Material Properties of Bulk Hydrophobic Concrete in a Nordic Environment
Time: Fri 2023-04-21 13.00
Location: B26, Brinellvägen 23, Stockholm
Video link: https://kth-se.zoom.us/j/67403522553
Subject area: Civil and Architectural Engineering, Concrete Structures
Doctoral student: Patrick Rogers , Betongbyggnad
Opponent: Professor Tang Luping, Chalmers University of Technology
Supervisor: Professor Johan Silfwerbrand, Betongbyggnad
Concrete in its unaltered form allows the mass transfer of fluids into and out of its microstructure. These fluids can contain detrimental solutes which change the chemistry of the cement paste and/or the corrosion properties of the reinforcement bars, most noticeably hydrogen carbonates (HCO3-), oxygen (O2) and chloride ions (Cl-). Water and its solutions containing salts, mostly sodium chloride (NaCl), can also cause physical damage due to phase changes (freezing and thawing).
External application of hydrophobic agents onto the cement paste surface is a well-known method to alter the mass transfer at this interface. Bulk application of hydrophobic agents in ready mixed concrete is also a possible route but alters the entire cement paste.
This thesis presents relevant aspects concerning the use of bulk hydrophobic agents in concrete within a spectrum water to cement ratio (w/c) = 0.40-0.50. The main focus was on triacylglycerides (TAG) and alkyl alkoxysilanes (“silanes”) with application rates 1-3% based on cement weight.
Alterations to the compressive strengths have been observed and documented over a three-year period. The relative drop in mechanical strength is inversely proportional to w/c. The higher the addition rate, the lower the compressive strength. Chemical differences within the hydrophobic groupings (TAG or “silanes”) resulted in different outcomes. This was most noticeable in the water absorption, compressive strengths and chloride diffusion.
Freeze thaw testing did show noticeable differences, the use of “silanes” was detrimental in these tests even in deionsed water. The exact mechanism is unknown, but thin section analysis shows a lack of air entrainer (even when added on the fresh concrete mix) and extensive cracking in the entire cement paste. The scaling in concrete with TAGs was smaller but needs further improving.
The main properties intended with these agents were the ability to alter the mass transfer of water or solutions into the cement paste. Capillary suction and diffusion were examined. Increasing the w/c reduces the effectiveness of the hydrophobic agents to resist water uptake. This was seen in capillary suction and uni-directional chloride diffusion testing.
Processed TAGs were more effective in reducing chloride diffusion than the unprocessed chemical whereas, in some cases, the “silanes” actually increased the amount of chloride ions transferred into the cement paste. Only a slight positive effect can be seen at the lower inclusion rate (1%). Increasing the w/c reduces the resistance to chloride ion diffusion with the same dosage rate.
A field test station close to vehicular traffic was also established in 2018 and 2019, but the specimens have not been tested at this point in time. It is hoped that these and other future studies will lead to a complete PhD project.