Bond behaviour of deformed steel reinforcement in lightweight foamed concrete

De Villiers, Johannes Petrus (2015-12)

Thesis (MEng)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Lightweight foamed concrete is a low density concrete that utilizes the entrapment of air generated from a protein based foam mix constituent. It is produced with the base constituents: water, cement and fly-ash, whereafter stable foam is added to change the density. Lightweight foamed concrete is a relatively new building material with many economic advantages, but it does not reach some of the typical engineering properties of normal weight concrete. The latter is thoroughly researched, characterized and documented in design standards used throughout engineering practice, whereas the paucity of literature on lightweight foamed concrete adds to the slow progress in its development. Recent progress in mix design and density control of lightweight foamed concrete have led to the notion for its use in structural application i.e. steel reinforced lightweight foamed concrete. This study characterizes the bond behaviour of deformed steel reinforcement in lightweight foamed concrete, quantifies several of its engineering properties and presents prediction models. The concretes used for testing were a benchmark normal weight concrete and lightweight foamed concretes with casting densities of 1200, 1400 and 1600 kg/m3 and they were evaluated for compressive strength, Youngs modulus, indirect tensile splitting strength and wedge splitting fracture energy. Pull-out and beam-end tests were used to quantify the characteristic bond behaviour. The design of these tests comprises of the loading arrangements, specimen preparation, slip measuring techniques, equipment configuration for accurate slip control and a method for determining a design bond stress correlated to a physical occurrence. Even though compressive strengths suitable for structural application were achieved for some of the lightweight foamed concretes, its other strength properties and bond behaviour results did not reach the predicted or measured values of normal weight concrete. Compared to normal weight concrete, the fracture energy of lightweight foamed concrete is at least an order of magnitude smaller. The intrinsic brittleness of lightweight foamed concrete becomes more with increased density and is shown to have a considerable effect on the bond behaviour. The beam-end tests on the 1600 kg/m3 concrete showed early onset of bond deterioration as a result of internal cracks, intensified by its brittle nature. Evaluation of the bond-slip envelopes show the denser lightweight foamed concrete performing well when considering bond stress magnitude, but lacking ductility in failure. The least dense lightweight foamed concrete showed excellent ductility during failure, but lacked sufficient bonding stress. A new concept, named the Bond Integrity (BI), was introduced to quantify early bond disturbances such as internal cracks. It was formulated from the measured rate of change in applied moment and rate of change in the bar tensile force during the beam-end test. Prediction models for the design bond stress of lightweight foamed concrete were derived from the stress conditions at the steel-concrete bond interface, taking into consideration two failure modes; pull-out and splitting failure. The outcome of this study not only quantifies but also emphasizes the lack of sufficient bonding of deformed steel in lightweight foamed concrete. The inadequate crushing strength of the least dense concrete and the brittleness observed for the denser concrete, both diminishes bond integrity and permits significant bond development. It can be concluded that lightweight foamed concrete is not suitable for structural application and that further development of the material is required to increase strength, reduce brittleness and introduce innovative reinforcing systems that will lead to improved bond performance. Apart from providing standardized and novel results, together with detailed guidelines for the experimental setup and quality control, this study may well become the future basis for further development of lightweight foamed concrete as a structural material.

AFRIKAANSE OPSOMMING: Liggewig-skuimbeton is ’n laedigtheid beton wat van lug-gegenereerde, proteïen-basis skuim gebruik maak om lug in die betonmensel vas te vang. Dit word vervaardig uit die samestellende bestanddele: water, sement en vlieg-as, sowel as ’n gekontroleerde byvoeging van skuim om ’n verlangde digtheid te verkry. Liggewig-skuimbeton is ’n nuwe boumateriaal met baie voordele, maar dit skiet te kort in baie eienskappe wat eie is vir normale-gewig beton en wat geskik is vir strukturele elemente. Laasgenoemde beton is reeds goed gevestig in die ingenieursbedryf en word ondersteun en gestaaf deur omvattende navorsing, eksperimente vir die karakterisering van die materiaal en dokumentasie. Die gebrek aan literatuur betreffende liggewig-skuimbeton dra by tot die stadige vordering in die materiaal-ontwikkeling. Onlangse vordering in die ontwerp van liggewig-skuimbeton mengsels en effektiewe beheer van die giet-digthede, het gelei tot die oorweging om hierdie beton as ’n strukturele boumateriaal te gebruik d.w.s as ’n gewapende liggewig-skuimbeton. Hierdie studie klassifiseer die verbindingsgedrag van tipiese vervormde staalbewapening in liggewigskuimbeton, deur sommige eienskappe met gespesialiseerde toetse te meet en kwantifiseer. Nuwe modelle vir die voorspelling van die verbindingsgedrag word aangebied. Die tipes beton wat getoets is sluit in normale-gewig beton en liggewig-skuimbeton met gietdigthede van 1200, 1400 and 1600 kg/m3, waarvan eersgenoemde se resultate as referensie gebruik is. Hierdie materiale is getoets vir vergruisingssterkte, Young’s modulus, indirekte treksplytingsterkte en wig-splyting kraak-energie. Uittrek- en balk-einde toetse is gebruik om die karakterisering van die verbindingsgedrag te doen. Die ontwerp van hierdie toetse sluit in die belastings, monstervoorbereiding, tegnieke vir glipmeting, kalibrasie van die toetsmasjien vir akurate glipbeheer en ’n metode om die ontwerpsbindingskragte te verkry, gebaseer op ’n fisiese gebeurtenis tydens die toetsing. Genoegsame druksterktes vir beton, geskik vir strukturele toepassing, is verkry met sommige van die liggewig-skuimbeton toetse, maar alle ander toetse, insluitend die verbindingsgedragtoetse, het nie die voorspelde of gemete spesifikasies van die normale-gewig beton bereik nie. Die energie benodig om liggewig-skuimbeton beton te kraak is ten minste ’n orde kleiner as dié van normale-gewig beton. Die intrinsieke brosheid van liggewig-skuimbeton word meer met toename in digtheid en die invloed daavan op die verbindingsgedrag is ekperimenteel bepaal. Die balk-einde toetse op die 1600 kg/m3 beton het ’n vroeë verlies van die verbinding a.g.v. interne krake aangedui, wat verder verswak met toename in digtheid/brosheid. Die gemete verbindingsgedrag-glipgrafieke toon dat die digter liggewig-skuimbeton ’n relatiewe hoë verbindingspanning lewer, maar dat die styfheid gou verlore gaan a.g.v. splyting. Die minder digte liggewig-skuimbeton lewer daarenteen duktiele faling, maar teen ’n baie lae verbindingspanning. ’n Nuwe konsep, genaamd bindingsintegriteit (E: Bonding Integrity), BI, word geformuleer om vroeë versteurings in die bindingsgedrag, soos interne krake, te kwantifiseer. Dit word gedurende die balk-eind toetse gedoen met die meting van die tempo van verandering van die toegepaste moment en die tempo van verandering van trek-kragte in die staal staaf. Voorspellingsmodelle vir die ontwerp-verbindingspannings word gerapporteer en is geformuleer uit die spanningstoestand by die beton en staal koppelvlak. Die model neem twee falingsmodes in ag, naamlik uittrek- en splytingsfaling. Die resultate van hierdie studie kwantifiseer en beklemtoon die tekortkomings van liggewigskuimbeton om strukturele aanvaarbare verbindingsgedrag te lewer. Die minder digte liggewigskuimbeton het enersyds te min vergruisingsterkte en te veel brosheid aan die ander kant van die digtheidspektrum. Beide gevalle lei tot verlies van verbinding, maar veroorsaak deur verskillende meganismes. Gevolglik word dit aanbeveel dat liggewig-skuimbeton nie gebruik word in strukturele toepassings nie. Verdere ontwikkeling word benodig om die vergruisingsterkte te verhoog en die brosheid te verlaag. Nuwe innoverende bewapening-sisteme kan lei tot die verbeterde verbindingsgedrag vir gevolglike veilige aanwending van liggewig-skuimbeton. Hierdie studie verskaf nie net resultate en riglyne om gewapende liggewig-skuimbeton te analiseer en karakteriseer nie, maar is ook ’n oorbrugging van die akkute tekort in die spesifikasies van die materiaal en vervaardigings- en toetstegnieke. Dit kan dus dien as die toekomstige basis vir die verdere ontwikkeling daarvan as ’n strukturele materiaal.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/97835
This item appears in the following collections: