Early-age Cracking of Concrete

A study into the influence of stress relaxation on early-age cracking of concrete structures under imposed deformations

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Abstract

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.