Save a texture after a pass in shader

Hi !

I'm trying to concatenate add a texture in a bigger but I fail to keep the result of the bigger texture.

I have a shader that take some variable like a position for the smaller texture in the bigger, the smaller texture and of course the bigger one. So basically, my shader work correctly, in result I see the smaller texture in the bigger à the right position. But my bigger is never save, so all the precedent pass is ignored.

How can I save the result to send it again in the shader to concatenate all the small texture ?

I try this : Graphics.Blit(_RTConcat, _RTConcat, _concat);

RTConcat is my bigger texture, it's a RenderTexture, I've tried to create it both in editor or in script but the result is the same and _concat is the material using the shader.

Thank in advance :)

PS : I'm using unity 4.5.3 and can't upgrade, it's a pro version.