Taxable and exempt aggregate 20 5 Taxable aggregate (1) Tax is to be charged only on taxable aggregate. (2) In this Part a quantity of aggregate is taxable aggregate on any occasion that it is subjected to commercial exploitation, except to the extent that it is exempt aggregate under section 6. 25 6 Exempt aggregate (1) A quantity of aggregate is exempt aggregate to the extent that it falls within subsections (2) to (8) of this section. (2) Aggregate is exempt under this subsection if it is, or derives from, any aggregate that has already been subjected to a charge to tax. 30 (3) Aggregate is exempt under this subsection if it has previously been used for construction purposes (whether before or after the date on which this section comes into force). (4) Aggregate is exempt under this subsection if it is aggregate that, on the date on which this section comes into force, is on a site...