Bricks, Masonry, Asphalt and Concrete - Beaty Bark in New Concrete Slab - Whose Fault?

We recently had a 12 x 12 concrete slab poured and it appears that beauty bark somehow got into the concrete. The bark eventually rose to the surface leaving tiny holes about the size of a toothpick. A few holes were larger, the ones with a bigger chunk of bark. I was wondering what the odds are that the ready-mix was somehow contaminated or if it's more likely the contractor somehow transferred the matter from his boots or tools. We do not have beauty bark anywhere near the slab. The concrete was pumped. Could the pump be involved? Fortunately, we have not paid for any labor yet.

Thanks, Diana

The contaminant may have shown up in any of the ways you mention.