Ingenieurwissenschaften und zugeordnete Tätigkeiten
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Alkali silica reaction (ASR) is a major concrete durability problem resulting in significant maintenance and reconstruction costs for concrete infrastructures all over the world. To determine whether an aggregate is potentially reactive, accelerated concrete tests are used. Aim of this study is the chemical and microstructural characterization of ASR-products formed under the performance conditions of motorway pavements and during subsequent tests on the remaining ASR susceptibility. Samples were taken from concrete motorway pavements. Some of the samples already showed first indications for beginning damages whereas others showed deteriorations only after applying a performance testing procedure for the estimation of future risk of ASR damage. In a first step ASR-products were identified by polarized light microscopy in thin sections. The reaction products where then analyzed by Raman Spectroscopy and finally the chemical composition of the ASR-products was identified by SEM with EDX.
The results indicate that most reaction products show an increasing Ca/Si-ratio with progressive crack length. If an ASR-product develops inside an aggregate and moves through the cement paste, it has a high (Na+K)/Si-ratio at the beginning which decreases with increasing distance from its place of origin. The Raman spectra of the ASR-products are characterized by two broad bands indicating specific Q-species of different amorphous gel networks. Within the scope of the analysis distinctions in the microstructure and the chemical composition of the ASR-gels were detected relating to various conditions of accelerated testing.
Ultra high performance concrete (UHPC) is characterised by its high compressive strength of more than 150 MPa and its high durability. Due to thermal treatment at 90°C a strength comparable to the 28-days-strength can be achieved immediately after the treatment and in some cases can be even further increased up to 30 %. The explanations for the increase in strength are the accelerated hydration of the clinker minerals and the intensified pozzolanic reaction contributing to a denser microstructure and hence, a high performance in compressive strength.
Former research shows that thermal treatment can lead to inhomogeneities in form of a visible zonation within the cross-section. The width of the margin increases with shorter pre-storage time before the thermal treatment and with omitting protective measures against desiccation during the treatment. Specimens exhibiting a zonation typically show a lower compressive strength compared to the undisturbed reference whereas changes in chemistry, mineral content and microstructure were not reported in detail.
In this study the zonation of thermally treated UHPC is investigated with respect to its chemistry, mineral composition and microstructure to allow predictions on durability and strength development. Measurements show a change in pore sizes, minerals phase composition and element distribution leading to the visible zonation and weaker bending tensile strength compared to the reference.
Ultra high performance concrete (UHPC) is characterised by its high compressive strength of more than 150 MPa and its high durability. Due to thermal treatment at 90°C a strength comparable to the 28-days-strength can be achieved immediately after the treatment and in some cases can be even further increased up to 30 %. The explanations for the increase in strength are the accelerated hydration of the clinker minerals and the intensified pozzolanic reaction contributing to a denser microstructure and hence, a high performance in compressive strength.
Former research shows that thermal treatment can lead to inhomogeneities in form of a visible zonation within the cross-section. The width of the margin increases with shorter pre-storage time before the thermal treatment and with omitting protective measures against desiccation during the treatment. Specimens exhibiting a zonation typically show a lower compressive strength compared to the undisturbed reference whereas changes in chemistry, mineral content and microstructure were not reported in detail.
In this study the zonation of thermally treated UHPC is investigated with respect to its chemistry, mineral composition and microstructure to allow predictions on durability and strength development. Measurements show a change in pore sizes, minerals phase composition and element distribution leading to the visible zonation and weaker bending tensile strength compared to the reference.