Durability In Concrete: Understanding The Influence Of Capillary Pressure For Controlling Shrinkage And Cracking In Early Age Concrete

Abstract

Shrinkage cracking is a major concern for concrete durability, leading to the ingress of corrosive agents and increased infrastructure maintenance costs. This problem is particularly critical in the early ages when concrete has not attained sufficient strength. Capillary pressure is the main cause of shrinkage in concrete, and controlling it during the early days can reduce cracking potential. However, capillary pressure measurement has been limited to 100 kPa due to insufficient sensor capacity, restricting data collection to about 7 hours and hindering understanding of its relationship to shrinkage. Additionally, models predicting shrinkage based on capillary pressure lack validation due to insufficient data at early age.
This study employs High Capacity Tensiometers (HCTs) to monitor capillary pressure in self-consolidating concrete up to 2000 kPa over several days. An extensive experimental program also investigated concrete’s chemical, physical, mechanical, and microstructural properties, including shrinkage and cracking potential, to relate these factors to HCT results. The effects of key design parameters, such as water-to-cement ratio (w/c), ground granulated blast furnace slag (GGBS), and shrinkage reducing admixture (SRA), on capillary pressure were also investigated. The data was then used to develop a deep neural network (DNN) model to predict capillary pressure across varying w/c ratios, GGBS levels, and SRA dosages up to 2000 kPa.
A novel four-stage capillary pressure evolution model has been developed to investigate the governing mechanisms of capillary pressure during the early stages of concrete. The model identifies distinct stages controlled by different processes: the initial two stages are governed by bleeding and setting times, during which minimal capillary pressure develops, while the latter two stages are dominated by evaporation and the hydration process, where significant capillary pressure evolves. The study reveals that proper curing might manage moisture loss and capillary pressure during the later stages of concrete. Additionally, it was observed that concrete at the end of semi-plastic phase is highly susceptible to cracking due to evolution of capillary pressure and shrinkage, and reduced tensile strain capacity, underscoring the necessity of curing during this critical period. Early curing, immediately following casting, may be also crucial as significant shrinkage occurs, facilitating future crack propagation.
The research further demonstrated a capillary pressure gradient across the depth of concrete elements, which is particularly pronounced in deeper structural elements, potentially leading to element distortion, a factor that must be accounted for in design considerations. In terms of mix optimization, while SRA was effective in significantly lowering the rate of capillary pressure evolution, increasing the dosage beyond 1.5% yielded minimal additional benefits. Cement replacement with GGBS was shown to effectively reduce capillary pressure, with a 20% replacement offering comparable results to the use of 1.5% SRA. This indicates that both approaches can be employed based on specific performance requirements in concrete applications.
Finally, the ability of the proposed model to predict the capillary pressure and capture the net effect of multi-parameters on it, provides insights into the optimum design of more durable concrete mixtures with the lowest capillary pressure evolution and guides the implementation of appropriate cost-effective shrinkage-mitigating strategies.

Date of Award	28 Nov 2024
Original language	English
Awarding Institution	Northumbria University
Supervisor	Joao Mendes (Supervisor), Brabha Nagaratnam (Supervisor) & Michael Lim (Supervisor)