دانلود رایگان مقاله انگلیسی نظارت بر انقباض داخلی در ملات سیمانی با زیرساخت مغناطیسی هسته H یک طرفه - الزویر 2018

ABSTRACT

Moisture transport into concrete and mortar surfaces strongly influences the durability of structures. Transport processes are foremost controlled by the material's porosity. Cementitious materials, such as concrete and mortars, exhibit distinct changes in pore structure while drying and subsequent rewetting due to the internal shrinkage and swelling of the layered calcium-silicate hydrates. Mortars with different w/c-ratios and binders were dried and rewetted and the subsequent time dependent changes in water filled pore structure were monitored with single-sided 1 H nuclear magnetic resonance. This technique combines a localized depiction of all pore water with a good time resolution. The NMR signal is split into the four pore species C-S-H interlayer water, gel pore water, interhydrate water, and capillary water by multi-exponential analysis. Two consecutive running processes in the swelling of cementitious mortars were identified and discussed based on recent models of the cement microstructure as well as its implications on the sorptivity.

5. Conclusions

In this study, dried mortar samples have been rewetted and the change in porosity has been continuously monitored using single-sided 1 H nuclear magnetic resonance. The relaxation signals have been interpreted and assigned to four different pore species [14, 19]. The following conclusions are drawn:

• Single-sided nuclear magnetic resonance allows the characterisation of water bound in cementitious mortars and with it, the characterisation of porosity. It is not necessary to determine the absolute water content of samples by drying. Other than previously used enclosed magnet setups, this allows investigation of the porosity of heterogeneous samples or samples not suitable for enclosed magnet setups.

• Upon drying, a considerable amount of water bound in C-S-H interlayer spaces is evaporated. Looking closer at the porosities immediately after rewetting, a reduction of fine pore space is seen up to a drying temperature of 40 °C causing the opening of larger cavities. At 105 °C, C-S-H interlayer water is further emptied. No further open porosity volume is developed, though C-S-H interlayers are further consolidated.

• After rewetting of mortars dried at 40 °C, three processes happen simultaneously, though seen subsequently in the measurements. First, water reinvades empty pores immediately. Second, a fast reinvasion of beforehand shrunk gel pores causes swelling and reduces the overall coarser porosity. Third, a much slower reinvasion of consolidated C-S-H interlayer spaces proceeds over several weeks, further reducing gel and capillary pore space. In mortars dried at 105 °C, gel pores are consolidated to an extent that no fast swelling is observed.

• Links between the results of porosity change here and the change in sorptivity in capillary water uptake experiments have been drawn, that will be further addressed in future experiments. Altogether, based on ours and recent findings in the literature [30] the role of drying and its influence on transport properties and the durability of concrete structures might be worth investigating further.