Lifetime-Oriented Structural Design Concepts- P7 ppsx

138 3 Deterioration of Materials and Structures Number of cycles Strain Measuring point 2 Measuring point 1 Measuring point 3 Fig. 3.19. Development of strains in bending [662] Stress Deformation Fig. 3.20. Degradation process of relevant concrete properties due to tensile load- ings [429] process in metals consists of a microcrack initiation and afterwards microc- rack propagation phase. The fatigue degradation may culminate into macro- cracks and cause fracture after an adequate number of fluctuations or loads cycles. The part of fatigue modeled here is the propagation of the microcracks. Altough most materials on a scale of a few dozen grains are still anisotropic, and microplasticty certainly plays a role in the propagation of such microc- racks in most cases it is tried to build up a phenomenological continuum dam- age model based on the small scale yielding approach of linear elastic fracture mechanics. So the propagation of microcracking can be described with stress intensity factors of the cracks near tip field embedded in an isotropic material with the properties of the macroscopic scale. 3.1 Phenomena of Material Degradation on Various Scales 139 Flexural strength Related cycle ratio Fig. 3.21. Degradation process of relevant concrete properties due to flexural load- ings [866] Fig. 3.22. Stiffness reduction by high cycle fatigue Brittle Damage by Microcracks As in this context only damaging processes caused by microcracks, which are triggered by elastic stresses, are regarded no macroscopic plasticity has to be considered. Imagine such a member with growing microcracks undergoing a process, in which it is deformed by a total deformation, a certain part of this will be elastically recoverable, and another part can be induced by damage. When these loads are released, the member will have, in contrast to plastic- ity, not any remaining permanent deformation. Nevertheless, the state of the member could have changed; its elastic stiffness could have been reduced by the growth of microcracks. For a process which involves no further damag- ing, the total deformation is an elastic one, but starting from a state with 140 3 Deterioration of Materials and Structures Fig. 3.23. Model for brittle damage by microcrack growth changed elastic properties. The underlying micromechanics for a continuum point and the corresponding macro-stresses and strains are sketched in Fig- ure 3.22. Starting with an unstressed member, containing a crack of length 2a and the resulting average stiffness E (the stiffness of the matrix remains unchanged by crack growth), up to a certain load the crack will not grow in length but only open its width. Beyond this threshold the crack length will increase and the average stiffness decreases. When the member is unloaded again the crack will close and no further growth occurs. For the same stress a greater strain will result, due to the reduced stiffness. In the stress free state there is - as already mentioned - no permanent deformation. Only the stiffness remains on a lower level than in the initial stage. The free energy W stored at the end of the process and the energy dissipated by crack growth W crack are also sketched in Figure 3.22. This means the process of stiffness degradation can be modelled by finding a correct representation for the energy dissipated by crack growth. This will be the basis for the continuum damage model for high-cycle fatigue of metals presented in Section 3.3.1.2.2.2. 3.1.2 Non-mechanical Loading Authored by Otto T. Bruhns and G¨unther Meschke 3.1.2.1 Thermal Loading Authored by Rolf Breitenb¨ucher and Hursit Ibuk 3.1.2.1.1 Degradation of Concrete Due to Thermal Incompatibility of Its Components If the thermal behaviour and the thermal properties of the various concrete constituents are quite different from each another, in cases of significant 3.1 Phenomena of Material Degradation on Various Scales 141 temperature changes, incompatibilities in the deformations of the different materials cause internal stresses between the aggregates and the cement paste, wich further can result in internal cracking. For this purpose the coefficients of thermal expansion (α T -value) of concrete constituents can become important. However, under normal conditions, in practice, differences in the thermal expansion coefficient are not necessarily deleterious when the temperature does not exceed the temperature range of about 4 to 60 ◦ C. However, if the two relevant α T -values (aggregates, cement paste) differ seriously (much more than 5.5 ·10 −6 K −1 ) from each another the durability of concrete concerning freezing and thawing may be affected [567]. 3.1.2.1.2 Stresses Due to Thermal Loading Much more important for microcracking and degradation processes in concrete structures are restraint stresses, caused by restraining of thermal deformations ( T = α T ·Δt). Such restraint can be external as well as internal. In most cases temperature profiles over a cross-section are not constant or linear, but more or less stochastic and non-linear (Figure 3.24). Thus, the re- sulting stresses can be divided into longitudinal, warping and internal stresses. For longitudinal stresses with constant magnitude an external restraint and constant temperature changes over the cross-section of concrete are respon- sible. Such an external restraint is caused in practice e.g. by bond to a stiff foundation or an already hardened concrete member. A linear distributed tem- perature gradient results in warping stresses, since the bending deformations usually are restrained already by the deadload or also by external restraint. Internal stresses are formed by the restraint of non-linear thermal deforma- tions. In this case the restraint is internal, as the cross-section cannot deform unevenly (Bernoulli-hypothesis). In context with restraint stresses due to restrained thermal deformations especially in thicker concrete members often already the load-case ”heat of hydration” becomes relevant. Fig. 3.24. Stresses in a concrete slab at one-sided, non-linear cooling from the top [145] 142 3 Deterioration of Materials and Structures 3.1.2.1.3 Temperature and Stress Development in Concrete at the Early Age Due to Heat of Hydration When concrete has been placed, initially the temperature remains unchanged, because the hydration process is still in its rest period (stage I) (Figure 3.25). A few hours after starting of hydration also a moderate temperature raise can be observed, however (also in case of restraint) without developing significant compressive stresses. At this stage II the concrete has not yet set and is there- fore still plastically deformable. Along with further hydration the stiffness of the concrete increases and may lead – if the deformations are restrained – to compressive stresses (stage III). The concrete temperature at the beginning of this third stage is called the first zero-stress temperature (1. T z ). However, also in this stage the relaxation of the young concrete is still high, so that in spite of a significant temperature rise only small compressive stresses are raised. In the consequence of this high relaxation at the end of stage III the maximum of the compressive stresses is obtained in general some time before the temperature maximum. After exceeding the temperature maximum the remaining compressive stresses decrease rapidly (stage IV). Only a few degrees below the temperature maximum the second zero-stress temperature (2. T z )is obtained. Already starting from this point tensile stresses are caused during Time [t] Time [t] T crack 2. T Z 1. T Z T concrete Stage -s +s Longitudinal stress Temperature [°C] I II III IV V T=T air fresh concrete Fig. 3.25. Temperature and stress development during the first hydration phase in restrained concrete elements [763, 145, 466] 3.1 Phenomena of Material Degradation on Various Scales 143 further cooling (stage V). When the not yet significantly developed tensile strength is exceeded in this cooling period, at an age of only a few days, first cracks will be formed at the so-called cracking-temperature (T crack ). [145]. Especially in mass concrete structures the internal restraint and thus the resulting internal stresses can become a dominant cause for thermal cracking. If the heat of hydration is not controlled and large temperature differences between the inner core and the surface are raised, internal stresses with tension at the surface develop in the concrete member. Thus, a surface map-cracking in the surface-zone can occur, whereby the crack-width usually is very small. It is evident, that the described cracking also at such thermal loadings doesn’t develop suddenly. Furthermore it has to be considered, that also in such cases a complex micro-cracking is preceding the macro-cracking formation. Thus also by this way degradation processes can take place, even if the tensile strength is not exceeded, i.e. when the ambient temperature is achieved before macro- cracks could be formed. In this case the concrete structure remains on a high tensile stress level and micro cracks (with resulting degradation) develop. 3.1.2.2 Thermo-Hygral Loading Authored by Max J. Setzer and Rolf Breitenb¨ucher 3.1.2.2.1 Hygral Behaviour of Hardened Cement Paste Authored by Max J. Setzer and Christian Duckheim Due to its nano- and microporous structure hardened cement paste inter- acts strongly with its environmental humidity. This gain or loss of water has a deep impact on durability and material properties below 0 ◦ C (e.g. frost) as well as above 0 ◦ C (e.g. creep and shrinkage) [633]. Even if further research is required, freeze-thaw-resistance of concrete structures and the corresponding mechanisms have been investigated extensively in the last years and can be ex- plained well today. In contrast, despite numerous different analyses creep- and shrinkage-mechanisms are only fragmentarily understood up to now. Amongst others, this fact can be attributed to the manifold parameters which influence experimental results (such as sample composition and shape or the measur- ing setup and procedure) but most of all to the complex colloidal structure formed by nano-sized CSH-particles, where only complicated ascertainable surface interactions play a decisive role. Drying shrinkage and swelling as a basic hygric property of hardened cement paste (w/c =0, 35; 0, 40; 0, 50 and 0, 60) has been investigated over the complete humidity range by means of a newly developed laser supported measuring principle. This new technique allows the speedy, precise measurement of the pure material characteristic of several filigree samples with an accuracy of about 20 nm. Further mainly novel methods have been applied for examining sorption behaviour as well as inner volume and density change. Measurement data have been analysed with 144 3 Deterioration of Materials and Structures -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 0 102030405060708090100 relative humidity (%) w/c = 0,35 (1st Des.) w/c = 0,35 (1st Ads.) w/c = 0,40 (1st Des.) w/c = 0,40 (1st Ads.) w/c = 0,50 (1st Des.) w/c = 0,50 (1st Ads.) w/c = 0,60 (1st Des.) w/c = 0,60 (1st Ads.) Fig. 3.26. Hygric strains vs. relative humidity -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 02468101214161820 water content (%) w/c = 0,40 (1st Des.) w/c = 0,40 (1st Ads.) w/c=0,40(2ndDes.) w/c=0,40(2ndAds.) Fig. 3.27. Hygric strains vs. relative humidity & vs. water content respect of the prevailing mechanisms on the nano- and microscale as surface energy, disjoining pressure and capillary tension. In Figure 3.26 hygric strains of four samples with different w/c-ratios during first de- and adsorption are illustrated. Figure 3.27 shows the relation between the measured deformations (w/c =0, 40) and water content of 3.1 Phenomena of Material Degradation on Various Scales 145 -9 -8 -7 -6 -5 -4 -3 -30 -25 -20 -15 -10 -5 0 surface free energy change (J/g) w/c = 0,35 (1st Des.) w/c = 0,35 (1st Ads.) w/c = 0,40 (1st Des.) w/c = 0,40 (1st Ads.) w/c = 0,50 (1st Des.) w/c = 0,50 (1st Ads.) w/c = 0,60 (1st Des.) w/c = 0,60 (1st Ads.) Fig. 3.28. Hygric strains vs. surface free energy change. For further details (calcu- lation of surface energy and deformations due to capillary tension) see [239] the structure including a second desorption-adsorption-cycle. The total deformation, which grows with increasing w/c-ratio, lies in between 7 mm/m and 9 mm/m. Examining the results, in the range from 0 % r. h. to 100 % r. h. different sections (desorption: 100% → 35% → 25% → 0%; adsorption: 0% → 60% → 100%) with each varying dominating mechanisms can be found. A close connection between water content of the structure and studied properties is demonstrated with only a marginal hysteresis between drying and wetting as well as the influence of capillary condensation. It could be proved that in the lower humidity range shrinkage and swelling are indeed proportional to changes in the surface free energy indeed (Figures 3.28 to 3.31). However, an energy reduction during adsorption does not lead to an expansion as assumed up to now (Munich Model), but rather to a contraction of csh-particles (Figure 3.31), while the pore volume increases simultaneously and vice versa during desorption. Solid density which is nearly independent from w/c varies between about 2.3g/cm 3 (dry) and 2.5g/cm 3 (wet). For this reason the influence of surface energy has to be attributed to the dispersive component of disjoining pressure which prevails in the lower humidity range, whereas in the range of condensation repulsiv components (electrostatic and structural component) and capillary tension dominate the processes in hardened cement paste. Consequently here a distinct linear relation-ship exists between hygric strains and water content (Figure 3.27). Irreversible strains have to be attributed merely to first drying. 146 3 Deterioration of Materials and Structures -5 -4 -3 -2 -1 0 0 102030405060708090100 relative humidity (%) w/c = 0,35 (calculated) w/c = 0,35 w/c = 0,40 (calculated) w/c = 0,40 w/c = 0,50 (calculated) w/c = 0,50 w/c = 0,60 (calculated) w/c = 0,60 Fig. 3.29. Hygric strains vs. surface free energy change & comparison between mea- sured hygric strains and hygric strains calculated by capillary tension. For further details (calculation of surface energy and deformations due to capillary tension) see [239] 0 5 10 15 20 25 30 0 102030405060708090100 relative humidity (%) w/c = 0,35 (1st Des.) w/c = 0,35 (1st Ads.) w/c = 0,40 (1st Des.) w/c = 0,35 (1st Ads.) w/c = 0,50 (1st Des.) w/c = 0,50 (1st Ads.) w/c = 0,60 (1st Des.) w/c = 0,60 (1st Ads.) Fig. 3.30. Sorption isotherms vs. relative humidity 3.1 Phenomena of Material Degradation on Various Scales 147 2,2 2,3 2,4 2,5 2,6 0 102030405060708090100 relative humidity (%) w/c = 0,35 (1st Des.) w/c = 0,35 (1st Ads.) w/c = 0,40 (1st Des.) w/c = 0,40 (1st Ads.) w/c = 0,50 (1st Des.) w/c = 0,50 (1st Ads.) w/c = 0,60 (1st Des.) w/c = 0,60 (1st Ads.) Fig. 3.31. Solid density vs. relative humidity The presented findings and additional results are merged in a schematic diagram (Figure 3.32) which describes the change of various hygric proper- ties qualitatively and illustrates the effects of the two different mechanisms (disjoining pressure and capillary tension) on the system of hardened cement paste during first desorption and adsorption. Elaborate explanations and fur- ther details can be found in [239]. 3.1.2.2.2 Influence of Cracks on the Moisture Transport Authored by G¨unther Meschke Cracks, irrespective of their origin, have a considerable influence on the moisture permeability of cementitious materials. As a consequence, the trans- port of aggressive substances is promoted and the degradation process is further accelerated. The significant influence of fracture on the transport prop- erties of porous materials was first recognized in the context of the coupled mechanical and hydraulic behavior of fractured rock masses. Experiments by Zoback & Byerlee [874] indicate an increase of the permeability of granite caused by microcracking. Particularly for materials with very low moisture permeabilities, such as granite and shale, flow through the connected pore space was found to be insignificant compared to flow through fracture zones. The role of cracks on the transport properties of cement-based materials has been investigated in e.g. [92, 155, 309, 41, 310], see Breysse and G´erard [153] for a state-of-the-art survey. It has been shown, that the problems of moisture transfer change the scale, in fact that the permeability is increased by several orders of magnitude, when cracking is considered. [...]... materials (Subsection 3.2.1.1), laboratory tests of concrete and soil subjected to cyclic loading (Subsections 3.2.1.2 and (Subsections 3.2.2) and structural experiments of steel-concrete composite structures (Subsection 3.2.3) 3.2.1 Laboratory Testing of Structural Materials Authored by Otto T Bruhns and G¨nther Meschke u 3.2.1.1 Micro-macrocrack Detection in Metals Authored by Henning Sch¨tte and... concrete structures Cooling towers, containments for nuclear or other waste disposal, cement-bound coatings of drinking water reservoirs, grouted anchors and tunnel linings are examples for structures and structural 3.1 Phenomena of Material Degradation on Various Scales (a) (b) (c) 153 (d) Fig 3.35 Schematic illustration of the dissolution and loading induced long-term deterioration of concrete: (a) Reduction... This indicates, that the calcium ion conductivity of the pore fluid increases as the calcium concentration c decreases with propagating chemical dissolution The influence of chemical degradation on the structural behaviour of concrete specimens have been investigated by Le Bell´go et al [477, 479, 478] e In these experiments mortar beams have been exposed to ammonium nitrate solution on the front and... sufficient Typical indicators of ASR are at the beginning random map cracking and in advanced states attendant spalling of concrete Petrographic examination can conclusively identify ASR [146, 768] The basic structural unit of all forms of silica is a silicon ion (Si4+ ) surrounded by four oxygen ions (O2− ) with the arrangement of a tetrahedron [403] In crystalline, the rather low reactive form of silica,... regions which, in turn, lead to the opening and propagation of cracks and to the disruption of the affected concrete This results in a drastic reduction of the mechanical properties and consequently to structural degradation [373] By comparing the time scales of the swelling of synthetic gels (e.g Struble & Diamond [779]) and of concrete specimens (e.g Larive [469], it can be concluded, that the imbibition... strain amplitudes are relatively small (i.e the cyclic stress path does not touch the failure line) Usually these conditions are fulfilled for soils under high-cyclic loading since the foundations are designed to keep the stress path (including the amplitudes) away from the failure condition The effects of compaction due to cyclic loading result in strengthening and stiffening of the soil They are even... techniques and therefore it is somewhat misleading to use the term ”deterioration” or ”fatigue” for soil However, excessive settlements of foundations under cyclic loadings may result from unsuitably designed foundations (e.g inappropriate dimensions, missing soil improvement) Especially, in statically indeterminate structures large differential settlements may accelerate the deterioration processes... liquefaction (Nc = 10 - 100) is much smaller than the Nc values usually considered in studies on the life time of structures Therefore, this ”deterioration” phenomenon is irrelevant for lifetime oriented design concepts The liquefaction phenomenon may also be utilized for passive isolation of structures or for soil improvement techniques (Section 2.5.1) Under drained conditions a ”deterioration” may take... average stress ratio above the critical one In both cases dilatancy occurs leading to a reduction of the strength and the stiffness of the soil However, such stress paths should be avoided by an appropriate design of foundations considering also the cyclic part of the loading While the extensive experimental study presented in Section 3.2.2 has significantly improved the understanding of the accumulation phenomenon... due to the additional mechanical load Considerable progress was achieved in material-oriented research on environmentally induced degradation mechanisms, which led to a better understanding of the microstructural mechanisms and to analysis tools to simulate the relevant processes In Subsections 3.1.2.3.2, 3.1.2.3.3, 3.1.2.2.2 the main experimental findings associated with long-term degradation processes . lower humidity range, whereas in the range of condensation repulsiv components (electrostatic and structural component) and capillary tension dominate the processes in hardened cement paste. Consequently. envi- ronmentally induced degradation mechanisms, which led to a better under- standing of the microstructural mechanisms and to analysis tools to simulate the relevant processes. In Subsections 3.1.2.3.2,. of drinking water reservoirs, grouted anchors and tunnel linings are examples for structures and structural 3.1 Phenomena of Material Degradation on Various Scales 153 (a) (b) (c) (d) Fig. 3.35.