What is Thermal Movement?

Concrete moves with changes in temperatures, expanding when heated and contracting when cooled.  While the amount of movement is not significant, stresses induced by thermal expansion can be. 

If the concrete slab was free to move without restraints, stresses at the edge of the slab would be low.  However, if ACO Drain Systems are positioned to restrain slab movement, the stresses would be high and therefore the channel system would be susceptible to damage.

The type of aggregate used in the concrete is the major factor influencing thermal expansion.  The following table is from “SAA HB 64-2002 Guide to Concrete Construction – Table 19.5 Coefficients of expansion”, published by Cement & Concrete Association of Australia.

Aggregate type Coefficient of expansion of resulting concrete   (microstrain / °C) (mm/m / °C) Quartz 11.9 0.0119 Sandstone 11.7 0.0117 Gravel 10.8 0.0108 Granite 9.5 0.0095 Basalt 8.6 0.0086 Limestone 6.8 0.0068

The following calculation shows the effects of thermal expansion on a concrete slab with the change in temperature on a typical day.

Assuming:

                        Thermal Expansion = (0.0095 x 15 x 40) = 5.7mm

If the concrete slab is restrained from movement in all directions, then cumulative stresses induced on the restraints can be calculated using the asPittions above:

                        Strain = ((9.5 x 10-6) x 40) = 0.00038
                        Stress = (25,000 x 0.00038) = 9.5 N/mm2

Equating to a force per metre of:

                        Force = (9.5 x 200 x 1000) = 1,900,000N = 1,900kN (approximately 194 tonnes)

No surface drainage system would be able to withstand these forces and so the encasement supporting it must either be structurally designed to direct the stresses away from the drain or be physically isolated with the inclusion of longitudinal expansion joints.