In my earlier post, I talked about the types, classifications and examples of stones/rocks as a traditional civil engineering material.

In this post, let us look at some of the properties of stones for engineering works.

How would you identify a good stone for engineering applications?

The following are some of the properties of stones/rocks that should be considered before choosing them for engineering works.

Strength: Strength is an important property of rocks that should be looked into before selecting a building stone. IS code specifies a minimum crushing strength of at least 3.5N/mm2 for a building block. A factor of safety of 10 is used to find the permissible stress in stones due to non uniformity of materials. As a result, even laterite can be used to safely build a single storey building because stresses developed in such a building are highly minimal and hardly exceed 1.5N/mm2. Care should however be taken to check the stresses when concentrated loads are placed on laterite walls especially in stone masonry.

Find below the crushing strengths of common building stones:

Laterite                –                  1.8 – 3.2N/mm2

Limestone            –                 55N/mm2

Appearance: If a stone has a uniform attractive colour with compact grains, such a stone is durable. Granite and marble get good appearance when painted in buildings hence they are often used for face works in buildings.

Texture: There is a tinge of attractiveness in fine grained stones with homogenous distribution. They are often durable and strong and are utilized for carving works.

Density: Denser stones are stronger while less dense stones are weak. As a result, stones with specific gravities less than 2.4kg/m3 are generally not suitable for building works.

Percentage Wear: This is an important property of stones to be considered in selecting stones for road work and railway ballast.

Percentage wear of stones is determined by attrition test. A good stone should not show more than 2% wear.

Hardness: Hardness is an important property to be considered in choosing stones for pavement and flooring purposes. Coefficient of hardness of stones is determined from standard specimens tested in Dory’s Testing Machine. For pavement construction, coefficient of hardness should not be less than 17 while for flooring and building purposes, coefficient of hardness of less than 14 is generally not suitable.

Dryness/Seasoning: Stones which are obtained from quarries contain pores filled with moisture. Moisture is removed from stones through a process known as seasoning which improves the strength of stones. Natural seasoning is the preferred method. This entails exposing the stones to atmospheric conditions like wind and sunlight for a period of 6 to 12 months. This is highly required for laterites.

Absorption & Porosity: All stones have pores hence, absorb water. When water reacts with the material constituent of stones, it causes disintegration of stones. As a result, stones used for construction should have a low moisture absorption percentage, generally not more than 5. This is determined by absorption test which entails immersing stone in water for a period of 24 hours and then determining the percentage of water absorbed after this period.

Weathering: Stones usually undergo weathering due to the action of wind and water. This can deter the appearance of stones. Stones with good weather resistance should be used for face works of buildings and constructions.

Ease in Dressing: Cost of dressing contributes to masonry cost to a great extent. Stones with lesser strength are easier to dress than stones with high strength hence, an engineer should consider choosing stones with sufficient strength rather than stones with high strength while choosing stones for building works.

Fire Resistance: Sand stones have a higher resistance to fire. Though argillaceous materials have poor strength characteristics, they have good resistance to fire.

Toughness: This is a measure of the resistance of a stone material to impact. It is usually determined by impact test. For roadworks, stones should have a toughness index of 19 and above. Medium toughness stones are stones with a toughness index of 13 – 19 while stones with a toughness index less than 13, are generally poor stones.