Mortars

Tutor Copy

Below are some web sites that may be useful for students studying construction:-

• www.ibstock.co.uk Bricks and related sites.• www.bca.ogr.uk British Cement Association• www.buildingdesign.co.uk Building design services • www.igltd.co.uk IG steel lintels.• www.catnic.co.uk Catnic lintels.• www.constuction-index.com Manufacturers• www.mortar.org.uk Mortar industry

association.• www.insulation-installers.co.uk Directory and related

links.• www.trada.co.uk Timber research• www.tra.org.uk Timber trussed rafter • www.hse.gov.uk Health & Safety Executive.• www.ucatt.org.uk Union of Construction Allied Trades & Technicians.• www.cskills.org.uk ConstructionSkills.• www.sheffield.gov.uk Planning application

information.• www.ebuild.co.uk Self-build / land for sale etc.

Mortar

Characteristics of Materials

General purpose mortar can consist of:• Sand.• Ordinary Portland cement.• Water.• Plasticiser.• Retarder.• Accelerator.• Pigments or colouring agents.

Requirements of a Mortar Mix

• The main requirement of a good mortar mix is workability.

• Bricklayers require a mortar mix which is described as ‘fatty’. This means it hangs on the

• trowel without being sticky, it spreads easily and it does not stiffen too quickly or too slowly.

• The mortar should meet other requirements to ensure that it retains its strength and

• durability during the life of the brickwork or block work.

These requirements include:

• 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.• The workmanship of the bricklayer.• The protection of the materials and brickwork against

adverse weather.

SANDUsed to create the ‘bulk’ for mortar. There are two main types of sand used for construction purposes.

WASHED BUILDING Sand (FOR MORTARS)Pure, clean sand – has had all impurities ‘washed out’ : e.g. soils / silts / clays etc.

GRIT SandLarger grain sand – used in the production of concrete works.

SILVER sand very fine sand – used for brushing into the joints on block paving.

Sand used for mortars should pass through a 5mm sieve.

• Poorly graded sands, with single-size aggregates, contain a greater volume of air, and will

• require a greater amount of binding agent to fill in the spaces between each grain and make

• the mortar workable. Mortar made from poorly graded sand will be weaker and is more

• likely to shrink, leaving cracks into which rain can penetrate.

ALWAYS USE GOOD GRADED SAND

The Silt Test for Sand

• To carry out the test, a 250ml measuring cylinder should preferably be used.

• Fill the cylinder approximately up to the 50ml mark with a salt/water solution (one teaspoonful of salt to 750ml of water).

• Pour in the sand until the level is up to the 100ml mark.• Add more salt solution until it reaches the 150ml level.• Shake the mixture well.• Stand the cylinder on a level surface and tap it until the

top of the sand is level.• Leave the cylinder to stand for three hours.• Measure the volume of the silt layer, and the volume of

sand.

To calculate the percentage silt content, the following formula is

used:

SPECIAL CEMENTS

• As previously discussed cement is mostly made from Limestone, Shale and Gypsum. It is used as the ‘binding agent’ in mortars and concrete.

• The most widely used cement was developed in 1824 by John Aspdin, using a limestone called Portland Stone. This commonly used cement is known as Ordinary Portland (O.P.) cement.

• O.P. cement is mainly used for concrete mixes but may be used for bricklaying. If used for bricklaying, to make the mortar more workable, it would require you to add a ………………………………………

• Over the years the development of cement has created different types, for different purposes.

Types Available• MASONRY cement• O.P. cement with a Plasticiser or an Air-entraining Agent included.

Produces a ‘fatty’ mortar with high workability suitable for brickwork, blockwork, stonework and rendering.

• SULPHATE RESISTING cement• Sulphates are corrosive elements found naturally in the ground.

Sulphate crystals can enter walls below ground, or walls retaining ground and attack O.P. cement. This attack takes the form of causing the cement to expand causing cracking and splitting of the masonry.

• RAPID HARDENING cement• Chemical additives make this type of cement set and harden in half

the time of O.P. cement.

• PORTLAND BLAST FURNACE cement• Finely ground O.P. cement and granulated blast furnace slag

(waste). Slower rate of setting than O.P. cement therefore not generating the same amount of heat. Useful for very thick concrete walls, slabs or mass-filled foundations. Greater resistance to heat, seawater and acids.

• HIGH ALUMINA cement• Made from Chalk and Bauxite – black in colour. Develops high

strength in 24 hours. Great heat produced during hardening which can seriously affect its strength. Must be sprayed with water for 24 hours. Ideal for ‘emergency work’.

• WHITE cement• Special selection of ingredients – used in the manufacture of pre-

cast concrete products, reconstructed stone, renderings and for the production of cement paints.

• COLOURED cements• By mixing chemically inert pigments with white or O.P. cement.

Cement strength slightly reduced, so compensate by adding an extra 10% to the mix.

MORTAR ADDITIVES• As previously discussed there are PLASTICISERS and

AIR-ENTRAINING AGENTS produced in either liquid or powder form.

• Lime can also used for this purpose, but as mentioned earlier, lime is a very hazardous material to work with.

• Bricklaying sand can be purchased with a desired amount of lime already added. It is crucial that this type of sand is kept covered from rain at all times.

• ACCELERATORS• Added to a mortar mix to speed up the setting

process. Ideal for work in cold climates.• RETARDERS• Opposite to Accelerators – slow down the setting

process. Used in pre-mixed mortars.

• WATERPROOFERS• Chemical (liquid or powder) added to mortar.

Used in rendering or work below ground.• ANTI-FREEZERS• Supposedly allows mortars to be used when

temperatures fall close to freezing.• COLOURING PIGMENTS / AGENTS• To colour mortar – in powder form - very

expensive – tends to ‘bleach’ over time –needs care when ‘batching’.

• NOTE: All additives can weaken mortars. Should not be used below ground level.

Mix Proportions

• The crudest method is gauging the materials by the shovel-full. The amount on the shovel can vary according to the material, e.g. shovels of sand are often greater than shovels of aggregate.

A slightly more accurate method is to use a bucket to measure the materials. As each bucket full is the same size, it will have the same volume.

Volume mixing by hand using a gauge box

Dry-Silo Mortar System

• The modern equivalent of the traditional weight batching mixer is the dry-silo mortar system that is becoming very popular. The system involves the delivery of a large stand-alone silo that is charged with the correctly constituted mix materials in a dry state.

• The correct amount of water for a mix is automatically measured and added each time by a trained operator.

Machine mixing

• In general terms, this 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.

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 plasticiser etc has the necessary effect.

• Mixed mortar should never be ‘retempered’ or ‘knocked up’ with added water because this dilutes the cement/lime element of the mortar which will result in a weaker mix.

Typical Mortar Mixes

• Mortar is a mixture of the following materials in different combinations:

• Sand, cement and water.• Sand, lime and water.• Sand, lime, cement and water.• Sand, cement, plasticiser and water.

• In site practice terms, it 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 on it. The mortar strength should roughly match that of the brick or block and in no case should it be stronger than it.

Design of Mortar Mixes

Pre-mixed MortarsAdvantages

• No space required for the mixer and storage of materials.• Less transportation plant and smaller labour force required.• Quality control of the materials is the responsibility of the ready-mix

supplier.• Economic for medium volume requirements.• Consistency of colouring is easier to achieve.

Disadvantages

• Part loads are uneconomical, so the system may not be suitable for small works.

• Waiting time for delivery may cause delays.• More expensive than site mixing, so it is important to order the correct

amount to avoid wastage.• If the mortar is supplied by site mixing methods it can be delayed or

halted to compensate for any delays due to inclement weather, whereas mortar supplied pre-mixed may not have this flexibility.

Mixing Mortar by Hand on Site

• Using a shovel, repeatedly turn the ingredients into a pile making sure that all the materials are thoroughly mixed together.

• Form a hole or indentation in the middle of the pile of dry mix.

• Add water to the mix. The amount of water should be approx. 0.6 of the weight of cement (see water cement ratios).

• The materials are mixed with the water from the middle until all the water has been incorporated into the mix.

• The materials are then turned repeatedly until the mix is a fatty workable mass.

• The mix is now ready for use.• Adding more water to the mix will make the mortar easier to lay but it will reduce the strength by up to 30%.

Mixing Mortar by Hand on Site

Mixing Mortar Using Mechanical Mixers on Site

• Mixers can be powered by:• • Petrol.• • Diesel.• • Electricity.

Procedure for mixing using mechanical mixers

• Set up the mixer according to the manufacturer’s instructions, or those of the site supervisor.

• Ensure that the correct power supply is available.• Ensure sufficient materials and equipment are available.• Carry out all safety checks and start mixer.• Add the required water.• Add the pre-gauged materials, adding the sand content

first.• Add more water if required.• Allow the materials to mix thoroughly for approximately

three minutes; never longer as segregation may occur.• Turn out the mixed materials into a container or

wheelbarrow.• When work is complete, clean out the mixer.

Water/cement ratio

• The strength of mortar can be affected by the ratio of the weight of water in the mix to the weight of the cement.

• Water/cement ratio = Weight of water in the mix

• Weight of cement in the mix

• If a mortar mix has 50kg of cement, and the total water does not exceed 25 litres

• (1 litre = 1kg), the ratio will be:

• 25kg Divided by 50kg

• = 0.5

Example:

• If the ratio of a mix is specified as 0.45, then the quantity of water required for each 25kg bag of cement added to the mix will be:

25 x 0.45 = 11.25 litres (11.25kg)

EFFLORESCENCE• Sometimes, especially new brickwork, can be

seen to have a white ‘fur’ / dust deposit on the face. This unsightly effect is known as Efflorescence.

• It is caused by ‘mineral salts’ trapped in the materials used to make the brick / block etc.

• Whilst the bricks are dry these ‘salts’ are inert but when water strikes the wall they can dissolve and cause the staining. Eventually these salts will completely dissolve but the effect on new walling can be very disturbing.

• Even though efflorescence has no damaging effect to a structure it may deter prospective buyers.

• Not all bricks contain these salts so it is imperative that whilst bricks are stacked on site they are kept from contact with the ground as salts in the ground could enter the bricks.

• Efflorescence can also appear on the mortar joints. This could be caused by the sand initially containing impurities or poor site storage of sand

WORK IN COLD WEATHER

• Building materials are quite capable of withstanding effects from bad weather and low temperatures. It is usually a poor design fault that causes materials to fail e.g. walls being constantly wet.

• If materials are constantly saturated their pore structures are virtually filled with water. When water freezes it attempts to expand and if the materials are unable to resist the stresses they will break down. (SPALLING)

• This effect is known as Frost Damage.• Building materials should be kept as dry as possible,

before use, and after construction.

• Mortars are particularly vulnerable (contain water in the mix), before they have set, as once frozen they will be unable to set. The mortar will be permanently damaged

• Bricklaying should be stopped when a falling air temperature reaches 3 degrees Celsius

Remember• WALLS DRY – cover with a waterproof

sheet – polythene can create dampness underneath – think about the wind?

• WALLS WARM – cover with HESSIAN sheeting – use insulation slabs.

• SAND DRY & WARM – keep covered – store inside if possible – use hot air blowers.

JOINTING & POINTINGWhen bricks have been laid to form a wall, the surplus mortar is scraped flush with surface of the brick face. This type of surface finish is called

Flush Finish.

Why Point• The main purpose of any joint finishing is to

improve the wall’s resistance to rain or the ingress of water through the joint, as well as enhancing the appearance of the wall.

• This is achieved by packing mortar into the joints so that it has close contact with surface of

the brickwork.• Jointing up brickwork is a critical part of building

face work, and it should not be hurried.

• One of the most important aspects of jointing up brickwork is to avoid smudging or staining the surface of the bricks.

• Timing is probably the most important aspect of jointing up. The correct time to joint-up will be determined by the suction rate of the bricks being used and the weather conditions when the bricks were laid.

• The consistency of the mortar is also crucial as to when pointing takes place. If the mortar is too wet, the joint will not be crisp and the mortar may be dragged out from between the bricks.

JOINTING• The term used for ‘finishing off’ mortar

joints, of the brickwork, before the mortar hardens. (as the work proceeds)

Profiles of Mortar Joints

Keyed / Half round

Raked or square-recessed

Weather struck

POINTING• Cutting out and pointing back at a later

date

TO RE-POINT AN EXISTING PROPERTY

• Carefully remove old mortar, to a depth of 10mm, trying not to damage the surrounding bricks.

• The old mortar must be removed ‘squarely’ off all the brick arrisess.

• Use a sharp plugging chisel or hand grinder. • Wear the appropriate P.P.E. and consider

other people.• Remove all traces of old or loose mortar and

brush joints well, to remove dust. Work from the top, downwards.

• Mix mortar to specified mix and workability. ( 1:4 )

• Use a fine water spray to dampen joints for a small area (0.5 square metres)

• Using pointing trowel and hawk, force mortar fully into joints and neatly finish off.

• Protect pointing from weather, until set.