Around 25 minutes when Rick wakes up from a dream of Hershel, he holds his chest with his left hand but there is no blood on his hand. In the second scene you can see that his hands are blooded.
As a career US Army Engineer whose expertise was the construction and demolition of bridges, its obvious that there would be no need for dynamite on this construction site since the only two uses for explosives would be to prepare abutments (which were still intact) or felling trees for land clearing or lumber. Land clearing isn't necessary on an existing bridge site and if needed for timber acquisition, the explosives wouldn't be needed anywhere near the bridge site.