The economic literature has been investigating the positive relation between public infrastructure spending and the productivity of the private sector since Munnell (1992). We have thus introduced this relation into the recursive dynamic computable general equilibrium model of Quebec to assess the economic impacts of scaling up infrastructure on the economy. We use we draw our assumptions from Estache et al. (2010) combined with sectoral elasticity parameters from Harchaoui and Tarkhani (2003) based on Canadian estimations. We conduct a comparative analysis between a scenario without positive external effects of infrastructures, and another with positive externalities. The investments are financed by debt. The externalities help attenuate the negative macroeconomic effects associated with scaling up of infrastructure and amplify the positive effects.