Cracking in reinforced concrete structures can occur at any stage during their lifetime, and is anticipated for during its design. It is crucial to control crack widths by applying sufficient reinforcement to prevent impairments to the structure’s functionality. Cracks may develop due to external loads applied to the structure or due to imposed deformation. Imposed deformation occurs when movement is restrained, for example due to adjoining structures or internal thermal strain gradients in thick concrete sections. With the increased use of high-strength concrete since the late 20th century, autogenous shrinkage has emerged as a significant source of early-age cracking as well.
In practice, observed crack widths due to a combination of imposed deformation and mechanical loading during the early age of concrete are often smaller than predicted by structural design codes. Thissuggests that less reinforcement could be used in concrete structures, offering potential durability benefits. However, literature lacks sufficient validation of this observation, as well as an understanding of its origin and how it can be incorporated into design practices.

Liang (2024) developed a mini-TSTM specifically for plain mortar and validated its setup using results
from the regular TSTM, which has been extensively documented in the literature. This setup allows for precise control over both deformation and temperature development and is integrated with an Instron loading machine. By restraining the mini-TSTM specimen during its early age, stresses due to autogenous shrinkage and thermal deformation are induced.

The following research question was explored: How do crack widths develop in reinforced concrete
under the combination of imposed deformation and mechanical loading?
The first method used to answer this research question was to compare experimentally observed crack widths due to the combination of imposed deformation and mechanical loading to predictions from five structural design codes and guidelines.
Secondly, the impact of increasing the fraction of imposed deformation relative to mechanical loading on crack width in reinforced concrete was investigated.
Initially, the experimental setup and procedure, including reinforcement, were developed, followed by three experiments conducted at different times after the initial setting time and at various levels of imposed deformation. Using data from the Instron and a DIC analysis, experimental crack widths were obtained and compared to predictions from five structural design codes and guidelines.The optical microscope analysis provided further insights into crack locations.
Model Code 2010 proved to be the most accurate and consistent during the crack formation stage, while Van Breugel’s method was most accurate and consistent during the stabilized cracking stage.
Crack widths due to the combination of loading is generally overestimated by the design codes and guidelines, highlighting the importance of understanding their underlying principles and theories for accurate predictions.
There appears to be a trend indicating that an increase in the fraction of imposed deformation relative to mechanical loading results in a decrease in the final crack width in reinforced concrete. However, the varying weighted maturities across experiments also influence the predicted crack widths, making it difficult to confirm this hypothesis with certainty.

The development of the reinforced mini-TSTM experimental setup opens up new research opportunities. It significantly reduces the required workforce compared to the regular TSTM and, with the addition of the Instron, facilitates mechanical tensile testing of reinforced mortar.