Concrete curbs shall be installed along every street. The standard curb section to be used shall be not more than ten (10) feet in length and shall be set in accordance with approved lines and grades, and radial curbs shall be formed in an arc segment, in a smooth curve. Chord segments are prohibited. Concrete curbs shall be eight by six by eighteen (8 x 6 x 18) inches (six-inch exposed face), using Class B concrete having a twenty-eight (28) day compressive strength of four thousand (4,000) pounds per square inch and shall be air-entrained. At locations specified by the approving authority and at all intersections, the curbing shall have a barrier-free design to provide a ramp for bicycles and/or wheelchairs, details for which may be obtained from the Engineer. In certain instances it may be necessary or desirable to construct alternate curb types. For example, these may be required by the Board on the perimeter or channelizing islands or in areas of unusually heavy gutter drainage flow or may be desired by the subdivider for decorative purposes or to preserve vegetation, e.g., granite block curb, rolled concrete curb, etc. If alternate curb types are to be permitted, an appropriate construction detail shall be submitted for approval with the preliminary and final plat.