On a recent job the concrete arrived in 13-cubic-yard batches in revolving-drum mixer trucks but the trucks were labeled "10 cubic yard mixer maximum capacity." We inquired about whether the concrete could be properly mixed if the drum was overloaded like that and were told that it is all right because the concrete had been plant-mixed. Does that make everything OK?
It does in this case. Standards for truck mixers are promulgated by the Truck Mixer Manufacturers Bureau affiliated with the National Ready Mixed Concrete Association. Their table shows that a truck mixer rated at 10 cubic yards maximum mixer capacity is permitted to carry up to 13 1/4 cubic yards if the truck is being used only as an agitator. Your concrete had already been mixed before the truck was loaded and the 13 cubic yards is below the maximum allowed for agitation. Copies of the complete standards are available from the Truck Mixer Manufacturers Bureau, 900 Spring Street, Silver Spring, Maryland 20910.