The bridge anchors will take 700 * 2 = <<700*2=1400>>1400 tons of concrete.
The roadway deck and bridge anchors need 1600 + 1400 = <<1600+1400=3000>>3000 tons of concrete.
Thus, Johnny found the supporting pillars need 4800 - 3000 = <<4800-3000=1800>>1800 tons of concrete.
#### 1,800