Based on previous testing and analysis, it is believed that North American concrete design codes yield conservative shear designs for precast/prestressed hollow-core slabs in the 203 to 305 mm depth range. The objective of this research program is to calibrate the Canadian code shear equations to precast/prestressed hollow-core slabs, through testing a series of full-scale slabs to failure in shear. A total of twelve hollow-core slabs from one slab producer, using two types of extruders were tested in shear. Test variables include the bearing length and the prestressing level. Results are presented in terms of experimental to predicted capacity using the Canadian and American concrete design codes, the failure mode, crack profiles, and the critical section location for shear. It was concluded that the Canadian code shear equations are inappropriate for use with hollow-core slabs with low prestressing levels in the 203 to 305 mm depth range.

Objective: To evaluate the effect of an in-office plus at-home bleaching protocol on shear bond strength of orthodontic buttons when using a fluoride-releasing sealant. Methods and Materials: One hundred and sixty extracted human molars were randomly divided into bleached (N=80) and unbleached groups (N=80). The bleached group was treated with 45% carbamide peroxide for half an hour, followed with five applications of 20% carbamide peroxide at 24 hour intervals. After two weeks, lingual buttons were bonded on the teeth in both groups using either Transbond XT primer or Pro Seal sealant. The teeth were then stored in artificial saliva and subjected to shear testings at 24 hours and 3 months using a Zwick Universal Test Machine. Results: The ANOVA analysis of the 24-hour results indicated a significant difference between the four subgroups (p<.0011). Further simple t-tests indicated that the differences were significant only between bleached and unbleached subgroups (p<.0011). The 3-month results showed the mean shear bond strengths of the unbleached group using Pro Seal sealant was statistically significantly lower than the others although still greater than clinically minimal suggested bond strengths. Interestingly, 15% of the bleached teeth exhibited enamel fracture at the 3-month testing. Conclusion: At 24 hours, both Pro Seal sealant and Transbond XT primer appear to be a reliable choice on both bleached and unbleached teeth. However, at the 3-month period, Pro Seal sealant yielded significantly lower shear bond strength on unbleached teeth, nevertheless well within the range of values considered to be "clinically acceptable".

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