BEHAVIOUR OF GRANITE SURFACE PLATES

BEHAVIOUR OF GRANITE
When we investigate the changes in granite surface plates over  time and with usage, we find that the surface topography is not retained with changes in atmospheric conditions like temperature. So the behavior of the plate has  to be studied under varying conditions like,
•  Temperature
•  Change in humidity
to determine their effect on the accuracy of the surface plates. 

First the surface plate is divided into squares of 100X100mm. An electronic level with a sensitivity of 0.0005/100mm placed on a bridge is used to measure the flatness of the surface. Readings are taken along and across the surface and the surface topography is computed to attain the contour. This bi-directional approach to every point gives accurate measurements of the surface.

Now the behavior of the granite is observed at various temperatures. We can see a significant change in the contour of the surface due to temperature rise. But the magnitude of the change cannot be explained by the coefficient of thermal expansion of Dolerite rock, which is 2x10 ^-6 to 4X10 ^-6 per degree centigrade. At 26°C, we can notice the surface contour seems to be concave with overall flatness accuracy of 5.5 microns. When there is a rise in temperature to 40°C the surface contour changes to convex with overall flatness accuracy of 8.6 microns. This is the behavior of granite due to change in temperature.

Changes in contour due to a change in atmospheric humidity is very insignificant. The change in contour of the surface with change in humidity of 20, 40 and 60% has been shown to be negligible.When the surface was soaked with water and measurement taken within a short period the variation in surface contour is negligible. However a change was observed when the surface was allowed to be covered and soaked with water for 12 hours. For a study, we took a granite plate of size 400X400X100mm. Before soaking in water the flatness was checked as 3.48 microns, when the top surface was soaked in water briefly, the flatness was recorded as 3.24 microns. Now the top surface was soaked in water for 12 hours, the reading of flatness was recorded as 2.34 microns and when all sides soaked in water for 3 hours, we got the flatness reading to be 1.91 micron. This was the effect of water on the accuracy of the surface plate.


However it is observed that for both the water soaked condition and as well as for temperature change the surface returns to the original surface topography, once the surface plate is brought to its original atmospheric condition. Studies on igneous rocks such as Granite and Dolerite showed that,
•  The igneous rock has pores and capillaries
•  The material consists of distinctly different crystals of different materials.

As stated above granite has pores and these can have with them entrapped moisture. The moisture content is maximum at the surface and decreases in the deeper layers. When the stone is soaked in water and the surface is dried and after some time it is broken into two halves, the newly exposed upper layers have distinctly higher content of water whereas it is less wet as the depth increases. We know that the igneous rock like Granite, Dolerite, etc., consists of various substances such as Quartz, Hornblende, Calcite, Orthoclase, etc., Due to this various ingredients of stone have uneven volumetric or linear expansion within them. For example quartz expands four times more than the feldspar and twice as much as hornblende. Due to rise in temperature the quartz exerts a pressure against its surroundings and causes an expansion of the surface, which even if small, affects the surface accuracy. The variation of the surface topography thus is an inherent phenomenon due to the properties of a stone. Its value changes from stone to stone. It is more in stones with higher porosity and quartz content.

Further investigation reveal that, granite expands only about 0.036% at 25ºC whereas water expands more than 0.32%, at the same temperature if it is not compressed. If the walls of the pores prevents water from expanding it can exert about 70 atmospheric pressure against the pore walls. Thus the entrapped moisture or water in the pores comes to an equilibrium pressure due to capillary action and there will be a pressure on the walls of the pores, which act as small pressure vessels causing the surrounding portion of the rock to expand due to this internal pressure. However this pressure will be maximum at top layer of the surface and reduces in the deeper portion. Thus the upper layers of the stone expand more than the lower causing the surface to swell.