Having "since there was no" and "due to lack of" in one sentece does sound redundant! The latter sounds more formal and probably that's what the context requires, so I'd keep that. You could even omit "access" since it's probably understood from context why scaffolding would be necessary for the tests: The tests were not performed on the surfaces of the below mentioned steel structures due to lack of scaffolding. Or make it simpler if you do keep "access": The tests were not performed on the surfaces of the below mentioned steel structures due to lack of scaffolding to access them. The tests were not performed on the surfaces of the below mentioned steel structures as they were inaccessible due to lack of scaffolding.