Acidic solutions of collagen are in vitro models of procollagen, the neutrosoluble precursor of collagen secreted by cells to build the extra-cellular matrix. Their viscoelastic properties may influence fibrils nucleation and growth during pH-triggered fibrillogenesis, and could thus be of particular interest for the engineering of artificial tissues as well as for the regulation of the structural properties of the extracellular matrix. Here we report on the rheological properties of acidic solutions of collagen over a wide concentration range, 0.6-300 mg mL(-1). At low concentrations, solutions display usual viscoelastic features consistent with those of dilute and semi-dilute solutions of macromolecules. At higher concentrations, both storage and loss moduli, G' and G '', scale as a weak power-law of the frequency omega(alpha) (alpha = 0.15 - 0.3), similar to what is reported for cross-linked actin networks, living cells and tissues. Creep experiments at high concentration reveal a weak power law regime at short times followed by a steady fluid-like regime at longer times. The transition between these two regimes appears at shorter times as stress amplitude increases. By rescaling creep responses, all time-domain data collapse onto a master curve. This defines a new time-stress superposition principle (TSS) and shows that fluidization occurs above a critical strain and after a lag-time scaling as sigma(-1/a). Power law rheology and strain-induced yielding are suggestive of soft-glassy rheology (SGR): concentrated collagen solutions would be close to a soft-glassy transition. Eventually, we compare the evolution of the rheological properties of acidic solutions from dilute to concentrated regimes to the non monotonous variation of fibrils diameter with collagen concentration in neutral gels.