The outer strands are aluminum, chosen for its excellent conductivity, low weight and low cost. The center strand is of steel for the strength required to support the weight without stretching the aluminum due to its ductility. This gives the cable an overall high tensile strength.
ACSR cables are available in several specific sizes, with multiple center steel wires and correspondingly larger quantities of aluminum conductors. For example, an ACSR cable with 72 aluminum conductors that requires a core of 7 steel conductors will be called 72/7 ACSR cable
The higher resistance of the steel core is of no consequence to the transmission of electricity since it is located far below the skin depth where essentially no AC current flows.