· Concrete Mix and Mortar/Screed Mix are similar in some respects.
· They are both made from the same basic materials: cement, water
· and aggregate (sand, stone or gravel).
· But, when it comes to their uses as building materials, concrete and screed mix are very different.
· : Quality concrete is used for construction and structural tasks.
· : While mortar/screed is used/applied as a top layer to a concrete/brick base.
Physical differences between concrete and screed:
Concrete and screed are both made from a mixture of cement, water and aggregate. The main difference between the two materials is the type of aggregate that is used.
When mixing concrete, you add coarse aggregate like gravel to the mix. These small stones are normally 20mm or less in size. This coarse aggregate gives the concrete its strength, durability and workability, and makes it suitable for structural work.
For mixing screed mix, you use fine, sharp sand particles, with a maximum grain size of about 2mm, cement, water and sometimes lime.
Sharp sand is more coarse than building/soft sand and is more perfect for mixing with other sands to prevent cracking during the drying process. It’s often used in situation where a slightly thicker layer of mortar is required such as chimney flaunching, bedding roof tiles and many garden projects will require sharp sand.
Plasterers sand is not as coarse as sharp sand, but not as fine as soft sand. Its washed to remove salts and clay residue that could cause efflorescence salting.
Lime can be used as an alternative to cement in some mixes or used in conjunction with it. Lime allows materials such as bricks to breathe, it’s commonly used on older period buildings with soft, permeable bricks that would likely crack if cement was used.
Plasticiser is a liquid that is added to the mix to make the materials easier to work with and also slows down the curing process slightly. It makes a typical mortar mix sticky and easier to point over.
Water proofer is different to plasticiser, it does what it says and is often used when rendering.
Frost proofer, accelerators and colour additives can be included in the mix as required.
What Happens if you Make the Mortar too Weak?
Not only will the mortar fail to bind sufficiently, it may also crumble after a short amount of
time or wash away after minimal weathering.
What Happens if you Make the Mortar too Strong?
Too strong a mix, e.g. made with too much cement and your mortar many dry to quickly, thus
it will dry to quickly, thus it will shrink and crack. Cracking can be avoided by reducing the
amount of cement in the mixture and ensuring the mortar is wet enough.
The best mix will depend on the type of brick and the location of the wall. For most domestic builds such as a house building, use one of these mixes:
· 4 parts soft sand with 1part cement, add water and plasticiser.
· For soft older bricks already bedded onto a lime based mortar, a lime and sand
mix should be used, the ratio will depend on the type of lime and how much water content it has.
· For retaining walls or anywhere likely to be in regular contact with water, use 3
parts soft sand and one part cement, 1 part lime is optional depending on the type of brick or block.
For chimney repointing in a modern home, the best mix will depend on how exposed the chimney is and its location. For chimneys in wind swept rainy parts of the country, use a mix of 3 parts soft sand and 1part cement. For softer or more permeable bricks, 4 parts sand, half part cement and 1 part lime can be used. Plasticiser is optional.
Chimney flaunching will receive a lot of rain so the mix should be strong, this prevents it washing out. Because flaunching is laid several centimetres thick, add sharp sand to the mix to
avoid cracking during the drying process… 2 or 3 parts soft sand, 1 part sharp sand, 1 part cement and half-part lime is optional.
For older period homes, a lime-based should be used instead of cement.
Paving Mortar Mix:
· For bedding under the slabs use 5 parts sharp sand, 1 part soft sand and 1 part cement.
· For pointing use 4 parts soft sand and 1 part cement.
· For high traffic areas, a strong mix of 3 parts soft sand and 1 part cement can be used.
Render Mortar Mix (Modern Buildings)
The first coat of render should ideally be slightly stronger than the second coat. While some tradespeople make the two mixes the same strength, the second coat should never be stronger than the first coat.
A pure soft sand mix shouldn’t be used when rendering. It’s preferable to use either sharp sand, plasterer’s sand or a mixture of the two. Soft sand can be added to the mix but it shouldn’t
comprise of more than 25% of the entire mix.
The first coat of render can be 2 parts plasterer’s sand and 2 parts sharp sand, 1 part cement and half part lime. Waterproofer can be applied to the first coat if the property is modern.
The second coat must be slightly weaker than the first, so 3 parts plasterers sand, 2 parts sharp sand, 1 part cement and half lime. Plasticiser can be used in the second coat.
Locations that are sheltered, a slightly weaker mix can be used. For the first coat 4 parts sharp
sand can be used and for the second coat 5 parts sharp sand can be used.
Modern buildings are constructed with thermal efficiency in mind and sealants, waterproofing materials and cement based products can be used.
Period properties are designed to breathe, floors and walls are often constructed with lime, being a material that lets the moister in the building escape. Covering breathable materials with
non-breatherable cement, gypsum plaster or waterproofing sealers lead to significant damp problems.
For period properties we suggest seeking the advice of a specialist who understands how older buildings are designed to breathe.
Floor Screed Mix:
For modern homes: 4 parts sharp sand and 1part cement. Although anything from 3-5 parts sharp sand to 1 part sand would be acceptable for most situations.
For period buildings: Lay a lime-based screed as the floor needs to breath. If you don’t, the
Cement in the screed will push moister towards the walls where the soft brick and plaster will absorb the water.
Best Mortar Mix for Roof Tiles:
Because roof tiles are located in an exposed location that’s likely to experience rainfall, a strong mix should be used. Unfortunately, some profiled roof tiles require a very thick bedding of mortar, so to reduce the risk of cracking, sharp sand should be introduced to the mix for most tiles.
· All tiles except Plain tiles ….2 parts soft sand, 1 part sharp sand and 1 part cement.
· For tiles where only a thin bedding of mortar is required (i.e. Plain tiles) 3 parts soft sand, plus 1 part cement and plasticiser.
· It is recommended choosing a soft sand that is fairly coarse, avoid soft sands that are at the “silty” end of the spectrum.
How Much Water is Too Much??
The firmness of the wet mortar can be altered by changing the type of sand, the sand/cement ratio and the amount of water added to the mix.
Most bricks and blocks are quite porous and as only a thin layer of mortar is required wet or ‘sloppy’ mix is often preferable.
Some roof tiles require a bedding layer of mortar that’s inches thick and a wet/sloppy mix would not be suitable, it would slump off the tiles.
For roofing projects, a firmer mixture is generally required.
Needless to say the mortar should never be so firm that it does not have enough water content to create a chemical reaction. Neither should it be so sloppy that it’s impossible to work with.
What’s the Lowest Temperature I Can Use Mortar??
Generally 5 degrees is the lowest temperature that mortar can be laid. Any colder and there is a risk of frost which will weaken the finished mortar. Frost proofers can be added to the mortar but shouldn’t be relied upon.
Colourants: Can be used but only use as much as you need and excessive amounts.
Accelerators: These speed up the drying time but often leave you with little time to work with the mortar before it starts to set. If you continue to work with mortar that has been set, you’ll reduce the strength of the finished mortar. Therefore, you should only use them when necessary.
Do not forget: Australian Standard Building Regulation/Codes for Roof Tiles:
Require all roof tiles, including ridges, valley tiles and verges must be secured with, screws, nails or clips…..in addition to standard water proof mortar.
Note: Forced Action Mortar Mixers are specifically design for mixing all types of compounds: Mortars / Renders/Screed Mixtures /Rein Mixtures and general Plaster mixtures.
This is what gives screed its fine, tightly packed texture and makes it suitable for applying as a top layer to a concrete floor.
concrete and fine aggregate for screed. We are able to precisely control the ratios of raw materials being mixed, which is how we create different types of concrete and screed. The concrete or screed is mixed fresh onsite and barrowed or pumped to where you need it. Concrete and screed mix designs can both be altered to meet your requirements. We can adjust the ratios or introduce admixtures to give the material different qualities. If you need a lightweight concrete or screed, for example, an aggregate like LYTAG could be used in the mix. Or, for a higher compressive strength, we can adjust the water-cement ratio as needed.
The benefits of concrete and screed
Concrete has the potential to hit a very high compressive strength, which makes it an ideal building material. It is used in everything from walls, driveways, patios and roads to piling, concrete floors and other building structures. As well as being immensely strong, concrete is durable, economical, long-lasting and versatile. It is also non-combustible, which means that it could be advantageous in the event of a fire. The main benefit of readymix concrete, in particular, is that it can be batched to your exact specifications and amount using volumetric mixers.
Screed is generally used to create a smooth, bump-free finish to a concrete floor or surface. As well as improving a concrete base aesthetically, it can also lengthen its life – the compact mix