Chemical Modification of Crumb Rubber to Increase Its Bonding with Concrete
The hydrophilicity of crumb rubber was increased with wet chemical modification and added to concrete to increase its adhesion with concrete, consequently the compressive strength relative to that of concrete with untreated rubber. Other modification methods were tested on crumb rubber and rubber sheets to yield an increase in hydrophilicity. Certain tests were less reliable for powder testing or tedious and time consuming and concrete materials and rubber may not be homogeneous.
For wet chemical modification, several approaches were attempted. First, crumb rubber was soaked in 5% sodium hydroxide, 5% potassium permanganate and saturated sodium bisulfite, tested for hydrophilicity using Sessile Drop, Washburn Capillary Rise and hydrophobic partitioning and added to concrete cylinders tested for compressive strength. The results from the Sessile Drop method show that water spread out and disappeared over the modified rubber indicating hydrophilicity. The results from the Washburn Capillary Rise show that the modified rubber has a contact angle of 89.4 ± 0.02°. The results from the Hydrophobic Partitioning method show that rubber became somewhat more miscible with water. Results of compressive strength testing show that the compressive strength of concrete containing unmodified rubber was 40% lower than that of regular concrete but the compressive strength of concrete containing modified rubber was 2% lower than that concrete containing unmodified rubber. Second, crumb rubber was soaked in 5% sodium hydroxide for 24 hours or stirred in saturated sodium hydroxide for 20 minutes and tested for hydrophilicity using Sessile Drop or hydrophobic partitioning. The results from the Sessile Drop method show that unmodified crumb rubber has a contact angle of 121.8 ± 13.2°, crumb rubber soaked in 5 % NaOH for 24 hours has a contact angle of 115.8 ± 8.0° and crumb rubber stirred in saturated NaOH for 20 minutes has a contact angle of 121.5 ± 8.1°. Results from the Hydrophobic Partitioning method show that unmodified rubber and rubber soaked in 5 % NaOH for 24 hours are not miscible with water at all (rubber soaked in saturated NaOH was not tested). To gain some basic understanding on how rubber can be affected by different treatment methods, rubber sheets were modified with sodium hydroxide, potassium permanganate, sodium bisulfite, combinations of the three, as well as by ultraviolet radiation or plasma radiation. Water contact angles on the treated rubber sheets were measured using the sessile drop method. The results show that rubber sheets were made somewhat more hydrophilic with the combinations of sodium hydroxide, potassium permanganate and sodium bisulfite treatments and significantly with UV or plasma but not with sodium hydroxide alone.