A Scientific Investigation into Concrete Pavement Durability

Abstract

Although concrete pavements offer many long-term performance benefits, there are still instances where premature degradation of pavements leads to unexpected and costly repairs. In addition to being a burden to transportation agencies and the driving public, these situations have the potential to unduly tarnish the reputation of concrete pavements.

We assembled a multidisciplinary team across multiple universities whose objective was to improve the durability of concrete pavements by improving the scientific understanding of pavement distresses. In particular, we sought to develop a quantitative understanding of the chemical reactions to the physical manifestation of concrete pavement damage from alkali–silica reaction (ASR) and freeze-thaw (FT). This will lay the foundation for connecting pavement material properties and fracture and durability prediction, while also helping to establish the potential for ASR and/or FT damage in a concrete pavement and the rate at which it would happen. In essence, it will identify the conditions that lead to ASR or FT damage.

The research approach involved a range of experiments including nano-scale chemomechanical characterization of ASR gels, mechanical and thermal characterization of cement paste after meso-scale FT cycling, and likelihood of ASR damage for concrete mixtures. The modeling approach included analytical and simulation models of ASR, FT, and fracture at the nano-, meso-, and micro scales.

The key outcome of this project is a unified theoretical framework for explaining both ASR and FT damage. Detailed explanations for both mechanisms are as follows.

FT: by combining mechanical and characterization experiments together with atomistic and mesoscale simulations, we found that there is NO direct impact of an ice phase in damage to the paste and concrete. FT damage in concrete/cement paste appears to result from a disjoining ionic pressure at the C-S-H/ice interface in the capillary pore network which is that fractures the C-S-H matrix. Computer-simulated effects with the type and concentration of ions in the pore solution agree well with experiments.

ASR: by combining mechanical and characterization experiments together with atomistic and mesoscale simulations, we found that the swelling of ASR gel is NOT the reason for damage to concrete. ASR damage is likely the consequence of a Na+ Ca2+ exchange mechanism between an initially formed alkali gel that is deficient in Ca (Ca-poor) and C-S-H. Over time this exchange creates a disjoining ionic pressure at the interface between C-S-H and calcified ASR gel (Ca-rich) in the capillary pore network that causes expansion and cracking within the C-S-H matrix.