Contractors often buy in ready mix concrete from local companies who deliver to site and pump as directed. The cost of a truckload varies according to the 'mix' ratio of cement to sand to aggregate. Different mixes for different applications. Suppliers will provide a certification of individual loads based on a mix sample.
Bottom line. Building contractor buys the cheapest mix which, when cured, does not have adequate strength. Add to that inadequate rebar and safety limits are compromised.