Superabsorbent polymer (SAP) for self-healing concrete. JEONG Yeonung, Ph.D. Research Fellow National University of Singapore

Transcription

1 Superabsorbent polymer (SAP) for self-healing concrete JEONG Yeonung, Ph.D. Research Fellow National University of Singapore

2 Content 1. Background 2. Concept of self-healing concrete 3. Introduction to SAP 4. Absorption kinetics of SAP 5. Experimental results 6. Summary

3 Background H 2 O Time + Cl - + SO 3 2- => serious durability problems

4 Concept of self-healing concrete Self-healing concrete is a concrete which heals itself when it comes in contact with air, water, or specific agents. Example of self-healing concrete using an expansive agent and an active geo-material T.-H. Ahn and T. Kishi, (2010) J. Adv. Conc. Tech. 8(2), Crack healing by formation of C-A-S-H gel

5 Concept of self-healing concrete Various approaches of self-healing concrete Approach Strategy Autogenous healing - Hydration of unhydrated cement particles - Dissolution and carbonation of calcium hydroxide - SCMs and expansive agents Vascular healing - Vascular approach Shape memory materials - Shape memory alloy (SMA) - Shape memory polymer (SMP) Capsule-based system - Encapsulation of chemicals - Encapsulation of bacteria

6 Concept of self-healing concrete Autogenous healing concept Good compatibility with cement matrix High amount of cementing components and water supplies are required Healing performance highly depends on age of concrete V. Li and E. Herbert, (2012) J. Adv. Conc. Tech. 10,

7 Concept of self-healing concrete Vascular healing concept Large amount of healing agent can be supplied Difficult to cast network of tubes on large scale Introduction of supply tubes may weaken the concrete structures K. V. Tittelboom and N. D. Belie, (2013) Materials 6,

8 Concept of self-healing concrete Shape memory materials G. Song et al., (2006) Eng. Struct. 28(9), Effectiveness under multiple damage events Significantly cost intensive Heating and shrinkage can lead to uncertainties

9 Concept of self-healing concrete Capsule-based system Healing agents release on necessity Difficulty in casting Negative effect on the mechanical properties of the cement matrix K. V. Tittelboom and N. D. Belie, (2013) Materials 6,

10 Introduction to SAP Superabsorbent polymer (SAP) is a hydrogel having the ability to absorb water up to 1000 times of its own weight. Most (>95%) of SAP is used for disposable diapers. Global demand (in 2016): 2.4 million tonnes. Price (in 2014): $ /kg (in the US), /kg (in Germany) and (in China). 200 times Structure of sodium polyacrylate after absorption [by BASF] SAP used for diaper

11 Absorption kinetics of SAP (a) (b) (c) Absorption Desorption Osmotic pressure Loss of internal RH First minutes Until initial setting After few days A new method to determine absorbency of SAP in cementitious materials Absorbency = (W swollen SAP - W dry SAP ) / W dry SAP Tea-bag test External solution Revomal of excessive solution Conventional method (Schröfl et. al., CCR, 2012) Filtrated cement slurry (w/c=4.3) Gently wiping with a towel New method (Kang et. al., CCR, 2017) Artificial pore solution (which reflects cement pore solutions) Centrifugation with 1000 rpm

12 Absorption kinetics of SAP Modified tea-bag test Excess solution ratio [g/g] y = 0.25x R² = Absorbency by conventional method [g/g] Absorbency (in DI water) [g/g] Absorption time [min] Absorbency (in solution) [g/g] Excess solution ratio in the conventional method Reproducibility of the new method 25 Absorbency [g/g] SAP in filtrated cement slurry (w/c=4) Absorption time [min] SAP in artificial pore solution Different sorption kinetics of SAP depending on external solution (by new method)

13 Experimental results Necessity of self-healing in UHPC Durability Strength Mix proportion UHPC structures can be used for years. Therefore, it has to be maintained for a long period. UHPC structure is very thin and slender. Thus, micro-cracks can affect the structure s performance. About half of cement does not hydrate, but merely acts as an expensive filler. This is due to the lacks of available space and water for hydration. Self-healing can be achieved by water released from SAP, consuming unhydrated cement particles and filling micro-cracks with newly formed products. The self-healing can contribute to a more sustainable use of UHPC. Mix proportion of UHPC with or without SAP Sample OPC Silica fume Silica flour Fine aggregate W/C SPPL (PCE) SAP/C Steel fiber [Vol.%] Slump flow [mm] UHPC UHPC_SAP % (AA) UHPC_SAP % (AM) 700

14 Experimental results Promotion of hydration Pore refinement 0.08 Initial setting w SAP w/o SAP Log differential intrusion [ml/g] w SAP w/o SAP Pore size diameter [nm] Application of SAP promotes hydration of cement, reducing UHPC porosity.

15 Experimental results Effect of SAP on compressive strength and durability Compressive strength [MPa] Time [d] w/o SAP w SAP Ref_0.215 [D] AA_0.255 [D] AM_0.275 [D] UHPC (98 columbs) UHPC_SAP1 (351 columbs) UHPC_SAP2 (122 columbs)

16 Summary Concepts of various self-healing approaches are introduced. For several years, our group has studied on the application of SAP in UHPC, especially for its mix design, internal curing and self-healing capacity. Self-healing is a potential way to make UHPC more sustainable. SAP can be a promising and practical material when considering its absorption capacity, productivity and reasonable price. The use of SAP in UHPC has no negative effects on strength and durability, while it improves the microstructure of UHPC.

17 Thank you