Mortar Required for Various Types of Walls

Type of wall	Amount required per m² wall
11.5cm brickwall	0.25m³
22.2cm brickwall	0.51 m³
10cm sand-cement block wall	0.008m³
15cm sand-cement block wall	0.01 1m³
20cm sand-cement block wall	0.015m³

Materials Required per Cubic Meter of Mortar

Type	Cement bags	Lime kg	Sand m³
Cement mortar 1 :5	6.0	-	1.1
Compo mortar 1:1:6	5.0	100.0	1.1
Compo mortar 1:2:9	3.3	13.5	1.1
Compo mortar 1:8	3.7	-	1.1
Compo mortar 1:3:12	2.5	150.0	1.1
Lime mortar 1:3	-	200.0	1.1

Mortar

Mortar is a plastic mixture of water and binding materials used to join concrete blocks, bricks or other masonry units.

It is desirable for mortar to hold moisture, be plastic enough to stick to the trowel and the blocks or bricks and finally to develop adequate strength without cracking.

Mortar need not be stronger than the units it joins. In fact cracks are more likely to appear in the blocks or bricks if the mortar is excessively strong.

There are several types of mortars each suitable for particular applications and of varying costs. Most of these mortars include sand as an ingredient. In all cases the sand should be clean, free of organic material, be well graded (a variety of sizes) and not exceed 3mm of silt in the sedimentation test. In most cases, particle size should not exceed 3mm as the mortar will be "harsh" and difficult to work with.

Lime mortar is typically mixed 1 part lime to 3 of sand. Two types of lime are available. Hydraulic lime hardens quickly and should be used within an hour. It is suitable for both above and below ground applications. Non-hydraulic lime requires air to harden and can only be used above ground. If smoothed off while standing, a pile of this type of lime mortar can be stored for several days.

Cement mortar is stronger and more waterproof than line mortar, but is difficult to work with because it is not 'fat' or plastic and falls away from the blocks or bricks during placement. In addition, cement mortar is more costly than other types. Consequently it is used in only a few applications such as a damp-proof course or in some limited areas where heavy loads are expected. A 1:3 mix using fine sand is usually required to get adequate plasticity.

Compo mortar is made with cement, lime and sand. In some localities a 50:50 cement-lime mix is sold as mortar cement. The addition of the lime reduces the cost and improves the workability. A 1:2:9, cement-lime-sand mix is suitable for general purposes, while a 1:1:6 is better for exposed surfaces and a 1:3:12 can be used for interior walls or stone walls where the extra plasticity is helpful.

Mortar can also be made using pozzolana, bitumen, cutback or soil. A 1:2:9 lime-pozzolana-sand mortar about equals a 1:6 cement-sand mortar. Adobe and stabilizedsoil blocks are often laid in a mortar of the same composition as the blocks.

Tables 3.16 and 3.17 provide information on the materials required for a cubic metre of various mortars and the amount of mortar per square meter for several building units.

Starting with cement mortar, strength decreases with each type, although ability to accommodate movement increases.

Reinforced Concrete

Concrete is strong in compression but relatively weak in tension. The underside of a loaded beam, such as a lintel over a door, is in tension.

Concrete subject to tension loading must be reinforced with steel bars or mesh. The amount and type of reinforcement should be carefully calculated or alternatively, a standard design obtained from a reliable source should be followed without variation.

Important factors relative to reinforced concrete:

• 1 The steel bars should be cleaned of rust and dirt before they are placed.
• 2 In order to obtain good adhesion between the concrete and the steel bars, the bars should be overlapped where they join by at least forty times the diameter. When plain bars are used the ends of the bars must be hooked.
• 3 The reinforcement bars should be tied together well and supported so they won't move when concrete is placed and compacted.
• 4 The steel bars must be in the tensile zone and covered with concrete to a thickness of three times the diameter or by at least 25mm to protect them from water and air which causes rusting.
• 5 The concrete must be well compacted around the bars. 6 Concrete should be at least C20 or 1:2:4 nominal mix and have a maximum aggregate size of 20mm.

Concrete floors are sometimes reinforced with welded steel mesh or chicken wire, placed 25mm from the upper surface of the concrete, to limit the size of any cracking. However, such load-distributing reinforcement is necessary only when loadings are heavy, the underlying soil is not dependable, or when cracking must be minimized as in water tanks.

Curing Concrete

Concrete will set in three days but the chemical reaction between water and cement continues much longer. If the water disappears through evaporation, the chemical reaction will stop. It is therefore very important to keep the concrete wet (damp) for at least 7 days.

Premature drying out may also result in cracking due to shrinkage. During curing the strength and impermeability increases and the surface hardens against abrasion. Watering of the concrete should start as soon as the surface is hard enough to avoid damage, but not later than 10 to 12 hours after casting. Covering the concrete with sacks, grass, hessian, a layer of sand or polythene helps to retain the moisture and protects the surface from dry winds. This is particularly important in tropical climates.

Temperature is also an important factor in curing. For temperatures above 0° C and below 40° C strength development is a function of temperature and time. At temperatures above 40°C the stiffening and hardening may be faster than desired and result in lower strength.

Construction Joints

The casting should be planned so that the work on a member can be completed before the end of the day. If cast concrete is left for more than 2 hours it will set so much that there is no direct continuation between the old and new concrete. Joints are potentially weak and should be planned where they will effect the strength of the member as little as possible. Joints should be straight, either vertical or horizontal. When resuming work, the old surface should be roughened and cleaned and then treated with a thick mixture of water and cement.

Mixing

Mechanical mixing is the best way of mixing concrete. Batch mixers with a tilting drum for use on building sites are available in sizes from 85 to 400 litres. Power for the drum rotation is supplied by a petrol engine or an electric motor whereas the tilting of the drum is done manually. The pear-shaped drum has blades inside for efficient mixing. Mixing should be allowed to proceed for at least 2.5 minutes after all ingredients have been added. For small scale work in rural areas it may be difficult and rather expensive to get a mechanical mixer.