Product Description GRK Caliburn Concrete Screws are professionally engineered fasteners with a patented thread design for ease of driving the screw in concrete and similar applications. Available in three different head designs for multiple applications. Recommended for use with GRK Drill bits. Caliburn Screws are available with a standard flat head design, pan head PH and washer head XL. From the Manufacturer GRK Caliburn Concrete Screws are professionally engineered fasteners with a patented thread design for ease of driving the screw in concrete and similar applications. Available in three different head designs for multiple applications. Recommended for use with GRK Drill bits. Caliburn Screws are available with a standard flat head design, pan head PH and washer head XL.