The objective of this experimental work is the investigation of the viscoelastic behavior of a hybrid matrix, fiber-reinforced building material. Hybrid matrix consisted of epoxy resin mixed with fine marble sand, whereas short steel fibers were used as reinforcement. The experimental procedure involved, first, the manufacturing of specimens using different hybrid matrix types and different reinforcement by weight ratios. Subsequently, bending relaxation experiments at room temperature were executed, under three-point bending test experimental configuration, at different strain levels and the variation of stress of the hybrid matrix material with time was monitored. The data obtained were used to (i) investigate the effect of reinforcement mass fraction contained in the composite on the viscoelastic behavior and (ii) to apply existed and newly developed viscoelastic models for the description of the observed viscoelastic behavior. More precisely, the four-parameter (Burgers) viscoelastic model and the modified Residual Property Model were calibrated and used to simulate the relaxation behavior of the materials manufactured and tested. Experimental results exhibited a clear influence of both reinforcement ratio and initial displacement on the viscoelastic behavior of the materials manufactured and tested, whereas the models proposed and used can adequately reproduce the variation of relaxation stress with time. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41429.