Being able to make the correct concrete mix and knowing how to mix mortar are essential skills for any ardent DIY-er. Concrete and mortar Mix ratios vary depending on the project you are working on. Concrete is used in many places around the home, for laying a firm sub-floor or a base for paving, whilst Mortar is the most important bit in any wall. Both Mortar and Concrete is made with a mix of sand and cement, with Concrete also containing coarse aggregate (small stones) for extra strength. This guide will explain the components of concrete and mortar, how to choose the right mix ratio and how to keep concrete looking good for years after it is laid.
The standard type of cement, the type you will most likely use for general DIY tasks, is OPC or Ordinary Portland Cement. OPC is air setting, meaning that the moisture in the air will cause it to harden if not sealed in bags. It is grey and usually available in 50kg bags. Although there are several other types of cement available, the other two main types you are most likely to come across are White Portland Cement and Masonry Cement. White cement is used in areas where the finished appearance is important, whilst masonry cement has additives to increase its usability in mortar and render mixes. If you need to mix large amounts of cement/concrete, it my be worth buying or hiring a cement mixer.
Cement is highly alkaline and the setting process is exothermic (it produces heat as it sets). This means that wet cement can cause painful “cement burns” if it gets in contact with your skin for even a short period of time. Dry cement powder can also be dangerous if it gets into your eyes or throat. Always wear gloves when handling cement, and take care when mixing or applying it.
Aggregates are used to give body and strength to a concrete or mortar mix. Coarse aggregate is small stones, usually less that 20 mm in diameter, and fine aggregate is basically sand. Sharp Sand (which has larger grains) is used for concreting or laying paving slabs and Soft Sand is used for bricklaying, etc. It is also possible to buy mixed aggregate containing both coarse and fine aggregates.
In general, the aggregate component of a concrete or mortar mix is usually several times that of the cement. For example, a simple sand and cement mix should be at least 3 part sand (the aggregate) to one part cement. A strong concrete mix would be something like 1:3:5 (Cement, Sand, Coarse Gravel). In this case, both the sand and gravel are the aggregate. In pre-mix concrete, the aggregate is already mixed with the cement.
Aggregates have been used in construction for thousands of years but it was the Romans who refined and, to some extent, perfected the process.
Mixing your own concrete or mortar with separate bags of cement and aggregates is often the cheaper option, but if you only have a small job to do, you can buy bags of ready-mix or Pre-Mixed Concrete and mortar. Pre-mixed cement and mortar simply needs water added to it in the correct quantities. For bigger jobs, you can also buy wet pre-mixed concrete and get it delivered. However, you need to make sure you have enough helping hands to get the wet concrete in place before it sets.
There are several things that can be added to a mortar or concrete mix. It is possible to get pigments to change the colour of both mixes, but these need to be used with care. Most pigments will affect the usability of the mix, and you might need to think about adding lime or plasticizer to balance this.
Concrete and Mortar Mixes
There are several different mixes you can employ. The proportions of each individual element you add are dependent on the job at hand. A strong concrete mix is one with a higher percentage of cement, whilst a weak mix uses less.
General Purpose Concrete – 1:2:3 mix
Ideal for most uses except foundations and exposed paving. It is composed of one part cement, two parts sand and three parts coarse aggregate. If using combined aggregate, this mix would be 1:4, one part cement to four parts combined aggregate.
Foundation Concrete – 1:2 ½:3 ½ mix
Ideal for wall foundations or bases and laying paving slabs, etc. One part cement, two and a half sand and three and a half coarse aggregate. If using combined aggregate, this mix would be 1:5.
Paving Concrete – 1:1 ½:2 ½ mix
Used for exposed paving such as driveways and garage floors. One part cement, one and a half parts sand and two and a half parts coarse aggregate. Combined aggregate would need a mix of 1:3 ½.
Standard Mortar 1:5 mix
Used for internal or sheltered bricklaying. Mix one part cement to 5 parts soft sand. Add a small amount of lime or one part plasticizer to increase the workability. This type of mortar mix is perfect for use in an internal block partition wall.
Strong Mortar 1:4 mix
Used for exposed brickwork. Mix one part cement to 4 parts soft sand. Again, add a small amount of lime or plasticizer to increase the workability.
Mixing Concrete and Mortar
It is always best to mix concrete or mortar on a piece of board (spot board). This helps to keep the mix free from stones or dirt on the floor, and also avoids leaving a concrete stain. Start by mixing the separate components thoroughly whilst dry. This applies to pre-mix as well. Once the mix looks uniform in colour, make a small well in the middle of the mix and slowly add water. Work the mix in from the edges until the first bit of water is absorbed. Now make a new well and repeat the process. Continue doing this until you have a uniform mix. If you draw your spade across the mix and it leaves a smooth finish with no water seeping out, the mix is just about right.
Concrete Curing Tips
Concrete can take a very long time to completely dry out (Cure), particularly if it is laid thickly. It is important that the curing process is not too fast (or too slow) if you want the best results. Ideally, curing should continue for at least 7 days during the summer and 10 days in winter.
Moisture Loss – You should cover wet concrete with a polythene sheet as soon as it is laid and level. This will help to stop moisture evaporating too quickly.
Cold Temperatures – Concrete should not be laid in very cold conditions. If you lay concrete and think that the overnight temperature will be below freezing, you should create an air gap between the polythene sheet and the concrete surface using battens of wood. The air gap will help to insulate it from the cold.
Maintaining Concrete Surfaces
There are very few houses in the UK which do not include some concrete in their construction or in the garden. Concrete, as with any other building material, will suffer from the effects of damp and weather if it has not been treated properly (and sometimes even when it has).
Concrete on driveways or the floors of garages often gets stained with oil and grease. Soak up fresh spills straight away and sprinkle dry sand over the top to absorb as much of the liquid as possible. Sweep the sand away after a day or two and then wash out the stain with white spirit or a degreasing solution.
Newly laid concrete has a very high alkali content and efflorescence can form on the surface as it dries. Let the concrete dry completely before attempting to apply any sort of surface finish. Concrete dries very slowly, indeed, in perfect conditions 4″ of concrete could take 4 months to completely dry. Remove any efflorescence as you would with brickwork and then treat the surface with a clear sealant solution or general purpose primer.
If the surface of concrete is over-trowelled as it is laid, a dusty and loose layer can form on top of it as the cement is drawn out. In the event that this happens, you should let the concrete dry and then treat it with two coats of a PVA bonding agent (1 part PVA to 5 parts water). The same solution can be used to prime any very pourous concrete surface.
Repairing Cracks or Holes
Clean out the crack with a stiff brush to remove any debris. A very thin crack (5mm or less) should be opened up slightly with a chisel so that any filling will hold. If you can, undercut the edges to form a lip that will help to hold any filler. Primer the inside of the crack with a bonding agent, diluted as instructed by the product manufacturer. When this is dry, apply the appropriate filler (depending whether the crack is outside or inside the house). If you are filling a larger hole, mix up a standard concrete mix using a fine aggregate. Brush out and prepare the hole as you would with a crack, and then fill with the concrete mix.
External concrete is prone to spalling. This happens when moisture invades the surface of the concrete and then freezes. As the frozen water expands it can cause the surface of the concrete to break away. If the concrete contains steel reinforcement, this effect can be accelerated as the exposed steel corrodes. You can fill the hole or crack in the same way as described above, but you will need to treat any exposed metal with a primer to stop the corrosion continuing. To prevent further spalling, treat the concrete with a bitumen undercoat and then paint over with a reinforced emulsion.