The cause of shielding is believed to be increased likelihood of inter-particulate collisions,the high collision probability between incoming and rebounding particles reducing the frequency and the severity of particle impacts on the target surface. In this study,the effects of particle concentration on erosion of a mild steel bend surface have been investigated in detail using three different particulate materials on an industrial scale pneumatic conveying test rig.The materials were studied so that two had the same particle density but very different particle size,whereas two had very similar particle size but very different particle density. Experimental results confirm the shielding effect due to high particle concentration and show that the particle density has a far more significant influence than the particle size, on magnitude of the shielding effect