When hardening concrete is subjected to imposed deformations, stresses maydevelop. If at any point in time this stress exceeds the tensile strength ofthe material, the concrete will crack. Early-age cracking of concretestructures may lead to issues related to durability, serviceability and aesthetics.During hardening of concrete the material properties are still in development.This means that to be able to assess the risk of early-age cracking for aspecific case, understanding of the hydration process and the stress- andstrength development is required. Besides, concrete is a viscoelastic materialwhich means that stresses are affected by mechanisms of creep- and relaxation. Theright approach for assessing the risk of early-age cracking in concretestructures is yet to be determined and the effects of creep and relaxation onthe stress development during hardening remain subject of debate. The aim of this study was to gain more insight in the effect of stressrelaxation on early-age cracking of concrete structures under imposeddeformations. For this purpose, the following research question was formulated: “How can early-age cracking inconcrete under imposed deformations be analysed taking into account stressrelaxation and what is the applicability of the models used?” To be able to answer this research question, first the relevant processesand mechanisms that play a role in the hardening of concrete had to bediscussed in more detail. The state-of-the-art of the subject was discussed aswell as the methods to take into account viscoelastic material behaviour in theanalysis. Next, the research was narrowed down by considering a single case fromengineering practice. It was decided to consider a dive-under that was beingconstructed near Zwolle, the Netherlands. This dive-under consists of multiplesegments which are all constructed in the same manner by first casting the slaband subsequently casting the walls on top. The hardening process of the wallscorresponds to a typical imposed deformations case as is also described inliterature.  In order to assess the risk of early-age cracking for the selectedcase, finite element software was used. The hardening process of the walls ofthe dive-under was modelled by considering a cross-section of the wall andslab. A parameter study was carried out to gain more insight in the effect ofaltering the different input parameters of the analysis on the resultingstress- and strength development during hardening.  Also, creep data which could be found in literature was analysed andthe Maxwell chain model was adopted to be able to model the viscoelasticmaterial behaviour. The Maxwell chain model consists of units of springs anddampers connected in series. When using several of these units in parallel, theviscoelastic material behaviour can be simulated. Evaluation of the Maxwellchain model was needed to be able to use the available creep data in theanalysis.  Because the outcome of the analysis was dependent on many inputparameters, several laboratory tests were carried out at the Stevin laboratoryof the TU Delft. This was done for the specific concrete mixture that was also beingused in the construction of the walls of the dive-under. The aim of these testswas on the one hand to simulate the hardening process of the wall of thedive-under and on the other hand to determine the strength- and stiffnessdevelopment of the material over time. Also, the autogenous deformations weremeasured in a ADTM test. The results of the tests were analysed and couldsubsequently be used to improve the accuracy of the finite element modelregarding the risk of early-age cracking.  Then, by making use a finite element model of the TSTM test and comparingthe outcome of this analysis with the results of the actual TSTM test, theviscoelastic material behaviour of the concrete could be derived. It was foundthat the effects of creep- and stress relaxation at early-ages were initially underestimatedand new creep curves were derived. An average early-age creep factor for thestress in a governing point in the bottom of the wall of around 2.4 was found. Next to the laboratory testing and computational models, temperaturemeasurements and visual inspections were done in practice on a segment of thedive-under. The temperature measurements could directly be used to improve themodel. The aim of the visual inspections was to determine whether early-agecracking would actually occur in the walls of the dive-under. During theinspections it was found that early-age cracking did not occur in any of theinspected walls. Cracking of the walls was eventually observed, however notwithin the time period that was regarded for early-age cracking. The results ofthe inspections were then used to be able to make judgements on the accuracyand suitability of the model for the assessment of the risk of early-agecracking. By making use of the collected data, the derived material propertiesand the temperature measurements from practice, the assessment of the risk ofearly-age cracking could be performed more accurately. This resulted in afigure which showed the stress- and strength development over time for agoverning point in the cross-section of the wall. Based on information found inliterature, a global risk of early-age cracking of the wall could then bedetermined.  When comparing the results of the above described finite elementanalysis with the results of the visual inspections, it was found that theresults of the model did not correspond to the observations from practice. The outputof the analysis suggested a high risk of early-age cracking, while in realityearly-age cracking did not occur in any of the inspected walls. Possible causesfor this difference were subsequently discussed. In the end, conclusions were drawn on the accuracy of the analysis, thematerial properties that were determined through laboratory testing and theeffect of viscoelastic material behaviour on the risk of early-age cracking ofthe walls. It was found that the material behaviour at early-ages (the first 48hours) is very important for the overall stress development. Also, the resultsof the laboratory testing suggested that the effects of creep- and stressrelaxation are generally underestimated in this period. Moreover, it wasconcluded that early-age autogenous swelling is of significant influence on thestress development over time. This period of swelling prior the autogenousshrinkage is however not taken into account in the current Eurocode.  More research is needed into the (viscoelastic) material behaviour atearly-ages. In this way the creep curves as proposed in this research can beverified. The fact that cracking of the walls only occurred at higher ages,suggests that the effect of stress relaxation at higher ages is limited. Thisbehaviour should be investigated in more detail. Also, the research methods thatare used in this study should also be combined for more cases in the future.Using the combination of a computational model, laboratory tests and observationsfrom practice creates the necessary conditions to be able to draw well-foundedconclusions on the accuracy of the used models and the material behaviour in practice.Testing methods should be developed/improved to be able to measure materialbehaviour at very early ages.   ... Read more

In this research an effort is made to contribute to the goals of the Betonakkoord. This is done by optimising the design of big and bulky structures consisting out of a lot of concrete, namely lock heads. Lock heads are part of the navigation lock. The main research question answered in this thesis is phrased as follows: How can the design of lock heads be optimised to increase the sustainability?
Different alternatives have been generated to try to increase the sustainability. Each alternative has been compared to the lock head in Empel to test their feasibility. The following alternatives are considered: Inhomogeneous cross section, Prestressing and Hollow sections. From the alternative study it becomes clear that none of the alternatives are effective. The alternatives show no significant decrease in cost and MKI. Therefore, the alternatives are neglected in the remainder of the research. Based on this conclusion the following question arises: Is it possible to increase the sustainability of a lock head design based on commonly used design rules? In the next step of the research a parametric model is developed in order to answer this question. The parametric model design is based on the rules prescribed by the 'Handboek voor het ontwerpen van Schutsluizen' and the 'Richtlijnen Vaarwegen 2017'. The parametric model takes into account two types of gates, being a single leaf gate and a mitre gate. Again, the lock head in Empel has been used to validate the parametric model. From the parametric model it follows that in general a mitre gate is more cost effective and sustainable than a single leaf gate. This is because a mitre gate is generally shorter than a single leaf gate, so less materials are used and the construction pit can be smaller. Furthermore, the parametric model shows that the global stability check horizontal bearing capacity is a key parameter in the design of a lock head. To account for the horizontal bearing capacity the length and the weight of the lock head are important factors. The next step is to compare the lock head designs from the parametric model with the lock head design in Empel. The lock head design in Empel deviates from the rules prescribed by the 'Handboek voor het ontwerpen van Schutsluizen'. By deviating from the rules a more cost effective and sustainable design is acquired than both the designs from the parametric model. The lock head in Empel is shorter and lighter than the lock head designs from the parametric model. This is due to the fact that the lock head in Empel derives its horizontal bearing capacity from the lock chamber. In order to design a more cost effective and sustainable lock head it is advised to incorporate the lock chamber in the stability calculations. Hereby a shorter lock head can be achieved. ... Read more