Durability of Concrete under Combined Action – Mechanical Load and Alkali-Silica Reaction

Alaud, Salhin Mohamed (2016-12)

Thesis (DEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: There are still many problems remaining unresolved when concrete structures exposed to combined actions are studied separately, such as, severe environment and mechanical loads. In practice, these conditions act simultaneously, and separate studies may underestimate the rates of deterioration. Alkali-Silica Reaction (ASR) is a major source of deterioration causing cracking in concrete and these cracks may lead to other problems, such as a reduced resistance to the ingress of gas, water and deleterious matter, which in turn have the potential to accelerate the ASR process, but also the carbonation, corrosion and other deterioration processes. An experimental study has been designed to investigate the durability of normal and reinforced concrete elements subjected to the combined action of mechanical loading and degradation due to ASR. Mechanical loading is simulated by the compression, direct tensile, wedge splitting and fatigue (tensile cyclic loads) in pavements due to repeated wheel loads. The environmental action is simulated by means of ASR, induced with the crystalline reaction products. Different mixes with two types of local aggregates, reactive (Greywacke) and non-reactive (Granite) were prepared. Also, ground granulated Corex slag (GGCS) was added to half of the specimens. Reinforced prisms of concrete were prepared to calculate the combined action of mechanical and ASR expansion. Strength and modulus of elasticity under various conditions of ASR were determined from cylinders concrete. Also, wedge splitting on cubes were conducted to monitor the role of ASR on the pre-mechanical cracks. Two methods were followed to determine the ASR effects under combined action; the first was the accelerated method according to the conditions of ASTM C 1260 using a device specially made to immerse specimens in sodium hydroxide solution at 80°C; the second method was carried out over a long period of time (65weeks) according to ASTM C 1293. The specimens in the second method were made with high alkaline and exposed to two conditions, namely partially submerged and high humidity at 38°C. A comparison was made between the behaviour of the specimens exposed to combined action and others exposed to ASR only. Test results indicated that the damage due to combined action is more significant than that due to ASR only. The Granite mixes were less deteriorated than Greywacke mixes. Also the Corex slag decreases this deterioration dramatically. Mechanical cracks were the largest in Greywacke mixes with a higher ASR expansion than the others and the smallest were in Granite mixes with Corex slag. The strength and E-modulus of concrete were reduced due to ASR effects over time. The results from the two methods were different, where the accelerated method (ASTM C 1260) was not an accurate reflection of the ASR of reinforced concrete, but an indicator of the potential deterioration due to ASR. The series method of ASTM C 1293 was more accurate in identifying the changes in mechanical cracks under ASR. Finally, all the results have been reflected in graphs and have been compared and modelled

AFRIKAANSE OPSOMMING: Indien die effek van aksies op beton apart bestudeer word, word die invloed van gekombineerde aksie verwaarloos. As voorbeeld, veroorsaak meganiese belasting krake, wat vervolgens kan lei tot vinniger agteruitgang van beton wat blootgestel is aan aggressiewe omgewings. In die praktyk tree hierdie omstandighede terselfdertyd op, en afsonderlike studies lei tot onderskatting van die tempo van agteruitgang. Alkali-Silika Reaksie (ASR) is ʼn belangrike bron van agteruitgang, wat krake in beton veroorsaak. Hierdie krake lei tot verdere probleme, soos verlaagde weerstand teen indringing van gas, water en ander nadelige stowwe, wat die potensiaal het om ASR te versnel, maar ook om karbonasie, korrosie en ander degradasie-prosesse te veroorsaak. ʼn Eksperimentele studie is ontwerp om duursaamheid van normale en bewapende beton-elemente te ondersoek, wat blootgestel is aan die gekombineerde aksie van meganiese belasting en ASR degradasie. Meganiese belasting word nageboots deur belasting in druk, direkte trek, wigsplyting en vermoeidheid (sikliese trekbelastings). Laasgenoemde simuleer byvoorbeeld snelweg brugelemente wat aan herhaalde wielbelastings blootgestel word. Die omgewingsinvloede word nageboots deur gunstige toestande vir ASR, wat ingelei word deur kristallyne reaksieprodukte. Verskillende betonmenge met twee soorte lokale aggregaat, reaktief (Greywacke) en nie-reaktief (Graniet) is voorberei. Fyngemaalde granulêre Corex slagment (GGCS) is gevoeg by die helfte van die proefstukke. Dit is bekend dat GGCS ʼn laer alkali-inhoud het, waardeur die ASR reaksie en -potensiaal verlaag word. Ook gewapende betonprismas is voorberei om die invloed van gekombineerde aksie te bepaal. Sterkte en styfheid van beton wat blootgestel was aan verskeie ASR toestande is bepaal op betonsilinders. Wigsplyting is ook uitgevoer op betonkubusse om die rol van ASR op vooraf gekraakte beton te toets. Twee metodes is gevolg om die effekte van ASR onder gekombineerde aksie te bepaal; die eerste is die versnelde metode volgens ASTM C 1260, waarvoor spesiaal vervaardigde apparatuur gebruik is om proefstukke te onderdompel in natriumhidroksied oplossing teen 80°C; die tweede is die metode van ASTM C 1293, uitgevoer oor ʼn periode van 65 weke. In laasgenoemde metode is die proefstukke vervaardig met hoë alkali inhoud en blootgestel aan twee toestande, naamlik deels onderdompel en hoë humiditeit teen 38°C. ʼn Vergelyking is gemaak tussen die gedrag van die proefstukke wat aan gekombineerde aksie onderwerp is, met dié van proefstukke wat slegs aan ASR-gunstige toestande onderwerp is. Toetsresultate het daarop gedui dat skade veroorsaak deur gekombineerde aksie meer noemenswaardig is as skade as gevolg van slegs ASR. Die Graniet-menge het minder agteruitgegaan as die Greywacke menge. GGCS lei ook tot verlaagde agteruitgang. Die invloed van meganiese krake was die grootste in Greywacke menge, met hoër ASR swelling as die ander menge. Die kleinste effek is waargeneem in Graniet-menge wat Corex slagment bevat het. Die sterkte en elastisiteitsmodulus van beton het met verloop van tyd verlaag as gevolg van ASR effekte. Die resultate van die twee metodes was verskillend. Die ASTM C 1260 toets het nie ASR in gewapende beton akkuraat gereflekteer nie, maar tog as indruk van potensiaal tot agteruitgang deur ASR gedien. Die ASTM C 1293 metode was meer akkuraat in identifisering van die veranderinge in meganiese krake onder ASR toestande. Ten slotte is alle resultate in grafieke gereflekteer, vergelyk, en gemodelleer.

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