The only way to do a real repair is to weld in a temporary brace, cut out the old one and make a new one. Block filler (just "special" concrete) will work short term, but you'll be right back where you started in very short order. Concrete has no tensile strength. Additionally, the rust can cause rust jacking and blow out the rest of the tube or fracture the concrete.
If a temporary brace and fabricating a new crossmember isn't possible, welding on angle steel across the length of the member is the next best option. It will still rust out, but will last longer than the concrete.
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