I just found out that my contractor used Quickrete, mixed on site, for the foundation of a small addition and for a patio slab. He was supposed to order pre-mixed concrete with a specific slump of 4-5. My architect and structural engineer got wind of this and are now calling for a $700 core test to verify the strength of the foundation. My contractor swears that he did nothing wrong - he even says he did me a favor by expediting the cement work instead of waiting for concrete to be delivered. Is the engineer over-reacting or is the contractor covering his you-know-what? This has blown up into a huge deal - all work on that area is halted until the test is completed. Help!