Weeds in cracks of sidewalk / patio made with rock slabs?

I re-did an old sidewalk a few years ago: it had been rock slabs a couple inches thick, but they were widely spaced and almost covered with gr. I dug down six inches or so, put down landscape fabric, then crushed gravel, then sand, laid new stones on top, and filled the cracks with sand. Also, had wood boards along the sides. It looked nice for a little while, but then after time and mowing with gr clippings etc. landing on it, gr began to grow in the cracks again. It pulls out relatively easily because of the sand, but takes most of the sand with it so you would have to be continually replacing the sand. Did I do something wrong? Is there any way to avoid the weeds, or would you have to fill the cracks with something hard (cement?)? I'm afraid the filler would crack.