Granite is one of the oldest and most utlilised building stones. Historically granite takes it’s most common form as solid masonry in load bearing walls and monumental features. In recent years the primary application of granite has changed with developments in structural framing and stone block sawing. Currently granite is most commonly utilised as a thin stone veneer on multi-storey buildings. These thin veneers must maintain their structural integrity of the design life of the building.
The performance of thin structural stone veneers has been under increased scrutiny of late as a number of highly publicised expensive failures have occurred. Long term environmental weathering is among the primary contributors to the deterioration of granite and is thought to be responsible for a good proportion of these failures.
At present the most reliable way to test the flexural strength of granite is by destructive test. Given that the performance of building stone subject to environmental weathering conditions is highly variable and environmental weathering conditions can differ greatly across the exterior of a building this type of test is not feasible for the evaluation of entire building façades.
This thesis is a continuation of previous research at The University of Queensland. The primary aim is to evaluate the effect of cyclic wetting and drying in combination with air pollution on the flexural strength of thin structural granite veneers. As a secondary objective evaluation of the direct transmission of ultrasound as a potential non-destructive test for strength and stiffness is also completed.
To best achieve these aims a critical review of the literature was completed and the following hypotheses were suggested:
(i) Ultrasonic techniques are suited to the identification and quantification of sub surface deterioration and flaws, however, the direct transmission of ultrasound is not well suited to the identification and quantification of surface deterioration and flaws.
(ii) Chemical weathering of granite has significant capacity to affect the design strength of thin veneer cladding panels.
(iii) Chemical weathering affects the design strength of thin veneer cladding panels by surface deterioration.
Sample populations of three different Australian granite types were then selected to undergo an accelerated Wet / Dry weathering procedure. A group of each granite type were also designated as unweathered or control samples. The samples to be weathered were tested ultrasonically before weathering, at an intermediate stage and at completion of the accelerated weathering schedule. Finally all samples were tested to failure using the Instron 5500R/6027 test machine and the ultimate load and deflection was recorded.
Destructive test data was refined to determine the Elastic Modulus and Flexural Strength. Ultrasonic pulse velocity data was used to obtain an estimate of the Elastic Modulus for each sample.
Upon comparison of the results the following conclusions were drawn:
o Freeze / Thaw (thermal effects and salt decay) have significantly greater capacity than weathering processes mobilised by cyclic Wet / Dry (hydrolysis, dissolution and oxidation) to affect the design strength of thin stone veneer cladding panels.
o No significant reduction in strength or stiffness as a result of the accelerated Wet / Dry weathering process was observed however in view of the literature and deficiencies identified within the experimental procedure it is believed that Wet / Dry weathering of granite has significant capacity to affect the design strength of thin veneer cladding panels.
o Chemical weathering affects the design strength of thin veneer cladding panels by surface deterioration.
o Direct transmission of ultrasound is useful for the quantification of internal damage provided results obtained are calibrated using destructive test data and the structural characteristics of the granite are deemed suitable.
o Direct transmission of ultrasound is not well suited to the identification and quantification of surface deterioration.
It is recommended that further research be conducted to accurately quantify the effect of Wet / Dry weathering processes on the structural integrity of thin stone veneers to minimise the impact of incorrectly designed stone façades in the future.