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["Jim Getaz" <jgetaz(--nospam--at)shockeyprecast.com>]

            Tiger,

                        I wish I had experience to offer you. The only time I was involved with a topping that sounded hollow under a chain drag, shear tests on cores turned up as much as 10 times sqrt f’c strength in the precast to topping plane.

                        You say “conventionally reinforced planks.” Matthew took that to mean prestressed (conventional reinforcing for hollowcore “plank”). I thought you meant mild-steel reinforced. I’m not sure this has any relevance to your question, I’m just curious.

                        Was the plank surface rough? Was there anything during construction that would have contributed to poor bond, such as: a long period between setting the plank and pouring the topping and construction dust not cleaned off; plank that dried a long time in hot sun and not dampened before placing the topping (which may be evident by a dark layer of topping at the interface – concrete with less water tends to be darker); or just hot plank that initiated a flash set of the topping concrete?

            Jim Getaz

            Precast Concrete Engineer