Numerical modelling of alkali silica reaction in concrete dams

Pourbehi, Mohammad Sadegh (2018-12)

Thesis (PhD)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Dams are important infrastructure components and an asset for any country. Past earthquakes have highlighted their vulnerability to damage and even failure which can have major socio-economic consequences, losses and other cascading effects (e.g. water supply, power generation and irrigation). Hence, considerable research has been done to evaluating the safety of the aged dams and in some cases to pursue a suitable remedial action and rehabilitation strategy. Alkali Silica Reaction (ASR), a deleterious chemical reaction between siliceous aggregate and cement paste in concrete, causes long term swelling and deterioration of concrete structures such as dams. Several important factors govern the ASR reaction kinetics, of which water and temperature are the most important. Numerical modelling should capture the impact of humidity, temperature, and confining stresses and material degradation during the ASR as the most prominent factors influencing ASR. In general, numerical models for modelling ASR and its effects fall into three categories: (1) macrostructural models concerned with the analysis of structures affected by the reaction; (2) microstructural models which link the chemical reaction to its impact at the material level; (3) mesoscopic models that consider the multi-phases of the aggregate, cement paste, void and ASR gel, whereby anisotropy is explicitly represented. In this study a chemo-damage-plastic model is developed, considering temperature, humidity, non-uniform time-dependent material degradation and 3D stress confinement effects on ASR evolution. The model is validated by modelling the structural behaviour and response of three concrete gravity dams namely Fontana dam, Kleinplaas dam and Koyna dam. Comparing the results with the actual data on macro crack appearance and crest displacement as well as with other numerical models available in literature. The model developed for Kleinplaas dam is calibrated with the measured data and the applicable ASR parameters are identified. An attempt is then made to simulate the past and current swelling behaviour of the dam and the associated damage. The results successfully predicted the displacements and ASR strain rates plus the development of irreversible plastic cracks. While the structural behaviour of ASR affected structures under monotonic and quasi-static loading has been extensively investigated over the last decades, limited research has addressed dynamic loading. The combined effect of old and new cracks under dynamic excitation may cause dam failure. Considering the predicted interaction between ASR and seismic loads, remedial measures can be adopted at the right point in time to safeguard the dam in the event of an earthquake. Fluid-Foundation-Structure Interaction (FFSI) also has received much attention in Finite Element Analysis (FEA) of dams. The SU-ASR (Stellenbosch University ASR FE code) model developed, is used to analyse and predict dynamic behaviour of the Koyna dam, damaged by an earthquake. The combined ASR and seismic action are investigated and comparisons are made through a comprehensive study of the damage development and crest displacement. The seismic response of the deteriorated dam is subsequently analysed based on the state of the structure at the end of the long term ASR analysis. The results show that this combined action can significantly change the dynamic behaviour of typical concrete dams. Finally, a rational prediction of the long term impacts of remedial actions such as the slot cutting technique and the justification of other remedial action based on analytical results, was done. This required the use of specialized software with an adequate constitutive law and time integration scheme. As an example, the slot cutting technique is implemented for a synthetic dam structure using the SU-ASR finite element based computer program code developed in this research. The results of this simulation are discussed and interpreted. The SU-ASR model predicted the behaviour of the dam before and after the slot cutting closure and the subsequent compressive stresses with good approximation.

AFRIKAANSE OPSOMMING: Damme verteenwoordig belangrike bates en ook komponente van die infrastruktuur van 'n land. Die voorkoms van aardbewings in die verlede het die vatbaarheid vir skade en selfs faling van hierdie strukture beklemtoon. Faling van damme kan drastiese sosio-ekonomiese gevolge, verliese en ander verdere gevolge hê (bv. watervoorsiening, krag opwekking en besproeiing). In die lig hiervan, word aansienlike pogings aangewend om die veiligheid van ouer damme te evalueer en ook om in sommige gevalle geskikte remediëringsaksies en rehabilitasie strategieë toe te pas. Alkali Silika Reaksie (ASR), 'n skadelike chemiese reaksie tussen silikahoudende aggregate en sementpasta in beton, veroorsaak langtermyn uitsetting en agteruitgang van betonstrukture soos damme. Verskeie belangrike faktore bepaal die ASR reaksiekinetika, waarvan water en temperatuur die belangrikste is. Numeriese modellering behoort die effek van humiditeit, temperatuur, en inperkende spanning en materiaaldegradasie tydens die ASR as die mees prominente faktore wat ASR beïnvloed, te ondervang. In die algemeen kan numeriese modelle vir die modellering van ASR en die effek daarvan in drie kategorieë geklassifiseer word: (1) makro struktuurmodelle wat verband hou met die analise van strukture wat deur die reaksie geraak word; (2) mikro struktuurmodelle wat die chemiese reaksie en die impak daarvan op die materiaalvlak met mekaar verbind; (3) mesoskopiese modelle wat die meer-fases van die aggregaat, sementpasta, lugopeninge en ASR-gel beskou, waarmee anisotropie eksplisiet gemodelleer word. In hierdie studie word 'n chemo-plastisiteit-skade-model ontwikkel, met inagneming van temperatuur, humiditeit, nie-uniforme tydafhanklike materiaaldegradasie en 3D-spanningsinperkings-effekte op ASR-ontwikkeling. Die model word gevalideer deur die strukturele gedrag en respons van drie beton swaartedamwalle naamlik Fontana-dam, Kleinplaasdam en Koynadam, te modelleer en die resultate te vergelyk met die beskikbare werklike makro-kraakvoorkoms en kruinverplasing data asook met ander numeriese modelle wat in die literatuur beskikbaar is. Die model wat ontwikkel is vir die Kleinplaasdam is gekalibreer met die gemete data en die toepaslike ASR-parameters is geïdentifiseer. Dan word daar gepoog om die voorafgaande en huidige uitsettingsgedrag van die dam en verwante skade te simuleer. Die resultate kon die verplasings en ASR-vervormingstempos asook die ontwikkeling van onomkeerbare plastiese krake, suksesvol voorspel. Alhoewel die strukturele gedrag en van ASR-strukture onder monotoniese en kwasi-statiese belasting die afgelope dekades omvattend ondersoek is, is beperkte navorsing oor dinamiese belastings gedoen. Die gesamentlike effek van ou en nuwe krake onder dinamiese aksie kan dam swigting veroorsaak. Met inagneming van die voorspelde interaksie tussen ASR en seismiese belastings, kan remediërende maatreëls op die regte tyd toegepas word om die dam te beveilig in die geval van 'n aardbewing. Daar is ook heelwat aandag aan Vloeistof-Fondament-Struktuur-interaksie (VFSI) in Eindige Element Analise (EEA) van damme gegee. Die SU-ASR model, wat ontwikkel is, is gebruik om die dinamiese gedrag van die Koynadam, wat deur 'n aardbewing beskadig is, te analiseer en te voorspel. Die gekombineerde ASR en seismiese aksie word ondersoek en vergelykings word gemaak deur 'n omvattende studie van die struktuurskade en kruinverplasing te doen. Die seismiese gedrag van die verswakte dam is vervolgens ontleed aan die hand van die toestand van die struktuur aan die einde van die langtermyn-ASR-analise. Die resultate toon dat hierdie gekombineerde aksie die dinamiese gedrag van betondamme aansienlik kan beïnvloed. Ten slotte is daar 'n rasionele voorspelling van die lang termyn impak van remediërende werk soos die groef-sny tegniek en die regverdiging van ander remediërende werk gebaseer op analitiese resultate, gedoen. Hiervoor is daar gebruik gemaak van gespesialiseerde sagteware met 'n voldoende materiaalwet modellerings- en die tyd integrasie skema vermoë. As 'n voorbeeld word die groef-sny tegniek geïmplementeer vir 'n sintetiese dam struktuur met behulp van die SU-ASR eindige element baseerde rekenaar program kode ontwikkel in hierdie navorsing. Die resultate van hierdie simulasie word dan bespreek en geïnterpreteer. Die SU-ASR-model voorspel die gedrag van die dam voor en na die slot-sluiting en die daaropvolgende drukspanning met 'n aanvaarbare vlak van akkuraatheid.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/104891
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