If the tank has already cracked, it's possible that the water pressure is too high and that the popoff, pressure relief valve might be stuck. Also, some states now require a relief bladder to be installed on the cold water line to allow for the hot water expansion, especially since most municipalities have check valves installed in the line that won't let the water pressure (expanding hot water) bleed back into the system.
You don't need a bladder to prevent overpressure due to expansion. Your tank must already have a pressure relief valve (usually a combined temperature-pressure valve and a second backup pressure relief valve).
The purpose of the bladder is to prevent the expanded water from being discharged via the pressure relief valve and wasted.
My apartment came with a cheap-ass unvented water heater. There is no bladder - the only provision for expansion is a domed top to the tank trapping an air bubble above the draw-off outlet. This isn't very effective, as in time, the bubble dissolves in the water. The expansion of the water is allowed for by having the pressure valve open and allow the excess volume to drip into the drain.
Good practice for installation of a water heater should address the following points:
1. A pressure regulating valve to reduce the mains water pressure to the pressure specified by the heater manufactuerer.
2. A filter to protect the pressure regulating valve
3. A check valve to prevent pressure rise in the tank discharging heated water into the mains supply/cold circuit
4. A pressure relief valve as specified by the heater manufacturer
5. A temperature/pressure relief valve as specified by the manufacturer
6. A visible drain for the relief valves to drain into
7. A drain system capable of safely draining water at the maximum flow-rate of the heater, and at the maximum temperature. (If the heater thermostat gets stuck on, the T&P relief valve will open due to the water temperature reaching 212 F. At this temperature, the relief valves opens full on. Boiling water will erupt from the vent at full flow rate until the heater becomes refilled with cold water) The drain must be capable of safely containing this flow, without spillage and must be made of material that will not melt or soften at 212 F (usually must be made of metal, although some codes do permit the use of specially authorized plastics).