A solution to concrete creep might be at hand with new research into its physical origins.
While concrete is a strong and ubiquitous building material, its tendency to ‘creep’ or deform progressively under mechanical stress means problems like crumbling bridges and cracked roads. Now researchers are working toward a better understanding of the physical origin of this mechanism.
The phenomenon of creep is inescapable, and despite the risks for the safety of infrastructure, engineers have had difficulty understanding why it happens. According to Gaurav Sant, an associate professor in the Department of Civil and Environmental Engineering at UCLA, this poor understanding of concrete creep forced engineers to estimate creep using empirical models, which are often poor predictors of creep behaviour.
The team at UCLA unified experimental and computational data, and found that creep originates from a dissolution-precipitation process, which acts at nanoscale contact regions of calcium-silicate-hydrate grains.
Calcium-silicate-hydrates are the binding phase that holds cement paste together. The researchers found that these tend to dissolve at higher-stress regions, and re-precipitate at low-stress regions.
“As a result of such dissolution-precipitation behaviour, a macroscopic, time-dependent ‘creep’ deformation manifests,” Sant said.
The dissolution-precipitation process is not a new idea: geologists use it to explain deformation in the earth’s crust. But this is the first time that same process has been shown to be relevant to concrete.
The researchers’ previous work includes developing vertical scanning interferometry methods to measure how quickly minerals dissolve on the nanometer scale. They also systematically examined the viscoelastic behaviour of calcium-silicate-hydrates, and developed ways to simulate long-term relaxation of disordered solids under stress. This research allowed them to comprehensively examine and isolate the variables at play in concrete creep.
The team’s analysis also uses molecular dynamic simulations to assess how the geometric arrangement of atomic networks influences the volume relaxation of calcium-silicate-hydrate compositions.
Their findings show that the chemical composition of calcium-silicate-hydrate can have an effect on creep and dissolution rates. It is possible to create calcium-silicate-hydrate compositions that effectively minimise the creep of concrete.
The researchers are going to put together a comprehensive description of concrete creep from the atomic to the macroscopic scale. They hope that this will help them develop mechanistic models for predicting creep behaviour.