Cracking in edge restrained concrete walls
File(s)
Author(s)
El Khoury, Karim
Type
Thesis
Abstract
There is growing evidence that current European guidance fails to predict and control restraint-induced cracking in reinforced concrete structures accurately. This is a critical concern for the construction industry, as excessive cracking, typically mitigated through steel reinforcement, can compromise durability, serviceability, and aesthetics.
Concrete structures undergo an initial expansion due to the exothermic hydration of cement, followed by contraction as they cool. This volumetric reduction is intensified by autogenous and drying shrinkage over time. Yet, most concrete elements are restrained either internally, due to reinforcement, or externally, by neighbouring elements cast before. These external restraints are classified as end, edge, and combined edge-end restraints. While end restraint is well-researched, edge and combined conditions are less understood. This research, therefore, focuses on the prevalent issue of edge restraint cracking in reinforced concrete walls.
Recent findings reveal that existing guidelines fail to accurately quantify crack width under edge restraint, sometimes leading to excessive cracking or, conversely, overdesign. Current UK guidance includes Eurocode 2 (2004, 2006, and 2023) and CIRIA C766, but field studies and experiments indicate limitations in these provisions. Previous research on edge restraint cracking is sparse, relying mainly on scaled models without full consideration of real-life parameters.
This study casts twelve full-scale, edge-restrained walls in the lab, varying geometry, reinforcement, and concrete mix. Experimental outputs—temperature, strain, restraint factors, and crack development—highlight significant impacts of wall geometry and formwork insulation on early cracking. Measurements show variation in restraint with wall height and reveal the influence of cracking on strain.
Non-linear finite element analysis was performed to extend the laboratory findings, identifying the role of horizontal reinforcement and bond quality in controlling crack width. Evaluating design codes, a new model was developed to enhance crack width prediction, iteratively calculating residual stresses and crack widths for more accurate edge-restrained cracking control.
Concrete structures undergo an initial expansion due to the exothermic hydration of cement, followed by contraction as they cool. This volumetric reduction is intensified by autogenous and drying shrinkage over time. Yet, most concrete elements are restrained either internally, due to reinforcement, or externally, by neighbouring elements cast before. These external restraints are classified as end, edge, and combined edge-end restraints. While end restraint is well-researched, edge and combined conditions are less understood. This research, therefore, focuses on the prevalent issue of edge restraint cracking in reinforced concrete walls.
Recent findings reveal that existing guidelines fail to accurately quantify crack width under edge restraint, sometimes leading to excessive cracking or, conversely, overdesign. Current UK guidance includes Eurocode 2 (2004, 2006, and 2023) and CIRIA C766, but field studies and experiments indicate limitations in these provisions. Previous research on edge restraint cracking is sparse, relying mainly on scaled models without full consideration of real-life parameters.
This study casts twelve full-scale, edge-restrained walls in the lab, varying geometry, reinforcement, and concrete mix. Experimental outputs—temperature, strain, restraint factors, and crack development—highlight significant impacts of wall geometry and formwork insulation on early cracking. Measurements show variation in restraint with wall height and reveal the influence of cracking on strain.
Non-linear finite element analysis was performed to extend the laboratory findings, identifying the role of horizontal reinforcement and bond quality in controlling crack width. Evaluating design codes, a new model was developed to enhance crack width prediction, iteratively calculating residual stresses and crack widths for more accurate edge-restrained cracking control.
Version
Open Access
Date Issued
2024-07-01
Date Awarded
01/11/2024
License URL
Advisor
Vollum, Robert
Izzuddin, Bassam
Publisher Department
Civil and Environmental Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)