Differential Drying Shrinkage Research Articles

Prediction of differential drying shrinkage of airport concrete pavement slabs

ABSTRACT In this study, strain gages were installed at the centre, edge, and corner of concrete pavement slabs of Incheon International Airport based on the slab depth, whereas thermometers were positioned at the slab centre to measure the slab behaviour according to the temperature change. A three-dimensional finite element model simulating the airport concrete pavement was generated, and the analysis was conducted by using the measured temperature and material properties as functions of the concrete age and slab depth as its input variables. The difference between the measured strain and predicted thermal strain was calculated as the drying shrinkage because the finite element analysis did not consider the moisture evaporation but rather only the temperature change. The calculated drying shrinkage was large near the top of the slab from where the moisture evaporated, and the differential drying shrinkage between the top and bottom of the slab increased until the concrete age of nearly one month. Consequently, an upward curling developed owing to the differential drying shrinkage. The upward curling was the most obvious at the slab corner because the position was more exposed to ambient air and less constrained than at other positions.

A practical creep model for concrete elements under eccentric compression

Many prestressed concrete bridges are reported to suffer from excessive vertical deflections and cracking during their service life. Creep softens the structure significantly, and therefore an accurate prediction of creep is necessary to determine long-term deflections in elements under eccentric axial compression such as prestressed concrete girders. This study proposes a modification to the creep damage model of Model Code 2010 to account for the effect of load eccentricity. The modified creep model considers damage due to differential drying shrinkage. Initially, the creep behaviour of small scale concrete specimens under eccentric compression load is investigated experimentally. Twelve small-scale concrete prisms were subjected to eccentric axial loading to assess their shrinkage and creep behaviour. The main parameters investigated include the load eccentricity and exposure conditions. Based on the experimental results, an inverse analysis is conducted to determine the main parameters of the modified creep model. Subsequently, a numerical hygro-mechanical simulation is carried out to examine the effect of load eccentricity on the development of shrinkage and creep, and on the interaction between drying, damage and creep. The results indicate that eccentric loading leads to different tensile and compressive creep through the cross section, which contradicts the current design approach that assumes that tensile and compressive creep are identical. The proposed model also predicts accurately the long-term behaviour of tests on reinforced concrete elements available in the literature. This study contributes towards further understanding of the long-term behaviour of concrete structures, and towards the development of advanced creep models for the design/assessment of concrete structures.

전력구 콘크리트 구조물의 건조수축 균열특성에 관한 연구

The purpose of this study is to predict the cracking behavior and suggest the method of controlling the cracking in concrete box culvert for power transmission due to differential drying shrinkage. Drying shrinkage cracking is mainly influenced by the moisture diffusion coefficient that determines moisture diffusion rate inside concrete structures. In addition to the diffusion coefficient, surface coefficient of concrete surface and relative humidity of ambient air simultaneously affect the moisture evaporation from concrete inside to external air outside. Within the framework of drying shrinkage cracking mechanism, it is necessary to perform the numerical analysis, which involves these three influencing factors to predict and control the shrinkage cracking of concrete. In this study, moisture diffusion and stress analysis cor responding to drying shrinkage on concrete box culvert are performed with consideration of diffusion coefficient, surface coefficient, and relative humidity of ambient air. From the numerical results, it is found that cracking behavior due to differential drying shrinkage of box culvert shows the different feature according to three influencing factors and the methodology of controlling of drying shrinkage cracks can be suggested from this study.

The impact of wet curing on curling in concrete caused by drying shrinkage

This paper examines the impact of wet curing with different durations on differential drying shrinkage of concrete beams. The work shows that in severe drying conditions the magnitude of curling increased with the duration of wet curing of concrete specimens. Testing results are presented along with an explanation for the mechanisms observed. These findings provide guidance for wet curing durations for concrete slabs that must be resistant to volume change from drying.

Precise observation of soil surface curling

The process of drying in soil is often associated with complex deformations. Due to the complexity of both the soil structure and the coupled hydro mechanical processes occurring during drying, different forms and shapes can be created during drying. Soil curling is recognized as one of the typical phenomena taking place during shrinkage, when the surface layer is curled up or down by different mechanisms triggered during drying. A precise non-contact electro-optical technique based on a 2D laser profile scanner with motion controller to systematically track the evolution of the exposed surface of a natural soil during controlled drying conditions was used in this research. Different stages of curling were identified at different elapsed times. These stages are discussed in detail. To gain a better understanding of the different curling stages and their associated mechanisms, a set of drying experiments were performed on artificially prepared mixtures of kaolin and silica sand. Particularly, two basic mechanisms were studied: differential drying and differential shrinkage effects. A simple conceptual model is also proposed and discussed to help in the interpretation of the test results involving the curling.

Influences of reinforcement on differential drying shrinkage of concrete

Shrinkage strain of concrete specimen with different reinforcement configuration was measured at various depths from the exposed surface by using several pairs of displacement sensors. Only one surface of the concrete specimen was exposed to dry condition during the experiment. The results show that differential shrinkage strain occurs in both plain and steel reinforced concrete specimens according to depths from the exposed surface. A higher reinforcement ratio results in a greater restraint against shrinkage of concrete nearby reinforcement rebar and a worse differential shrinkage strain distribution in the concrete specimen. The restraint against shrinkage of concrete becomes lower with the increasing distance from reinforcement rebar. Under the same reinforcement arrangement, a higher free shrinkage of concrete leads to a stronger restraint against shrinkage and a higher shrinkage stress formation in local concrete. The relationship between shrinkage strain and reduction of relative humidity in reinforced concrete structure is far different from that in plain concrete.

New Warping and Differential Drying Shrinkage Models for Jointed Plain Concrete Pavements Derived with Nonlinear Shrinkage Distribution

Warping can be quantified by the equivalent temperature difference required to deform a theoretically flat slab to the same shape as the actual slab. In the Mechanistic–Empirical Pavement Design Guide (MEPDG), the current warping model for jointed concrete pavements uses a triangular shrinkage distribution within the shrinkage zone near the surface of the slab as well as modification factors to account for ambient relative humidity (RH) and the amount of total shrinkage that is expected to be reversible. The current warping model was evaluated and found to predict improbable values. A new warping model based on an assumed nonlinear shrinkage distribution that followed the curve described by one-quarter of an ellipse was proposed. The elliptical shrinkage distribution was found to agree well with predicted moisture distributions through the depth of the slab at various locations around the United States. A new factor accounted for the fact that only drying shrinkage contributed to warping, and changes were made to the RH adjustment factor. The proposed model was derived from plate theory, rather than beam theory, to account for the two-way bending behavior of a slab. Only parameters that were currently inputs in the MEPDG were required for the new proposed warping model. With slight modification, the model could be used to predict differential drying shrinkage levels in jointed concrete pavements.

Numerical analysis of the early age behavior of concrete structures with a hydration based microplane model

To simulate the early age behavior of concrete structures composed of young concrete, a finite element analysis (FEA) was implemented with the hydration based microplane model. Structural behaviors were investigated with concrete age for a massive concrete wall and slab. From FEA, it was found that surface cracking at early age occurred via different crack driving mechanisms. In the case of combined hydration heat and shrinkage, later surface cracking was produced by differential drying shrinkage. A numerical analysis performed with the hydration based microplane model successfully simulated the typical cracking patterns due to edge restraint in the concrete slab.

Analysis of microcracking induced by differential drying shrinkage

Abstract The drying of concrete induces surface microcracking. Because drying is, especially, a slow process, differential drying between the surface and the core induces shrinkage, which provokes an increase of the tensile stresses on the surface and of the compressive stresses in the core (the resultant is zero if there are no external forces or external restraints). Often, the tensile stresses obtained by elastic finite element analysis exceed the tensile strength. Numerical simulations show that if creep strains are taken into account the damage due to differential drying shrinkage is less important (compared to the case if creep is not considered in the numerical analysis). Therefore, a little drop of mechanical properties is predicted due to drying, for the studied concrete. Therefore, one may assume that microcracking and the associated drop in the Young’s modulus as the concrete dries are due mainly to aggregate restraint. Besides, an elastic damage model for prediction of cracking is found to be sufficient to retrieve evolution of drying shrinkage and damage fields (but not the orientation).

Effect of differential moisture distribution on the shortening of steel-reinforced concrete columns

This paper presents experimental and analytical studies on the causes of differences between reinforced concrete (RC) and steel-reinforced concrete (SRC) column behaviour with a focus on moisture diffusion. The results of experiments on the long-term behaviour of RC and SRC columns have shown that the shortening of SRC columns tends to be overestimated by the methods generally used to predict the shortening of RC columns. Moisture diffusion coefficient and surface factor of the concrete used in the SRC column specimens were determined from experiments and an analysis on moisture diffusion was conducted to determine the progress of moisture diffusion in the SRC column specimens. The analytical results indicated that the progress of moisture diffusion in SRC columns is quite different from that in RC columns. Analyses on differential drying shrinkage and differential drying creep were also performed by analysing the progress of moisture diffusion at each point in an SRC column section. The results indicated that obstruction of the moisture diffusion route causes a reduction in both drying shrinkage and drying creep in the SRC columns. The differential moisture distribution should therefore be taken into account in order accurately to predict SRC column shortening.

Anchor Design to Prevent Debonding of Repair Mortar in Repaired Concrete Members

Reinforced concrete beams or slabs are often strengthened or repaired using polymer modified cement concrete Stresses can develop in the structure by ambient temperature changes because thermal coefficients of the repair material and the existing concrete are typically different. Especially, shear stress often causes debonding of the interface. In this study, a rational procedure was developed where anchors can be designed in strengthened or repaired concrete members to prevent debonding at the interface. The current design procedure considers thicknesses and elastic moduli of the repair material and existing concrete, ambient temperature change, length, and beam-vs.-slab action. The procedure is also applicable to stresses developed by differential drying shrinkage.

Transverse Joint Analysis for Mechanistic-Empirical Design of Rigid Pavements

With the further adoption of mechanistic-empirical design methods in the pavement industry, the calculation of critical responses and cumulative damage for a variety of parameters will be imperative. Traditionally, the critical tensile stress developed by loads at the midslab edge has been used as a mechanistic parameter to determine the required thickness in jointed concrete pavements. However, the inclusion of both temperature and shrinkage gradients in concrete pavement analysis can drastically alter the critical stress location and subsequent distress type that predicts pavement performance. Longitudinal and corner cracking have been found in California to be distresses as significant as transverse cracking. Most of the longitudinal and corner cracking can be explained by excessive differential drying shrinkage. Using finite-element analysis, this study compared the critical tensile stress near the transverse joint with the critical tensile stress at the midslab edge (relative reference stress) for California-type jointed plain concrete pavements. The analysis of the data showed that transverse joint loads were more significant in critical stress calculations for a considerable number of input parameters. These loads at the transverse joint can manifest themselves as top-down or bottom-up longitudinal, transverse, or corner fatigue cracks unlike the bottom-up transverse cracks traditionally predicted by midslab edge loads. The likelihood of critical slab stresses near the transverse joint was considerably increased with the use of negative temperature gradients and extended lane widths.

Mechanistic Design Framework for Spalling Distress

Spalling is a distress form in concrete pavements that often manifests as the breakdown of the joint of a slab within 15 cm (6 in.) of the joint or crack and can occur at both longitudinal and transverse joints. Efforts have been under way at Texas A&M University to formulate mechanistic spalling models derived from data gathered in recent Texas Department of Transportation studies related to spall development. Extensive field studies have led to the establishment of a spalling mechanism consisting of a step-by-step process that can be characterized with engineering mechanics. These findings indicate that spalling is the result of damage initiated in the form of a shear delamination that is oriented parallel to and at a shallow depth below the surface of the pavement. Conditions necessary for formation of the delaminations include low interfacial strength between the aggregate and mortar and sufficient evaporation of pore water from the hydrating concrete, resulting in differential drying shrinkage near the pavement surface. Delaminations have been noted to initiate early in the life of the pavement and, once formed, extend later into spalls as a result of incompressibles, freeze-thaw cycles, traffic loading, and other such effects. A design framework for delamination formation and subsequent spalling development is presented in a practical format in which to mechanistically design concrete pavement systems relative to spalling distress.

Prediction of differential drying shrinkage in concrete

The non-uniform moisture distribution in concrete causes the differential drying shrinkage. From this type of differential drying shrinkage, tensile stress occurs on the exposed surface of concrete structures and may result in crack formation. This residual stress is significantly affected by the creep of concrete. In this study, for the purpose of predicting the differential drying shrinkage, the analysis method was suggested, in which the creep of concrete was also considered. In addition, the differential drying shrinkage strain was measured at various positions in concrete by using embedded strain gauges. The internal drying shrinkage strain differs significantly according to the depth from exposed surface. The validity of analysis method was verified by comparing test results with analytical results. Finally it was found that analytical results were in good agreement with test results.

Effect of Graphite on Microstructure and Drying Shrinkage of Clay

Graphite specimens bonded by kaolinite‐illite clay, in which planar orientation of the graphite flakes and clay platelets had been produced by deformation, showed enhanced differential drying shrinkage with increased graphite content. The greater shrinkage was perpendicular to the plane of preferred orientation. Comparable additions of quartz grains to the clay made the drying shrinkage less differential. The volume drying shrinkage was first increased and then decreased by successive additions of graphite. The increase was ascribed to the increasing concentration of water in the clay bond as the clay content diminished. The subsequent decrease appeared because the drying shrinkage of the clay was reduced near the graphite and because the graphite itself did not shrink.

Anomalous Differential Drying Shrinkage of Clay‐Quartz Mixtures

Anomalous Differential Drying Shrinkage of Clay‐Quartz Mixtures