A simple concrete engineering project is to construct a loading ramp for a building. There are a number of fast form cardboard molds that can be used or a custom wood mold can be constructed in which to pour the concrete. Consult with your local state regulations on loading ramps as they are regulated in some states.
Retaining walls are supporting structures used to keep back earthen mounts. These can be constructed by first constructing the wall out of cinderblocks and then coating the wall in concrete. The concrete helps bind the blocks together and form a moisture barrier. For best results the concrete should be poured down into the center blocks so that it forms an internal concrete skeleton.
While it may seem relatively simple, a grain silo is actually a complex building. The loads on the walls can easily topple the tower if the concrete is not made strong enough and if the walls are not stout enough. You need to calculate the strength of the cement mix you are using and the loads each level of the tower will be supporting. In addition, you must calculate the expansion and contraction rates of the concrete so that a hot day does not shatter the silo.
Many experiments that involve explosives, like rocketry, require bunkers to hold sensors and observers. This is a great project for students to design models since the principles are easily scalable. For instance, slope structures shed kinetic and concussive force more easily than structures whose walls are perpendicular to the ground. In addition, this introduces the concept of rebar to the students. Rebar are steel reinforcing bars that are embedded in the concrete to make it stronger.