, it has a higher thermal conductivity than the lintel (galvanised steel), so the heat ‘bridges’ to the insulation and not outside the building. 10
This is very important concept of insulation, that also applies to cavity and solid walls where insulation is used.
Insulation – External Walls
Insulation boards are used to keep the heat within a room/building so that the living/working conditions are favourable.
We will have WT20 insulation boards (size is 1200x600x20mm) from warmtiles.co.uk
Thermal Conductivity (W/mK)
1200 x 600 x 20 mm
The U value is the amount of heat which will flow through 1 square metre of a wall for every degree in temperature difference between the inside and outside. Resistance is the resistance to heat flow of a material of a given thickness. Now I can calculate the U-value for the walls.
U value for cavity walls (Thickness/Thermal Conductivity = Resistance)
This transitions into weep holes; weep holes allow the water to exit the cavity. As we are only using blocks, we will use plastic weep holes that are placed every 450 mm. Weep holes, damp proof courses and the bucket handle mortar are excellent parts of the wall that allow it to prevent water penetration.
Cavity Walls – joining cavity walls to internal walls
This is how we will join the inner leaf to the internal walls, also where two internal walls (could be party walls) join, it will be toothed in.
Walls – Door and Window frames (Openings)
The building regulation below specifies the sum of openings (w1+w2) shouldn’t be more than 2/3 ‘L’. Opening heights should be less than 2.1m, and the width less than 3m. This ensures the wall is not weakened significantly under compression by openings.
We used this formula for a standard room (not disabled).
Length of whole wall
A very common part of walls, and obligatory for accommodation buildings are windows. Window sizes (British Standard: 2003)
*Purple segments are windows that open (white arrows indicate what direction they open), orange segments are fixed. This type of window is called ‘Left-hand multi light’.
One of the functions of the walls is to control the airborne sounds, such as aeroplanes and cars travelling outside of the building. The internal walls will be made out of dense concrete blocks, as this provides good sound protection. Celcon aerated blocks also have just as good of a level of sound protection.
Osbourn and Greeno (2007), say the Sound reduction index (SRI) is a way of measuring the effectiveness of construction insulating airborne sound. We will have a block cavity wall which is plastered, this has a SRI number of 49, and 150 mm dense concrete walls which have a SRI number of about 46. These are relatively acceptable SRI values to be used in construction.
There are different finishes you can have for aerated concrete blocks; plastered, rendered and painted. For the outside of the external walls we will render the walls (10mm wide). For internal walls (dense concrete blocks) we will have plaster boards (20mm wide).
After the mortar and blocks are bonded, render will be applied. For our design we are using aerated concrete blocks for the external walls, therefore we should not use strong renders (Duncan Marshall, 2008), because when the dense mix shrinks, it will go past the limit of stress that the blocks can handle, therefore cracking will occur.
Our building will have a very similar aesthetic appearance to this building (right), except for the colour.
David Littlefield, et al., 2008, Metric handbook, 3rd Edition,
Duncan Marshall ,2008, Construction Website 0809 (unframed construction) http://learn.lboro.ac.uk/file.php/1183/UWE08/index.htm
Dereck Osbourn and Roger Greeno, Mitchell’s Introduction to Building 4th Edition
Julian McKenzie (Lectures)
Sizes of concrete blocks – http://www.sigexpress.co.uk/Aerated-Concrete-Blocks/Celcon-100mm-Standard-Block.htm
Partial Fill Cavity Walls – http://www.insulateonline.com/index1.htm?walls2.htm~main
Insulation Boards – http://www.warmtiles.co.uk/category.php?id_category=17
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