Developing and testing a sprayable overlay of strain hardening cement-based composite for retrofitting of unreinforced load bearing masonry walls

De Beer, Leon Roode (2016-12)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: In the Western Cape Province (South Africa) a large number of multi-storey buildings, built with unreinforced load bearing masonry (URM) were constructed before seismic resistance became a design standard requirement in 1989. The region lies in a light to moderate seismic zone in South Africa. It is foreseen that these buildings perform poorly during seismic activities due to their brittle in-plane shear failure mode. Retrofitting of existing URM structures are no easy task and few solutions have been suggested. A spray-able Strain Hardening Cement-based Composite (SHCC) overlay as retrofitting material is proposed for the materials superior tensile and shear properties while applying the overlay is simple. SHCC is a fibre reinforced composite designed to form multiple fine cracks under tensile, shear and flexural load. The cracks are controlled to small widths, whereby significant toughness, or energy dissipation, is realised, while maintaining its resistance. This thesis report results of determining the applicability and performance of retrofitting URM walls with a sprayable overlay of SHCC produced with local available material. A spray-able SHCC was optimized for spray-ability as well as good adhesiveness to the masonry. Tensile strain capacity was tested throughout to determine the optimal mix having desirable fresh and hardened properties. SHCC free shrinkage, as well as the restrained shrinkage on the surface of the SHCC overlay were measured. The tests showed that restrained overlay shrinkage did not lead to visible debonding or cracking. The interfacial bond and multiple cracking for energy dissipation in a seismic event were investigated using the triplet shear test. The preferred failure mechanism is described and designed for by restricting the overlay thickness to allow multiple crack formation in the overlay, instead of overlay debonding. Design guidelines to ensure that energy dissipative failure mechanisms indeed occur during in-plane shearing action are presented. The tests showed that the SHCC overlay is a viable solution as the shear strength and energy absorption capacity increased manifold. Results from large scale shear wall validation tests are presented and the data was used to formulate a design model. The tests showed that multiple cracks form in the overlay, along with an increased in-plane shearing resistance compared with reference URM walls. It can be concluded that spray-able SHCC is a viable retrofitting solution for the seismic resistance of URM and further development and testing is encouraged to optimize and implement the use of this retrofitting strategy.

AFRIKAANSE OPSOMMING: In die Wes-Kaap (Suid-Afrika) is daar ’n groot aantal meer-verdieping geboue, gebou met ongewapende lasdraende messelwerk (URM) voor seismiese weerstand ’n ontwerpstandaard vereiste geword het in 1989. Die streek lê in ’n ligte tot matige seismiese streek in Suid-Afrika. Daar word voorsien dat hierdie geboue swak sal vaar tydens seismiese aktiwiteite as gevolg van hulle bros in-vlak skuif falingsmode. Versterking van bestaande URM strukture is geen maklike taak nie en etlike oplossings is al voorgestel. ’n Spuitbare vervormingsverhardende sement-gebaseerde saamgestelde materiaal (SHCC) as oorlaag materiaal word voorgestel vir die materiaal se uitstekende trek en skuif eienskappe terwyl die toepassing van die oorlaag eenvoudig is. SHCC is ’n veselversterkte saamgestelde material, ontwerp om menigvuldige fyn krake te vorm as trek, skuif of buig kragte aangewend word. Die krake se wydte word beperk, waardeur taaiheid en energie geabsorbeer word, terwyl die weerstand gehandhaaf word. Hierdie tesis rapporteer resultate oor die bepaling of ’n SHCC oorlaag, gemaak van plaaslike materiale, as versterking op URM mure toepaslik is, asook die ekstra weerstand en duktiliteit wat dit verskaf. Vrye krimp, asook belemmerde krimp op die oppervlak van ’n SHCC oorlaag is gemeet. Die toetse het getoon dat belemmerde krimp van ’n oorlaag nie lei tot sigbare krake of lostrek vanaf die bakstene nie. Die intervlak verband sterkte en veelvoudige krake vir energie absorbsie in ’n seismiese gebeurtenis is ondersoek met behulp van die ’triplet’ skuiftoets. Die voorkeur falingsmeganisme word beskryf en voor ontwerp deur die beperking van die oorlaag dikte, sodat veelvoudige krake in die oorlaag kan vorm in plaas van faling van die verband tussen die SHCC en bakstene. Ontwerp riglyne om te verseker dat die energie-absorberende falingsmeganisme inderdaad voorkom tydens in-vlak skuif aksie word verskaf. Resultate van grootskaalse skuif muur toetse word aangebied en die inligting is gebruik om ’n ontwerp model saam te stel. Die toetse het getoon dat meervoudige krake in die SHCC oorlaag vorm, saam met ’n verhoogde in-vlak skuif weerstand in vergelyking met die kontrole URM mure. Uit die studie kan die gevolgtrekking gemaak word dat spuit-SHCC ’n toepaslike oplossing vir die seismiese versterking van URM is en dat verdere ontwikkeling en toetsing aangemoedig word om die materiaal te optimeer en te implementeer as ’n versterkingsmetode.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/100318
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