I think black's mistake was to play 18...b4, allowing a3 to attack instead of having eventualy that 'a' pawn end up alone after trading pawns. Now the bcd pawns are able to form a deadly chain once the a pawn is traded, yet it requires preliminary maneuvers with the rooks and knight before openning up the 'a' column.

44. Rb7+ Ka8 45. Ra7+ Kb8 46. c7+ Kc8 47. Ra8+ Black resigns

44. Rb7+ Kc8 45. Ra7 also threatens Ra8+, 45. ... Kb8 just goes back to the previous line.

Yeah loomis I had this idea, but I thought taking the pawn would be safer without penalty, since the rook can easily defend the c pawn and black has very few possibilities anyway.

But thanks for pointing out that at this point there is no need to take the black pawn and I could have gone directly with Rb7+  killing the game some moves shorter.