Definitely the concrete.
There are two major factors that enter into the situation. Without knowing manufacturing and and curing methods are a couple of general categories.
If it was produced in a plant, the mix is usually well controlled and accelerated curing could could be stripped quickly to handle the panels and let the natural conditions add a little to meet and exceed the specified concrete strengths. Not all plants are set up for the manufacturing of Tees.
If they were produced in lesser controlled plant that uses some exterior variable curing it is common to over-load the mix to create enough to allowing stripping to get enough strength to strip the forms quickly and meet the specified strength. The further natural curing later may increase the strength to an unknown level and could be higher, but uncontrolled strength.
That is one of the problems with lab 28 day curing and testing. When Mother Nature gets involved there is addition strength and will depend on the age and conditions.
Older over designed concrete is definitely much tougher to deal with when it comes to drilling. By the time the panels are in place, the actual in-place strength could above what is specified or very much higher.
It is cheaper to add cement and possibly some additives to many concrete products than to change the process.
As everyone knows, concrete gains strength is several ways with time and conditions.
Dick
