Tuesday, March 25, 2014

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.

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.

Cement, lime with sand
Masonry Cement, sand
Cement, sand with plasticizer
Intended use

Class A engineering bricks and dense concrete blocks
1:21/2 to 31/2
1:3 or 4
Class B engineering bricks
1:1:5 or 6
1:4 or 5
1:5 to 6
Bricks and blocks below DPC level
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:

Quantities per 50 kg bag of cement

Fine aggregate (m³)
Coarse aggregate (m³
App water content (liters)
Very strong water tight concrete
Reinforced concrete for floor slabs, columns, beams etc
Foundations, screeds

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


Saturday, March 22, 2014

Timber work 03- Materials and Fixings

Obviously timber is the main material used to form timber items and components. Timber is a natural material which has many advantages over other optional materials.
  1. Timber being a natural product is a renewable source. With properly managed forestry plans trees are effective in reducing carbon dioxide levels in the atmosphere thus reducing global warming.
  2. Manufacture of timber products requires much less energy than alternative materials.
  3.  Does not cause any disposal problems
  4. Has a high strength weight ratio
  5. Aesthetically pleasing
  6. Has low thermal conductivity
  7. Easily worked with ordinary hand tools
  8. Strong joints can be made with many methods.

Trees from which timber is obtained can be divided broadly into two groups – Endogenous trees which grow inwards in longitudinal fibers such as banana, bamboo and palm trees, and Exogenous trees on the other hand which grow outwards by addition of concentric rings. Timber for engineering purposes is obtained mostly from exogenous trees.
Timber can also be divided into two groups namely hard wood and soft wood depending on the hardness, density and strength. Hardwoods are heavy, strong and dark in colour. Soft woods on the other hand are light weight, with average strength and light in colour. Mature trees are felled to obtain timber and converted into commercial forms such as planks, beams, posts or columns, roof members etc. Converted timber usually contains about 50% of moisture by weight. This moisture should be removed prior to use. Removal of moisture in timber to stabilize with the humidity of the environment is called seasoning. Seasoned timber discourages moisture movement, keeps away fungus, insects, prevents warping and shrinkage, and gives long life to timber. Seasoned timber when used in timber items and components should be preserved against insects, fungus and moisture movement by the application of chemicals, paints or waxes.
In addition to natural timber manufactured timber also finds wide usage in woodwork. Manufactured timber is usually made from wood waste. This helps to minimize cutting trees for timber and manufactured timber is being advocated for many engineering uses as an option for solid timber. Some of the manufactured timbers are:

1.  Plywood – made by pasting thin wood veneers of cheap timber faced on both or one face with veneers of good quality timber.  
2. Particle Board( Made out of wood chips, saw dust etc.)
3. Chipboard( Made out of wood chips, saw dust etc.)
4.  Medium Density Fiberboard (MDF) ( Made out of wood chips, saw dust etc.)
5. High Density Fiberboard (HDF) ( Made out of wood chips, saw dust etc.)
6. Hard board ( Made out of wood chips, saw dust etc.)
7. Lamin board (Made by framing cheap timber battens and facing with veneers)
8. Batten board  (Made by framing cheap timber battens and facing with veneers)
9. Synthetic timber – made out of synthetic polymers.

Manufactured sheet timber is usually 2.4m x 1.2m in size.

Components used for assembly of timber components are termed fixings. The different types of fixings used to assemble timber are

Wire nails - A nail is a pin-shaped, sharp object of hard metal or alloy used as a fastener. Formerly made of wrought iron, today's nails are typically made of steel, often dipped or coated with zinc to prevent corrosion.  Ordinary nails for wood are usually of a soft, low-carbon or "mild" steel (about 0.1% carbon, the rest iron and perhaps a trace of silicon or manganese).

Concrete Nails
Used to fix timber components to masonry. These are harder, with 0.5-0.75% carbon.]

Wood screws –
  1.Wood screws are the most commonly-used mechanical woodworking fasteners. They are used primarily for connecting wood to wood, and are popular for the clamping force that they provide to strengthen a joint. They can also be used to attach hinges, hardware, locks and other non-wood objects.
2.The gauge of screws range from No. 6 to No.12, No. 6 being the smallest.

3.Screw lengths are from 12mm (1/2”) to 75mm (3”).

4.Screws have three types of heads:
5.  Counter sunk
6. Raised head and
7. Round head.

8. There are three types of head slots:
9. Flat or single cut
      10. Philips head and

Bolts & Nuts
Bolts and nuts are a very strong method of fixing wood to wood or metals and other materials to wood. Sizes are specified by bolt diameter and length. There are many types of threads ranging from coarse to fine (on the No. of threads per inch).  Both left handed and right handed threads are made, but the majority being right handed (tighten on turning to right hand side)

Wedges are tapered pieces of wood which can be used as fixings in the assembly of timber parts. A typical example is the wedges in the figure used to lock the mortise &Tenon.
Dowels are rounded or square tapered pieces of timber driven through component parts to join them together.

The types of adhesives (or glues) available are so vast that any material can be pasted to any surface.
Adhesives are divided into two types depending on the method of setting.
Thermoplastic adhesives set either by loss of solvent or by cooling. It will soften again by applying the solvent or by re-heating.
Thermosetting adhesives set and solidify through a chemical reaction and the action is irreversible.

Types of adhesives

Animal glue – Made from hoofs, bones and hides of animals. Avilable in cake form or as small beads. Suitable for internal use and has high gap filling properties.
Fish glue – Made of fish offal and skins. Good for small repair work. Not suitable for structural work.
Casein glue – made from skimmed milk. The powder is mixed with cold water into a creamy paste. Good for internal work only.
Synthetic resins – Synthetic adhesives are very strong, durable and can be used both externally and internally. Some of the common types are:-
  •             Phenol Formaldehyde
  •            Resorption Formaldehyde
  •            Urea Formaldehyde
  •            Melamine Formaldehyde
  •            Polyvinyl Acetate
  •            Polyurethane glue

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