Frequently Asked Questions
We are always happy to provide advice to our customers. To save you a little time, we have collected together a list of the questions we get asked most frequently, together with the answers we give.
- Where can I get advice on the most suitable sand to use on my construction project?
- Why should I not use a Blinding Sand to make plaster or mortar?
- What does it mean when sands are described as sharp, soft, fine or coarse?
- Can I lay paving bricks onto Blinding Sand?
- What is the difference in strength of concrete produced using a 10mm and 20mm All-in Ballast respectively?
- I want to lay a gravel path in my garden. What size and grade of gravel should I use?
- My farm track is potholed and in need of repair. Which material should I use?
- What is the difference between a screed and a render?
Q: Where can I get advice on the most suitable sand to use on my construction project?
A: The British Standards Institution is an independent national body, incorporated by Royal Charter, which defines the characteristics and properties of materials for various uses. Each materials grouping is assigned a British Standard Number for reference. The published results - which are continually upgraded - are the result of exhaustive tests and research to ensure that the materials are fit for the purpose.
The British Standards Institution also publish Codes of Practice which define how the materials should be used or combined in practice. These standards and codes can be obtained directly from the British Standards Institution or via an Architect, Engineer or Local Authority Building Control Office; they are also available from some public libraries. For specialised work, various bodies such as The Sports Turf Research Institute, CADW, English Heritage, The Society for the Protection of Ancient Buildings, etc., can assist. Trade bodies such as The Mortar Producers Association, Interpave, The Brick Development Association, etc., also offer guidance.
See Resources for more on the trade bodies, building control, specifications bodies etc.
Q: Why should I not use a Blinding Sand to make plaster or mortar?
A: Blinding Sand is a very fine sand which does not meet the British Standards requirements for sands for mortars, plasters or concrete. Because the sand is very fine it has a large surface area. The cement content laid down for various mixes would not be enough to coat such a large area of grain surface, resulting in a very weak strength mix. The absence of larger grains would reduce the stability of any mix and cause the mortar to ooze out of joints when green and under load e.g. brick on brick or block on block. The fineness of the sand would not create an effective locking mechanism between adjacent layers of grains which makes the exterior surface susceptible to blistering. The high water retention characteristics of a fine sand leads to a high drying shrinkage which in turn causes crazing.
Q: What does it mean when sands are described as sharp, soft, fine or coarse?
A: Sharp and soft refers to the shape of the grains irrespective of size. Sharp means an angular or subangular grain. Soft means a rounded or subrounded grain. Fine or coarse, on the other hand, refer to the grain size irrespective of shape, with coarse referring to a larger grain size, and fine a smaller one.
Q: Can I lay paving bricks onto Blinding Sand?
A: Blinding Sand is not a suitable medium as a bedding course for brick or block pavers.
The retention of water within porous media is directly related to the size of the pore channels -- water retention is controlled by grain size. The bulk of the particles in Blinding Sand are in the 200 - 100 micron range, consequently it retains moisture and will swell when frozen. It is too stable when laid moist and would not 'settle up' into the joints on initial compaction, also the particle diameters are not compatible with the surface texture of the pavers and will fail to provide the proper frictional grip.
The most heavily trafficked pavements require sands to be virtually free of grains smaller than 75 microns. A Building Sand, for example, would be permitted to have 4% of grains of this size and would therefore not be fit for purpose. Crushed stone dust would normally have an even higher proportion of grains smaller than 75 microns. Suitable sands for brick and block pavers are specified in BS7533 part 3 and are listed in five categories (IA, IB, II, III and IV) according to duty.
See Laying Course Sands.
Q: What is the difference in strength of concrete produced using a 10mm and 20mm All-in Ballast respectively?
A:A 10mm All-in Ballast will generally have a greater surface area than 20mm All-in Ballast, and consequently requires slightly more cement to coat the grains to achieve an equivalent strength mix. However, the difference is slight - about one third of a ballast is made up of sand or fine aggregate, and this sand will make up the greatest part of the total surface area of the whole.
Q: I want to lay a gravel path in my garden. What size and grade of gravel should I use?
A: If you intend the pathway for pedestrian use only then a 10mm, 14mm or 20mm "single sized" gravel is the usual preference for aesthetic reasons. However, if for example a wheelbarrow is to use the route, then a single sized material is too easily displaced under the load of the wheel and a 20mm "graded" product would be preferred. (A graded product has a wider range of particle sizes than a single sized product).
Q: My farm track is potholed and in need of repair. Which material should I use?
A: Pothole repair is normally done by matching the existing base, sub base and capping layers displaced. Generally, a 40mm Crusher-Run or sub-base material will be adequate, but ensure that the edges of any shallow pothole are trimmed vertically. You can do this by running a pick around the edge before making the repair. This will lock the repair in place (a saucer shaped repair will be easily displaced again).
Q: What is the difference between a screed and a render?
A: Render is applied to perpendicular surfaces. It is applied in successive progressively weaker coats to absorb movement. Each undercoat is scratched to form a key except when underlying a gypsum skim. Its purpouse is to create a plane surface (iron out the bumps)and/or to protect the base wall. It can contain sand, lime, cement, gypsum, additives, fibres etc. Renders are normally about 20mm thick. Top coats can be steel trowelled to a smooth finish but are generally finished with a gypsum skim coat. Thick coats of render tend to fall off the wall or be dragged down and crack.
Floor screed is applied to generally horizontal surfaces. It can contain sand,lime,cement,gypsum,grit,additives fibres etc. It can also be a self levelling thin sand/polymer mix. It is usually applied to form a smooth level surface.
A sand/cement screed will usually be applied in a dryer state than a render. A screed needs to bond with the floor unless it is a thicker floating screed over say insulation or tanking. This bonding could be done by scabbling a key or nowadays using PVA. A keyed floor needs to be dampened with water so as not to extract the moisture from the screed. The floor would normally have been constructed to exclude moisture with an underfloor membrane linked in to the DPC. Otherwise it might be tanked with a bitumenised product before screeding again linking in to the DPC. Waterproofing the screed is another rarely used option but care must be taken to avoid moisture rising at the edges.