CGIndia has published a texture pack containing 15 medium resolution concrete textures. Not all of them are seamless but these should come in handy whenever you want to do some architectural modeling nonetheless.
From the announcement:
Keeping up with our commitment of bringing free and useful features for CG Professionals, CGIndia humbly announce absolutely free and optimized Concrete Texture maps. The Concrete Textures pack contains 15+ medium Resolution textures for use in 3d scenes and architectural visualizations. Now, The Free Texture’s Library has collection of very useful and handy Texture maps in five categories. Texture maps are downloadable through our website in .RAR format. All Textures are free for personal use only, Freelancers may use them for commercial purpose……..