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Methods and apparatus for screeding freshly placed concrete in which the acoustic impedance of the treating equipment is made substantially equal to the acoustic impedance of the concrete slab. Portions of the screed contacting the plastic concrete are matched to an acoustic impedance approximating

Methods and apparatus for screeding freshly placed concrete in which the acoustic impedance of the treating equipment is made substantially equal to the acoustic impedance of the concrete slab. Portions of the screed contacting the plastic concrete are matched to an acoustic impedance approximating the acoustic impedance of plastic concrete, promoting the energy transferred to the concrete. The acoustic matching material is fabricated from a material that has an acoustic impedance within 67% to 150% of the acoustic impedance of the concrete being finished. The acoustic matching material has an abrasion resistance of no greater than 150 measured using ASTM Method G-65 where steel is given a rating of 100. The preferred matching material comprises ultra-high molecular weight polyethylene (UHMWPE) plastic.

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What is claimed is: 1. A screed comprising: an elongated, rigid frame that traverses concrete to be finished, the frame comprising concrete contacting portions; wherein the concrete contacting portions comprise acoustic matching material; wherein said acoustic matching material is fabricated from a

What is claimed is: 1. A screed comprising: an elongated, rigid frame that traverses concrete to be finished, the frame comprising concrete contacting portions; wherein the concrete contacting portions comprise acoustic matching material; wherein said acoustic matching material is fabricated from a material that has an acoustic impedance within 67% to 150% of the acoustic impedance of the concrete being finished; and, wherein said acoustic matching material has an abrasion resistance of no greater than 150 measured using ASTM Method G-65 where steel is given a rating of 100. 2. The machine as defined in claim 1 wherein the acoustic matching material comprises ultra-high molecular weight polyethylene (UHMWPE) plastic. 3. A triangular truss screed comprising: a rigid frame adapted to be moved over plastic concrete; means supported by the frame for distributing plastic concrete; means for vibrating the frame to remove entrained air and promote consolidation; at least one concrete contacting portion vibrated by said vibrating means, said contacting portion comprising acoustic matching material, said material characterized by an acoustic impedance approximating the acoustic impedance of said plastic concrete; wherein said acoustic matching material is fabricated from a material that has an acoustic impedance within 67% to 150% of the acoustic impedance of the fresh concrete being placed; wherein said acoustic matching material has an abrasion resistance of no greater than 150 measured using ASTM Method G-65 where steel is given a rating of 100; and, whereby outputted acoustic energy is optimally transferred to the plastic concrete. 4. The triangular truss screed as defined in claim 3 wherein said acoustic matching material is made from ultra-high molecular weight polyethylene (UHMWPE) plastic. 5. A telescoping boom screed comprising: a rigid frame adapted to be moved over plastic concrete; an auger supported by the frame for distributing plastic concrete; means for vibrating the frame to remove entrained air and promote consolidation; at least one concrete contacting portion comprising acoustic matching material, said material characterized by an acoustic impedance approximating the acoustic impedance of said plastic concrete; wherein said acoustic matching material is fabricated from a material that has an acoustic impedance within 67% to 150% of the acoustic impedance of the fresh concrete being placed; wherein said acoustic matching material has an abrasion resistance of no greater than 150 measured using ASTM Method G-65 where steel is given a rating of 100; and, whereby outputted acoustic energy is optimally transferred to the plastic concrete. 6. The telescoping boom screed as defined in claim 5 wherein said acoustic matching material is made from ultra-high molecular weight polyethylene (UHMWPE) plastic.