USE OF SUPERABSORBENT POLYMERS IN CONSTRUCTION MATERIALS

Transcription

1 USE OF SUPERABSORBENT POLYMERS IN CONSTRUCTION MATERIALS O. Mejlhede Jensen Department of Civil Engineering, Technical University of Denmark, Denmark Abstract This paper gives an overview of some of the possibilities which are offered by the use of superabsorbent polymers in construction. Superabsorbent polymers, SAP, have some distinct properties that make them interesting to use in connection with construction materials. These properties include extreme and reversible water absorption, porosity formation, water blocking and controlled release of substances. The overview does not cover all aspects of SAP use, but is focused on topics the author has been dealing with. In particular examples of SAP use within concrete construction are elaborated. 1. INTRODUCTION Within construction materials one material has central importance: Water. Not as a selfcontained material, but through its significant interaction with construction materials. Exchange of water with the surroundings may cause porous construction materials to shrink, swell and crack; Water is an essential ingredient for the proper mixing and hardening of concrete; the presence of water may turn a nail into rust; and water is involved when wood is rotting. Obviously, control of water is important to construction materials (Fig. 1). Superabsorbent polymers, SAP, are polymeric materials that have the ability to absorb a large amount of liquid from the surroundings and retain it within their structure [2]. SAP are particularly developed for absorption of aqueous solutions, and may in extreme cases have a water take-up of 5000 times their own weight; standard, industrial SAP have a water absorption of typically g/g (Fig. 2). SAP belong to the group of so-called smart materials materials that in a controlled way significantly change properties due to external stimuli. When SAP are exposed to water they swell up, and when subsequently subjected to drying they reversibly shrink back. The extreme property of SAP can actively be used in relation to construction materials. 757

2 Figure 1: Relative humidities above approx. 80% will enable growth of dry rot in wood. For the case shown the decay was discovered when the double bed was moved during spring cleaning and foot was set on the wooden parquet floor underneath. The hole went all the way through to the basement [1]. Figure 2: An example of a sorption isotherm for SAP. Left: The maximum water absorption of SAP typically amounts to g/g. If the liquid has a high ionicity, such as is the case for cement paste pore fluid, this value may be reduced to g/g. Right: A strongly magnified view of the SAP sorption isotherm shown in the left graph. A representative example of a sorption isotherm for concrete is shown for comparison. Based on [3]. 758

3 In 1997 the author of this paper and the late Per Freiesleben Hansen initiated a series of experiments involving a new curing technology concept which we called water entrainment due to similarities with air entrainment. It was the culmination of a 10 years search for possibilities to mitigate autogenous shrinkage in high-performance concrete products [4]. Water entrainment is a technique for incorporation of designed, water-filled cavities in concrete. Straightforward and free of significant drawbacks, water entrainment can be induced with fine SAP particles as a concrete admixture. At the same time we also suggested a number of other potential uses of SAP in relation to concrete. These included frost protection, rheology modification and controlled release of admixtures. This paper elaborates the suggestions and mentions additional possibilities for the use of SAP in construction materials. 2. SHRINKAGE REDUCTION The shrinkage of concrete due to loss of water to the surroundings is a well known cause of cracking both in the plastic and the hardened state. This type of cracking can effectively be mitigated by slowing down or preventing the water loss. By acting as a water source, SAP may potentially be used in relation to this, but these types of shrinkage are basically surface related phenomena, and it may be difficult to focus the action of the SAP towards this interface. SAP, however, can ensure very efficient internal water curing of concrete [5]. Internal water curing has been used for decades to promote hydration of cement, and to control the shrinkage of concrete during hardening. Saturated lightweight aggregate, LWA, was previously the only material used as an internal curing agent. But there are some major problems connected to the use of LWA for internal water curing e.g. difficulties in controlling consistency and a significant reduction in strength and elastic modulus. These difficulties are minimized with the use of SAP. Fig. 3 shows how the autogenous shrinkage in an ultra highperformance cementitious binder is controlled by addition of very small amounts of SAP. Subsequent tests have shown that the shrinkage reduction due to the SAP addition is related to a corresponding increase in the internal relative humidity of the cement paste. Furthermore it was shown that the SAP addition results in a reduction or elimination of the stress build-up and the related cracking during restrained hardening of these high-performance cementitious systems [3]. SAP added to a concrete during mixing permits an active control of geometric and thermodynamic properties of the water phase. The water in the formed SAP inclusions is essentially free water, and the size and shape of the inclusions are governed by the initially added SAP particles. Water entrainment can thus be considered engineered water phase distribution. 759

4 Figure 3: Autogenous strain of cement pastes with different additions of SAP. The amount is given relative to cement weight. Basic w/c=0.30, 20% silica fume addition. First measurement is made 17 h after water addition. The test is done isothermally at 20 C. Previously unpublished data from FROST PROTECTION SAP may also be used as a means to engineer the pore structure of cementitious materials. During cement hydration, the SAP particles deswell and leave gas-filled voids. This can potentially be used for controlled air entrainment to improve the frost resistance of concrete. The method normally used in practice for air entrainment is connected with a number of significant technological difficulties, including coalescence of air bubbles in the fresh concrete, loss of air during vibration or pumping and compatibility between air entraining admixtures and superplasticizers. The use of SAP offers the possibility to actively control the entrained air content in the hardened concrete, the spacing of the air bubbles and their size we refer to this as Engineered air-entrainment of concrete. Fig. 4 shows the influence on SAP addition on the freeze-thaw resistance of concrete. The reference concrete has a scaling during the freeze-thaw test which is more than twice the 56- days performance criterion acceptable according to the Swedish standard [6]. By addition of SAP to this concrete the amount of scaled material is reduced by a factor of 10 and thereby significantly improving the freeze-thaw resistance. For these concretes the designed porosity generated by the SAP addition is in the range of % of the concrete volume, i.e. slightly lower than but comparable to the typical level for traditional air-entrainment. 760

5 Scaled material [kg/m 2 ] Reference Acceptable 0.5 Good SAP Cycles [-] Figure 4: Cumulated scaled material for concretes subjected to freeze-thaw cycles according to the Swedish standard [6]. The reference concrete has a w/c of 0.42 and no entrained air. The SAP concretes are identical to the reference apart from additions of dry SAP in the range % of the cement weight, and the extra water needed for the swelling of the SAP. Based on [7]. The 28 days compressive strength for the reference concrete was measured as 51 MPa, whereas the concretes with SAP-addition had an average strength of 51.5 MPa. This is in agreement with previous observations [8] and suggestions [3]: as regards strength the improved curing conditions due to the SAP addition seem to fully compensate for the increased porosity. If the porosity was generated with traditional air-entrainment a strength in the range MPa would be expected; the loss of strength is typically 5.5% per percent of entrained air. 4. RHEOLOGY MODIFICATION By addition of SAP to a concrete during mixing a considerable change in rheology can be created. For example, with a SAP water absorption around 15 g/g dry SAP [3] addition of just 0.4% SAP relative to cement weight will lead to a lowering of the free w/c-ratio of As an example, this change in w/c-ratio will cause the yield stress to triple and the plastic viscosity to increase by 25% for a concrete with an initial w/c-ratio of 0.4 [9]. In addition to this pure water binding effect a further increase in the yield stress and plastic viscosity will be caused by the physical presence of the swollen SAP particles. If the thickening effect caused by the SAP is unwanted, it may be mitigated by addition of plasticizing admixtures. Or the effect may on the contrary be used actively; for example, SAP 761

6 may be utilized as a pumping aid. Particularly for shotcreting by the wet process the thickening effect may be useful. A number of technological difficulties are connected with shotcreting by the wet process. To allow the pumpability of the concrete a high slump is needed. However, to minimize rebound and to allow a proper build-up thickness during shotcreting a low slump is needed. In practice the producer of the concrete may need to balance on a knife blade with a slump of 80 mm. In addition to careful control of the fresh concrete slump it is normally necessary to use a set-accelerating admixture, which is added in the nozzle. Unfortunately, the set-accelerating admixture leads to marked reductions in long term compressive strength [10]. Furthermore, it is very difficult to control the air-entrainment of the placed shotcrete, i.e. it is difficult to ensure the frost resistance of shotcrete. These difficulties can potentially be avoided by dry addition of SAP in the nozzle during shotcreting. This idea has been tested in practice (Fig. 5). The up-take of water by the SAP created a viscosity change during placing and allowed the build-up of thick layers without the use of set-accelerating admixtures. In this case the SAP was added for the sake of rheology modification, however, in relation to shotcreting other effects caused by SAP addition may potentially be positive side-effects, such as internal water curing and mitigation of autogenous shrinkage. Figure 5: Nozzleman shotcreting successfully with SAP as a rheology modifying admixture. Air-entrainment of shotcrete is a special case. During placing of the shotcrete major changes in the air void structure will occur if it is based on normal, air-entrained admixture. With SAP it is possible accurately to design the final air void structure, since this technology is uninfluenced by the pumping and in particular by the placing procedure. 762

7 5. OUTLOOK As mentioned in the introduction SAP has an extreme water affinity, and this property makes it very interesting in relation to construction materials. Closely related to the water affinity, SAP may also be considered a means to control porosity which is another important property for construction materials. The previously mentioned examples of SAP usage all deal with concrete. The usefulness of SAP in concrete is particularly obvious since concrete is a designed material and in addition actively involves water. Both for concrete and other construction materials many additional uses of SAP can be suggested. For the examples given below some have been preliminarily tested, other are purely speculative: A special use of SAP in concrete could be as a controlled release agent. This mechanism is used in several other connections, e.g. within medicine where the release of medicine is triggered by the ph-change occurring from the stomach to the intestinal system. An example of beneficial use of controlled release in concrete could be for certain plasticizing admixtures which have a stronger effect if they are first released shortly after the initial contact between water and cement. Possibilities within this area have been examined by a major construction chemicals company. Some bituminous materials involve water during their preparation, such as cold asphalt mixes which consist of asphalt emulsified with water. For these materials there are water balance problems which are handled by water regulating agents. The use of SAP may be considered in this connection [11]. SAP can be used to create a construction sealant which is self-tightening when exposed to water. Such a material was used in construction of the Channel Tunnel between France and England [2]. SAP may also be used to generate controlled porosities in construction sealants which may increase their tensile strain capacity [12]. The swelling of SAP may be used for water-blocking in designed smart paints. For example, beech wood has vessels in the size range µm. If dry µm SAP particles are deposited in these pores they should be able to hydraulically block the pores during water exposure. The paint should consist of a suspension of SAP particles e.g. in an organic carrier liquid. The deposited SAP particles would inhibit quick ingress of water in the wood but not influence the evaporation of water from saturated wood. The principle of using SAP as a concrete admixture has received considerable interest from both the concrete research society and from the industry. On this particular subject an international conference is in preparation a full session has already been held at an international conference [13] and a technical committee has been established under the international materials research organization, RILEM: TC SAP, Application of Super Absorbent Polymers in concrete construction. Also, the first structures utilizing this technology have been constructed. This includes the FIFA World Cup 2006 pavilion in Kaiserslautern, Germany [5]. Surely, implementation in practice of new techniques as described in this paper is not simple. It requires extensive documentation of the desired effect as well as careful scrutiny of shortcomings and problems. The first problem that SAP addition was intended to remedy crack formation in high-performance concrete was realized by the concrete society only 763

8 years after the introduction of high-performance concrete. This unfortunate course should not be repeated by likewise premature introduction of new techniques. ACKNOWLEDGEMENTS The amusing cooperation with Prof. Preben Hoffmeyer and Mr. Luis Esteves on smart paint and on shotcreting respectively is gratefully acknowledged. REFERENCES [1] Lorentzen, B.T, Register of building defects (in Danish: Byggefejlregisteret), Rødovre, Denmark, 1985, 5: 56. [2] Buchholz, F.L. and Graham, A.T. (eds.) Modern superabsorbent polymer technology, WILEY- VCH, New York, [3] Jensen, O.M. and Hansen, P.F., Water-entrained cement-based materials II. Experimental observations, Cement and Concrete Research 2002, 32, 6: [4] Jensen, O.M., Autogenous Phenomena in Cement-Based Materials, Department of Building Technology and Structural Engineering, Aalborg University, Aalborg, Denmark, [5] Kovler, K. and Jensen (eds.), O.M., Internal Curing of Concrete, State-of-the-Art Report of RILEM Technical Committee 196-ICC, RILEM Report 41, Bagneux, France, [6] SS , Testing of concrete Hardened concrete Scaling by freezing (in Swedish: Betongprovning Hårdnad betong Avflagning vid frysning), Swedish Standards Institute, 4 th ed., [7] Laustsen, S. and Madsen, A.M., Engineered air-entrainment of concrete (in Danish: Kontrolleret luftindblanding i beton), MSc-thesis, Techn. Univ. of Denmark, Lyngby, [8] Lura, P., Durand, F. Loukili, A. Kovler, K. and Jensen, O.M., Compressive strength of cement pastes and mortars with superabsorbent polymers. In: Jensen, O.M., Lura, P., and Kovler, K. (eds.), Proceedings of the international RILEM Conference, Volume change of hardening concrete: testing and mitigation, Lyngby: RILEM proceedings PRO 52, Bagneux, France, 2006, [9] Banfill, P. F. G., Rheology of Fresh Cement and Concrete, Rheology Reviews, 2006, [10] Jolin, M. and Beaupré, D., Understanding wet-mix shotcrete: mix design, specification, and placement, Shotcrete, summer 2003, [11] Partl, M., EMPA, Switzerland, Personal communication, Sept [12] Tanaka, K., Tokyo Institute of Technology, Personal communication, Feb [13] Jensen, O.M., Lura, P., and Kovler, K. (eds.), Proceedings of the international RILEM Conference, Volume change of hardening concrete: testing and mitigation, Lyngby: RILEM proceedings PRO 52, Bagneux, France,