Development of self-healing strain-hardening cementitious composites for leak-proof basement walls

Abstract

Basement walls in tall buildings are generally designed as Water Retaining Structures (WRS) that are usually constructed using steel reinforced concrete (R/C). In contrast to conventional R/C structures, the design of WRS needs to satisfy additional criteria to ensure sufficient crack control for water-tightness. To ensure acceptably low crack widths at Serviceability Limit State (SLS), a higher amount of reinforcement than that required for Ultimate Limit State (ULS), is usually needed. This results in high reinforcement ratios and congestion which make concrete placement and compaction difficult. This may lead to construction defects that compromise structural integrity, whereby water often seeps in through flaws or cracks in the concrete. This issue is particularly severe for a basement wall as it needs to restrict water under high pressure from seeping into the interior space of the structure. Strain-Hardening Cementitious Composites (SHCC) have intrinsic crack control properties. Using this material in place of conventional concrete would eliminate the extra steel required to limit crack width in a reliable manner. Conversely, the wall section can be reduced to provide more usable space. SHCCs have been developed based on the theory of micromechanics to ensure tensile strain-hardening, multiple micro-cracking and durability through tight crack control. It has also been shown that concrete cracks which are sufficiently small can be sealed by autogenous healing under appropriate conditions. However, the high material cost associated with the fibres used in SHCCs and its low elastic modulus, compared to concrete, have hampered their wider usage as a construction material in real life structural applications. This study focuses on comprehensive design and development of a cost-effective SHCC to be used in place of concrete in the construction of WRS using a performance driven design approach. The key structural performance parameters in the design of the basement wall such as the flexural moment capacity of the section, shear strength, crack width, etc. are identified. The same are linked to the material properties such as tensile strength, strain capacity, elastic modulus and compressive strength. Finally, these material properties are achieved by judicious selection of matrix and fibre combination under specified processing conditions. Locally available conventional construction materials along with a lower cost alternative PVA fibre and a novel PVA coated PET fibre are used to produce a SHCC at a lower cost that meets the material property requirements derived from the structural performance criteria. The mechanical properties as well as the self-healing ability of the developed SHCC are evaluated. In addition, a design approach for flexural members in its ULS and SLS using reinforced SHCC is also proposed and verified by testing flexural elements of different scales. With the superior tensile properties of the developed SHCC, it is shown that there can be a reduction of reinforcement or member thickness compared to the R/C design. Hence, even though material cost of SHCC is still higher than ordinary concrete, the overall cost for the member can still be comparable. In addition, the reduction of reinforcement congestion facilitates concrete placement, labour cost saving and elimination of defects. In brief, this study highlights the feasibility of using reinforced SHCC in place of R/C in the construction of water retaining structures using basement wall as an example of a practical application.

Date of Award	2020
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology