The earliest reinforced concrete bridges built in Latvia over river Salaca and Gauja were built in 1908 and 1909. 100 years later both are still in service and require only certain maintenance. Bridges were built with intuition of parameters that produces good quality: concrete mix design, choice of shape and installation within formwork. After 100 years in service, both bridge deteriorations could have been avoided, for example, foundation protection for bridge over Salaca and expansion joints for both bridges. The concrete has been vulnerable to carbonation; during the 100 years of exposition the carbonation has reached 40 mm and the main reinforcement. Within this research performed carbonation calculation results show that the inverse effective carbonation resistance of concrete on site is μ=31.45•10-11,σ=5.32•10-11(m5/(s•kgCO2)).For concrete directly exposed to the wetting the weather function value is μ=3.24, σ =0.74. The calculation of carbonation process was modelled by using DuraCrete carbonation model and MonteCarlo simulations at full probabilistic level.