What is the best way to secure these deck boards over an existing concrete patio? Is there an adhesive that could be used?

Hello, and thank you for your interest in Trex! Trex decking can be installed over a flat surface once a "sleeper system" is added. Adding in a sleeper system creates a substructure between the Trex and the flat surface below which allows for drainage, airflow, and access. The minimum sleeper system height is 1.5". In addition the sleepers should be built using code approved structural material such as pressure treated lumber and spaced to Trex's joist spanning requirements for proper support of the deck boards. Please be sure that the sleeper system is level and secured in a manner that attaches the framing system as floating systems are not recommended. Please note that adhesive is not a recommended fastener for Trex. Thanks, Tori - Trex Customer Care Team