The right limestone for outdoors

The demand for limestones for outdoor use is rising constantly. It is easily overlooked that not every limestone can withstand the high loads involved. To avoid nasty surprises, a number of criteria must be observed when selecting the stone.
By Reiner Krug

The use of limestones outdoors has a tradition of thousands of years. Examples of this are the Egyptian Pyramids and innumerable bridges, churches and castles in Europe and especially in Germany. Limestone is also a popular building material in modern architecture. The demand is growing constantly particularly in landscaping – not least due to the trend towards construction based on typical landscape features. The applications are varied: from walling, dividing and heavy walls, steps, floor coverings, fountains and furnishings to sculptures.

A gradient is necessary
Limestones used outdoors are subject to extremely high loads. One major stress in outdoor areas is caused by moisture. All outdoor surfaces must have a gradient to discharge surface water. The rougher the surface, the steeper the necessary gradient. The gradient must be at least 1.5 per cent for normal surfaces and at least 2 to 3 per cent for rough surface finishes. Adequately dimensioned water gullies must also be available for drainage.

Coverings in outdoor areas are never watertight. This means the sub-bases must also be drained and should have the same gradient as the surface coverings. All levels of the sub-bases carrying water must also be connected to gullies or drainage pipes provided.
Water-permeable bedding layers of pebble/grit or cement/concrete have proved advantageous for this purpose. Drainage mats encourage rapid and systematic discharge of the moisture.

Frost resistance
The frost resistance of natural stones is also directly related to the moisture absorption. Stones that absorb and store lots of water via capillaries can be damaged by the effect of frost. A stone can be rated as suitable for use in terms of weather resistance if it has proved itself under climatic and installation conditions that were at least as unfavourable as those that can be expected during the intended use. If no experience is available regarding frost resistance, the stone must be tested for water absorption to DIN 52103 and frost resistance to DIN 52104 Method B with at least 25 freeze-thaw repetitions.

It is often (falsely) assumed that specially dense and firm limestones also have particularly good weather resistance, but this is not correct. Particularly very dense limestones can absorb a lot of moisture via fine capillaries and the ice pressure then causes chipping. On the other hand, the travertines with many pores are very frost resistant, as the large cavities are never filled completely with water and sufficient free space is still available when the ice forms. De-icing salts should not be used – also for environment protection reasons. Many limestones are sensitive to de-icing salts and this can result in chipping on the surface of the stone. Only sufficiently dry limestones should be laid at cold times of the year – when danger of frost exists.

 
Variations possible
As with all sediment stones, large variations of strength and weather resistance occur in limestones due to their creation process. Not every limestone is suitable for use outdoors under unfavourable climatic conditions. Different layers with different frost resistance are often even extracted in the same stone quarry. It is therefore necessary to ensure that the tested specimens correspond to the population of the limestones supplied.