Trowel Works 01-Mortar and Concrete

Mortar is the material used in bedding, jointing and pointing bricks and blocks in masonry walling

Constituents of mortar - Mortar is composed of:-

·         Binding material (cement, lime, clay, gypsum, plaster of Paris)

·         Fine aggregate

·         Water

Additives are sometimes mixed to mortar to improve its properties.

Other forms - Fine aggregates mixed with binding material and water also forms some important building finishes such as Plaster, Rendering.

Materials for mortar - General purpose mortar can consist of Sand, Ordinary Portland cement, Water, Plasticizer, Retarder, Accelerator, Pigments or coloring agents and Hydrated lime. Hydrated lime was once commonly used as a binder for mortar but it is rarely used today.

Requirements of a mortar mix – A good mortar mix should have:

·         Adequate compressive strength.

·         Adequate bond strength between mortar and bricks.

·         Durability – resistance to frost and chemical attack.

·         Joints sealed against wind-driven rain.

·         An attractive appearance.

The ability of the mortar to meet these requirements will depend upon the Materials specified for the mix, Workmanship of the bricklayer, Protection of the materials and brickwork against adverse weather.

Mix Proportions - Before mortar can be mixed, the ingredients have to be measured in their correct proportions. The ingredients can be measured by:  Volume or by Weight. Volume mixing can be carried out by hand or by machine, while weight mixing can only be carried out by machine.

Methods of Volume mixing

Materials calculated by volume should be accurately gauged or measured into the correct quantities for each specified mix. The methods used are- Measuring by shovel, Measuring by bucket (or pan) and Measuring by gauge box

Standard Gauge Box

The internal dimensions of a gauge box should be:-

400 x 350 x 250 mm = 0.035 m³

0.035 m³ is the volume of a bag of cement weighing 50 kg.

Mixing by Weight

By using a weight batch mixer, the weight of the aggregate is recorded as it is shoveled into the hopper. This is a more accurate method of batching materials than any of the previously described methods. The materials can be loaded into the hopper while the previously batched materials are being mixed. The weight of materials can be read on a dial by the operator who controls the weight, based on previous calculations for the constituent weights for the required mix. Water is added in liters (1 liter weighing 1kg), after calculating the requirement.

Mixing time - Mixing time depends on the method of mixing - Machine Mixing or Hand Mixing.

Machine Mixing - In general terms, the time of machine mixing should be between two and three minutes. On no account should the mix be allowed to stay in the machine longer because the materials will start to segregate

A concrete and mortar mixer

Cleaning the mixer – It is very important to clean the mixer after day’s work to prevent remnants of mortar or concrete setting and sticking into the inner parts of the mixer. Un-cleaned mixers should not be left overnight. After mixing some sand, coarse aggregate and water should be loaded into the mixer and the mixer turned for about 5 min. This will remove all cement/mortar/concrete stuck onto the blades and the drum thus maintaining a clean machine.

After cleaning the machine should be stored under cover on an elevated level to protect from elements and provide drainage.

Hand mixing

This should take as long as required to ensure that all the particles have been completely integrated together. A high degree of agitation is required when hand-mixing, to ensure that any added plasticizer etc has the necessary effect. Mixed mortar should never be ‘re-tempered’ or ‘knocked up’ with added water because this dilutes the cement/lime element of the mortar which will result in a weaker mix. Mortar is a mixture of the following materials in different combinations: 

                  Sand + Cement + Water

                  Sand + Lime + water

                  Sand + Lime + Cement + Water

                  Sand + Cement + Plasticizer + Water

Typical Mortar Mixes

In work sites, mortar is possibly the least understood and most abused material on the building site. The design strength of the mortar should be determined mainly by the strength of the brick or block to be bedded in it. The mortar strength should roughly match that of the brick or block and in no case should it be stronger than it.

Workability

Mortar will work more easily if the mix contains lime. The more lime the mix contains, the more workable it will be. If the mortar contains cement, it will stiffen more quickly, therefore the more cement within the mix the quicker it will stiffen and set. When mortar of high strength is required, the mix should be composed of cement and sand only.

Design of Mortar Mixes

Mortar mixes are designed to suit certain building requirements. The chart below shows the different mixes and their designated uses which range from 1 to 4. As the mixes progress through the designations of 1 to 4, they become progressively weaker, but with higher lime content they become more tolerant to structural movement.

Designation	Cement, lime with sand	Masonry Cement, sand	Cement, sand with plasticizer	Intended use
1	1:1/4:3			Class A engineering bricks and dense concrete blocks
2	1:1/2:4:4	1:21/2 to 31/2	1:3 or 4	Class B engineering bricks
3	1:1:5 or 6	1:4 or 5	1:5 to 6	Bricks and blocks below DPC level
4	1:2:8 or 9	1:51/2 to 61/2	1:7 to 8	Bricks and blocks above DPC level

Concrete mixes - Concrete mixes are identified in two methods:

1.      By the proportions in which the raw materials are mixed – 1:2:4 (20mm). (1 part cement + 2 parts Fine Aggregate + 4 parts Coarse Aggregate. 20mm denotes the maximum particle size of coarse aggregate). Identifying concrete by this method has disadvantages.

The mix proportion does not specify required strength of concrete and has no control over the quality of concrete.

To overcome this modern day concrete mixes are identified by strength.

2.      Concrete mix by strength - Mixes are identified by the strength which the concrete should possess on setting. These mixes are called Graded Concrete mixes.

Example s –

Grade 10 concrete             (strength 10 mpa)

Grade 15 concrete             (strength 15 mpa)

Grade 20 concrete etc.      (strength 20 mpa)

Concrete mixes and material requirements for a 50kg bag of cement:

Mix	Quantities per 50 kg bag of cement			Use
	Fine aggregate (m³)	Coarse aggregate (m³	App water content (liters)
1:1:2	0.035	0.070	20	Very strong water tight concrete
1:11/2:3	0.053	0.105	22.5	Do
1:2:4	0.070	0.140	25	Reinforced concrete for floor slabs, columns, beams etc
1:3:6	0.105	0.280	35	Foundations, screeds
1:4:8	0.140	0.280	40	Fillings

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Timber Works 04-Timber Joints

Timber joints are used to join timber parts in a structure. Timber needs to be joined in 03 positions when making timber structures:

1.      Lengthening

2.      Widening

3.      Angled or Cross

Halving Joints

A halved joint is formed by removing material from each at the point of intersection so that they overlap. The two members can be joined either on the flat or on the edge. When joined on the flat it is termed a lap joint.

The simple halved joint is created by cutting a slot in opposite edges of the members to be joined so that they slip together. Most commonly, the amount of material removed is equal to half the width of the members being joined, although this depends on the relative dimensions of the members. This joint is relatively weak and prone to splitting, due to the lack of shoulders which would otherwise prevent twisting. When extra strength is required, a strengthened version of the joint is called for. This involves a more elaborate cut out which incorporates shoulders to prevent twisting of the joint.

                                    

                 End Lap Joint                                                                       Tapered end lap joint

                                             

                           Halving joint with bolts                                             Angled halving joint

                                             

     Cross halving joint                Dovetailed cross halving                   Oblique halving

Mortise &Tenon Joints

The mortise and tenon joint is one of the oldest timber joints. This joint is used to join pieces of wood, mainly when the adjoining pieces connect at an angle of 90°. In its basic form it is both simple and strong. Although there are many joint variations, the basic mortise and tenon comprises two components: the mortise hole (female) and the tenon (male). A mortise is a cavity cut into a timber to receive a tenon. The mortise is usually 1/3 thickness of the member. A tenon is a projection on the end of a timber for insertion into a mortise. Usually the tenon is longer than it is wide. The tenon, formed on the end of a member generally referred to as a rail, is inserted into a square or rectangular hole cut into the corresponding member. The tenon is cut to fit the mortise hole exactly and usually has shoulders that seat when the joint fully enters the mortise hole. The joint may be glued, pinned, or wedged to lock it in place.

                                     

Haunched Mortise &Tenon                                                                   Wedged Mortise &Tenon

Bridle joint

Bridle joint is similar to mortise &tenon, in that a tenon is cut on the end of one member and a mortise is cut into the other to accept it. The distinguishing feature is that the tenon and the mortise are cut to the full width of the tenon member. The corner bridle joint (also known as a slot mortise and tenon) joins two members at their respective ends, forming a corner. This form of the joint is commonly used to house a rail in uprights, such as legs. It provides good strength in compression and is fairly resistant to Stacking, although a mechanical fastener or pin is often required. Corner bridles are often used to join frame components when the frame is to be shaped. Material can be removed from the joined members after assembly without sacrificing joint integrity.

A variation of the bridle joint is the T-bridle, which joins the end of one member to the middle of another.

             

Corner Bridle Joint

‘T’ Bridle joint.

Timber products.

Timber products are the structural or non-structural items made out of wood. These are widely used for construction items and house furniture and fittings. The examples are numerous:

1    1.      Doors
2.      Windows
3.      Stairs
4.      Tables
5.      Cupboards
6.      Roofs
7.      Partitions
8.      Floors

                                

Door                                       Window                                              Stair

                 

Table                                                              Cupboard

            

Timber Roof                                                              Timber Partition

 

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