General Purpose Concrete Block Paving Hints

If properly designed and constructed, Concrete Block Paving will last for decades with little or no maintenance being necessary.

Which thickness of paving should I use?

As a general rule use 50mm-60mm for domestic use, and 80mm for industrial use.

Should plastic sheeting be used over the bedding sand?

The use of plastic sheeting is NOT recommended. The sheeting can inhibit drainage and lead to excessive efflorescence.

Should cement be added to the jointing sand?

Cement should not be added to the jointing sand as this converts a flexible surface into a rigid surface not able to accommodate flexural or thermal movement stresses. Also cement tends to discolour the paving and is difficult to remove afterwards

What maintenance needs to be carried out?

Jointing sand should be replaced if necessary during the first months. Weeds originate from wind blown seeds lodging in the jointing sand and should be sprayed with a suitable herbicide. Oil stains should be scrubbed with a hard bristle brush and strong detergent and washed down with water.

What is efflorescence?

The most common form of efflorescence is lime bloom and it is particularly noticeable on coloured pavers. It is a white deposit which is apparent as white patches or as an overall lightening in colour. The latter effect is sometimes mistakenly interpreted as the colour fading or being washed out. The cause of lime bloom lies in the chemical composition of cement. When water is added to cement, a series of chemical reactions take place, which result in setting and hardening. One product of these reactions is lime in the form of calcium hydroxide. Calcium hydroxide is slightly soluble in water and, under certain conditions, can migrate through damp concrete or mortar to the surface and there react with carbon dioxide from the atmosphere to produce a surface deposit of calcium carbonate crystals. This deposit is similar to a very thin coat of white-wash and gives rise to the white patches or lightening of colour mentioned previously. The surface deposit is normally extremely thin and this thinness is demonstrated by the fact that, when the concrete is wetted, the film of water on the surface usually makes the deposit transparent and the efflorescence seemingly disappears. The occurrence of lime bloom tends to be spasmodic and unpredictable. Nonetheless an important factor is the weather. Lime bloom forms most readily when concrete becomes wet and remains damp for several days, and this is reflected in the fact that it occurs most frequently during the winter months. Extended periods of rain and cold weather in particular are conditions most likely to precipitate a severe manifestation. Concrete paving is normally only prone to lime bloom in the early stages of its service life. In general concrete, which has been in service for a year without being affected, can be considered immune. Lime bloom is a temporary effect and, given time, usually disappears of its own accord. It is purely superficial and does not affect the durability or strength of the paving.

How can efflorescence be removed?

Lime bloom is a transient phenomenon and can be expected to disappear with time. It typically disappears within a year. If immediate removal is required, this can be achieved by washing with dilute acid. This is a relatively simple operation, but care should be taken on two counts. Firstly, acids can be hazardous and appropriate safety precautions must be taken. Secondly, acid attacks concrete and over applications to a concrete surface can result in acid etching altering the texture and appearance.

Generally a 5% solution of hydrochloric (pool) acid is used. Before the acid is applied, the surface should be dampened with water to kill the initial suction. This prevents the acid from being sucked into the concrete before it has a chance to react with the surface deposit. The acid is applied by brush or spray and a typical application rate is 1 litre of acid solution to 5-10m². Following the acid application the surface of the paving should be allowed to dry out and is then inspected. Often one wash with acid is sufficient but in more stubborn cases the treatment is repeated as necessary until the lime bloom disappears. Finally it is normal practice to give the paving a final wash with water. When carrying out acid washing, always test the effect on an inconspicuous area. Protective clothing should be worn at all times when working with acid.