My feeling is that there is no fixity at the ends of the span. Considerable reinforcement would have to be developed from the slab into the abutment to carry moments at the span ends. This would have allowed a longer span and the abutments could have been built on land.
A coffer dam was placed to dewater the area where the abutments were built. Note that the slab is slightly raised over the abutment to allow room for the falsework and forms to support the reinforcement and wet concrete.
Slab bridges make inefficient use of concrete but provide a good riding surface.
Mexico's Bridges: Puente de la Ciudad de Coquimatlan (3) by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.