Abstract: When the grouting pressure and time meet the specification requirements, the main factors affecting grouting-reinforcement effect are the strengths of rock mass before and after grouting and of hardened cement paste. Based on the existing test data of pure hardened cement paste compression strength, to express the relation between the 28 d-strength and the water-cement ratio for P.O. 42.5 cement, an empirical formula is proposed by means of the nonlinear fitting method. Based on the existing test data of rock mass strength before and after grouting, using the non-dimensional analysis method, two dimensionless parameters are proposed: strength growth rate and strength ratio of original rock mass to hardened grouting, and then an empirical formula to describe the compression strength growth rate of the broken rock mass reinforced before and after grouting is proposed by use of the nonlinear fitting method. The equations for the shear strength parameters growth rate before and after grouting are derived according to the Mohr-Coulomb strength theory. Filed tests on grouting into broken rock mass are conducted. The results show that the compression strength growth rates obtained from the formulae are basically consistent with those obtained from experiments, and that they meet the needs of rock grouting design.