Cavity Wall Construction - Advantages & Dis- Advantages

What is Cavity Wall?

Cavity walls are those which are constructed in that way that an empty space or cavity is left between the single walls. They are also known as Hollow Wall.

Cavity walls are two walls constructed as a single wall. These two walls having little space between them for insulation purpose are known as leaves of the cavity wall.

The outer wall is called an external leaf, and the inner wall is called an internal leaf. The empty space or cavity size should be in between 4 to 10 cm.

The internal and external leaves should have 10 cm thickness. These two leaves of cavity wall are interconnected by links or metal ties for a strong bond.

Cavity Wall Construction:-

       In general, cavity wall doesn’t require any footings under it, just a strong concrete base is provided on which cavity wall is constructed centrally. Two leaves are constructed like normal masonry, but minimum cavity must be provided in between them.

       The cavity may be filled with lean concrete with some slope at top up to few centimetres above ground level as shown below :

        Cavity Walls are useful in two ways. Firstly, the cavity prevents the dampness from the outer leaf percolating into the inner leaf. Secondly, they provide excellent insulation from heat and sound.

         Weep holes are provided for outer leaf at bottom with an interval of 1 m. Normal bricks are used for inner leaf and facing bricks are used for outer leaf. Different masonry is also used for cavity wall leaves. The leaves are connected by metal ties or wall ties, which are generally made of steel and are rust proof.

           The inner leaf is found to take a greater portion of the imposed load transmitted by floor and roof. Hence, the two leaves of the wall are bonded together with wall ties usually placed 900 mm apart vertically and 450 mm horizontally in every 6th course staggered.

          The wall ties are provided in such a way that they do not carry any moisture from outer leaf to inner leaf.

          These wall ties are made from mild steel wires of 3 to 4 mm diameter or MS bars and fabricated to std. shapes. They are dipped in hot tar and sanded or made from galvanized steel to prevent rusting. In very important works, copper may be used.

          Wire ties are placed with their twisted end down to allow water that may seep inside, to drip down in the cavity.

          The bond to be used for both the leaves, when the thickness is half brick, is the stretcher bond. Where the inner leaves are made thicker for carrying heavy loads. English bond can be used for that part.

           To prevent mortar dropping in cavity, wooden battens are provided in the cavity with suitable dimensions. These battens are supported on wall ties and whenever the height of next wall tie location is reached, then the battens are removed using wires or ropes and wall ties are provided.

Important Points to be Observed in Cavity Wall Construction:-

1) The cavity should extend to 15 cm below the damp-proof course level.

2) Below the ground level, the walls are built solid, or preferably the cavity should be filled up to 15 cm below the damp-proof course with fine concrete.

3) Under no circumstances, should the D.P.C be laid to the span of one leaf only. It should cover both leaves of the wall.

4) The upper part of the wall where it ends should also be built solid for two or three courses below the wall plate or roof line, to stiffen the head of the wall and distribute the load over both leaves.

5) The wall ties must be kept free from mortar droppings by means of a timber batten suspended in the cavity and raised as the work proceeds during its construction. Some bricks may be temporarily left out at the ground floor level to form openings to permit the bottom of the cavity to be cleared of mortar droppings at the end of each day’s work.

6) In exposed positions, it is desirable to leave a few vertical joints in the outer leaf open, at the
bottom of the cavity to permit water to drain away.

7) A certain amount of ventilation to the cavity is desirable to prevent stagnation of air and excessive humidity. It can be provided by vents, say 150 x 75 mm (6″ x 3″) at intervals, near the base and top of the wall, by leaving a few joints open.

8) The cavity walls should not be built solid at the jambs [the sides of door and window openings] to D.P.C are only permissible in fairly sheltered sites and where the wall surface is rough- cast.

9) A lead, galvanized iron or other suitable material made to form a trough or gutter, may be placed in the cavity above all openings for exposed doors and window to collect water which may drive through the outer leaf.

10) The cavity wall should not be built solid below window sills also, and a damp-proof course is desirable at this point also.

11) Damp proof course is provided for two leaves separately. In case of doors and windows, weep holes are provided above the damp proof course.

Advantages of Cavity Walls:-

1) Sound waves travel faster in solid walls as compared to hollow walls. Hence, the cavity walls are also best for sound insulation.
 
2) Cavity walls give better thermal insulation than solid walls. It is because of the space
provided between two leaves of cavity walls is full of air and reduces heat transmission
into the building from outside.
 
3) The construction cost of the cavity wall is about 20% less than the construction of solid walls. Hence, economically they are cheaper than solid walls.
 
4) Moisture content in outer atmosphere is does not allowed to enter because of hollow
space between leaves. So, they also prevent dampness.
 
5) They also reduce the weights on foundation because of their lesser thickness.

6) Outer Efflorescence is also prevented.

Disadvantages of Cavity Walls:-

1) Highly skilled labour and masons are required for cavity wall construction.

2) Require standard supervision during its construction.

3) A vertical damp proof course is also necessary for it.

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Types of Shallow Foundations & Its Use

Shallow foundations are constructed where hard soil layer is at shallow depth (up to 1.5 m). According to Terzaghi`s, the foundation is shallow if its depth is less than or equal to its width.

   

                                             

Types of Shallow Foundations:-

               

Spread Foundation :-

Spread footing foundation is basically a pad used to ‘‘spread out’’ loads from walls or columns over a sufficiently large area of foundation soil. Spread footing required to support a wall is known as a continuous, wall, or strip footing, while that required to support a column is known as an individual or an isolated footing.

Strip Footing (Continuous Footing) :-

             A strip footing is provided for a load-bearing wall. A strip footing is also provided for a  row of columns which are so closely spaced that their spread footings are overlap or nearly touch each other. In such case, it is more economical to provide a strip footing than to provide a no. of isolated footings in one line.               

Isolated Footing (Individual Column Footing) :-

              Isolated footing is also known as pad footing or individual column footing. An isolated footing may be square, circular, or rectangular in shape in plan, depending upon factors such as the plan shape of the column and constraints of space. Isolated footing has two small categories namely 

a.  Isolated Footing with Uniform Thickness or depth

b.  Isolated Pad Footing

                                
                                                         

                                         Common Arrangements of Strip Footing and Isolated Footing

Strap Footing :-

A ‘strap footing’ comprises two or more footings connected by a beam called ‘strap’. This is also called a ‘cantilever footing’ or ‘pump-handle foundation’. The strap beam is so connected that the whole assembly behave like a single unit. The individual footings are so designed that their combined line of action passes through the resultant of total load.

This may be required when the footing of an exterior column cannot extend into an adjoining private property. A strap footing is economical than a combined footing when the allowable soil pressure is relatively high and the distance between the columns is large. Common arrangements of strap footing are as shown in fig.

             

                  

                                                  Common Arrangements of Strap Footing

Combined Footing :-

A combined footing is supports two columns. It is provided when two columns are so close to each other that their individual footings may overlap. It is also provided when the property line is so close that a spread footing would be eccentrically loaded for kept entirely within the property line. Combined footing may be rectangular or trapezoidal in shape. Common arrangements of combined footing are as shown in fig.                  

      

                    

Common Arrangements of Combined Footing

Mat or Raft Foundations :-

A mat or raft foundation is a large slab supporting a no. of columns and walls under the entire structure or a large part of the structure. A mat is required when the allowable soil pressure is low or where the columns and walls are so close that individual footings would overlap or nearly touch each other. Mat foundations are useful in reducing the differential settlements on non-homogeneous soils or where there is a large variation in the loads on individual columns. Common arrangements of mat footing are as shown in fig.

             

                        

                         Common Arrangements of Raft or Mat Footing (Flat Slab Type)

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Types of R.C.C. Column Failure

            Columns are the most important parts of a structure. They transfer loads of the structure to the surrounding soil through the foundations. So we need to build strong columns, otherwise, failure will occur.

            Column is a long cylindrical member subjected to axial compression. Column carries self-weight and load coming on it. You know that columns are used to transfer the load vertically to the horizontal beam. Column is categorized based on its height. Every material used in the building experience both compression and tension stress that’s how we designed the modern buildings.

            Columns consist of two major material one is concrete and second one is steel. Before designing the columns, civil engineers should calculate total stress due to live and dead load of the building. When the applied stress exceeds the permissible stress (calculated) the structure will fail.

             There are three types of concrete columns based on its height and lateral dimension. Long columns are those whose ratio of height to least lateral dimension is more than 12. When the height to least lateral dimension is less than 3, it is called a pedestal and if it is between 3 and 12, it is called as a short column.

              The load carrying capacity and modes of failure of a reinforced concrete column is based on the slenderness ratio. Slenderness ratio is the ratio of the effective length (Le) and least lateral dimension of the column as per Indian and British Standards. But as per American Concrete Institute Code of Practice, the slenderness ratio is defined as the ratio of effective length of column to its radius of gyration, which is same as used for structural steel design as per IS Code. Effective length of a column depends on its support conditions at ends.

In this article, different types of column failures are discussed.

Types of Column Failure :-

Based on the slenderness ratio of the column, there are three modes of failure of reinforced concrete columns. The columns are assumed to be centrally loaded (no eccentric loads).

1) Compression Failure

2) Buckling Failure

3) Shear Failure

1. COMPRESSION FAILURE:-

             This type of failure is also called as “Column Failure due to Pure Compression”. When columns are axially loaded, the concrete and steel will experience some stresses. When the loads are greater in amount compared to the cross-sectional area of the column, the concrete and steel will reach the yield stress and failure will be starting without any later deformation.

            The concrete column is crushed and collapse of the column is due to the material failure. In this type of failure, the material fails itself, not the whole column. This type of failure mostly occurs in shorter and wider columns. 

             To avoid this, the column should be made with sufficient cross-sectional area compared to the allowable stress. This type of failure is generally seen in case of pedestals whose height to least lateral dimension is less than 3 and does not experience bending due to axial loads.

2. BUCKLING FAILURE:-

             This type of failure is also called as “Column Failure due to Elastic Instability”. Buckling failure generally occurs in long columns. Because they are very slender and their least lateral dimension is greater than 12. In such condition, the load carrying capacity of the column decreases very much for given c/s area and % of steel.

             When such type of concrete columns are subjected to even small loads, they tend to become unstable and buckle to any side. So, the reinforcement steel and concrete in such cases reach their yield stress even for small loads and fail due to lateral elastic buckling.

              This type of failure is unacceptable in practical concrete constructions. Building Code prevents usage of such long columns for slenderness ratio greater than 30 (for unbraced columns) for the use in concrete structures.

3- SHEAR FAILURE:-

            This type of failure is also called “Column Failure due to Combined Compression and Failure”. In engineering, shear strength is the strength of a material or component against the type of yield or structural failure where the material or component fails in shear. A shear load is a force that tends to produce a sliding failure on a material along a plane that is parallel to the direction of the force.

            Short columns are commonly subjected to axial loads, lateral loads and moments. Short columns under the action of lateral loads and moments undergo lateral deflection and bending. Long columns undergo lateral deflection and bending even when they are only axially loaded.

             Under such circumstances when the stresses in steel and concrete reach their yield stress, material failure happens and RCC column fails. This type of failure is called combined compression and bending failure.

                                                                    

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Air Entrained Concrete

             A major advance in concrete technology in recent years is the introduction of tiny disconnected air bubbles into concrete. The process which involves the introduction of tiny air bubbles into concrete is called as air entertainment. And the concrete formed through this process is called air entrained concrete. Using air entertaining Portland cement or air entertaining agents such as admixture, air entertainment is done in concrete. 

            The amount of air in such concrete is usually between 4 to 7 % of the volume of concrete. It is measured by galvanometric method, volumetric method and pressure method. The air bubbles relieve internal pressure on the concrete by providing chambers for water to expand when it freezes.

Adding entrained air to concrete provides important benefits in both plastic and hardened concrete, such as resistance to freezing and thawing in a saturated environment. Air entrapped in non air-entrained concrete fills relatively large voids that are not uniformly distributed throughout the mix.

Development of Air Entrained Concrete :-

                   As we know, volume of ice is about 10% greater than corresponding volume of water.
This causes a lot of problems in areas with extreme weathers. During the winters, water in the pores of concrete freezes. The ice formed occupies larger volume and hence exerts pressure. During the summer, the ice melts, and in the next winter, the pore water freezes again. This cycle is called freezing and thawing of concrete. 

              The pressure developed usually causes surface scaling. To overcome the ill effects of freeze thaw cycles, air entertainment in concrete was developed. Microscopic air bubbles were introduced in concrete. These bubbles give room for the ice formed to expand without exerting pressure on the concrete.

The dark areas in the picture above are air bubbles, the lighter areas are coarse aggregates, suspended in a cement mortar

Air entrainment has the following effects on concrete:

• Resistant to freezing and thawing

• Improved workability and durability

• Decrease in strength 

Properties of  Air Entrained Concrete :-

The following are properties of air entrained concrete:

1. Workability

The improved workability of air entrained concrete greatly reduces water and sand requirements, particularly in lean mixes and in mixes containing angular and poorly graded aggregates. In addition, the disconnected air bubbles reduce segregation and bleeding of plastic concrete.

2. Freeze-thaw durability

The expansion of water as it freezes in concrete can create enough pressure to rupture the concrete. However, entrained air bubbles serve as reservoirs for the expanded water, thereby relieving expansion pressure and preventing concrete damage.

3. De-icers resistance

Because entrained air prevents scaling caused by de-icing chemicals used for snow and ice removal, air-entrained concrete is recommended for all applications where the concrete contacts de-icing chemicals.

4. Sulphate resistance

Entrained air improves concrete’s resistance to sulphate. Concrete made with a low W/C ratio, entrained air, and cement having low tricalcium-aluminate content is the most resistant to sulphate attack.

5. Strength

The voids to cement ratio basically determines air-entrained concrete strength. For this ratio, voids are defined as the total volume of water plus air (both entrained and entrapped). When the air content remains constant, the strength varies inversely with the W/C ratio. As the air content increases, you can generally maintain a given strength by holding the voids to the cement ratio constant. 

To do this, reduce the amount of mixing water, increase the amount of cement, or both. Any strength reduction that accompanies air entrainment is minimized because air-entrained concrete has lower W/C ratios than non air-entrained concrete having the same slump.

However, it is sometimes difficult to attain high strength with air-entrained concrete, such as when slumps remain constant while the concrete’s temperature rises when using certain aggregates.

6. Abrasion resistance

Air-entrained concrete has about the same abrasion resistance as that of non air-entrained concrete of the same compressive strength. Abrasion resistance increases as the compressive strength increases.

7. Water tightness

Air-entrained concrete is more watertight than non air-entrained concrete since entrained air prevents interconnected capillary channels from forming. Therefore, use air-entrained concrete where water tightness is a requirement.

Advantages :-

1. Workability of concrete increases.
   
2. Use of air entraining agent reduces the effect of freezing and thawing.
3. Bleeding, segregation and laitance in concrete reduces.
4. Entrained air improves the sulphate resisting capacity of concrete.
5. Reduces the possibility of shrinkage and crack formation in the concrete surface.

Disadvantages :-

1. The strength of concrete decreases.
2. The use of air entraining agent increases the porosity of concrete thereby reducing the unit weight.
3. Air-entrainment in concrete must not be done if the site control is not good. This is due to the fact that the air entrained in a concrete varies with the change in sand grading, errors in proportioning and workability of the mix and temperatures. 

 

 

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