Self-sealing of cracks in concrete using superabsorbent polymers

Cracks in concrete can self-heal when exposed to prolonged wetting, but this is limited to narrow cracks. In practice, cracks > 0.2 mm cause leakage and impair performance of structures. The potential of superabsorbent polymers (SAPs) to self-seal such cracks was investigated via transport experiments, microscopy and modelling. Forty samples containing SAP and through-thickness cracks were subjected to 0.12 wt.% NaCl at 4 m/m pressure gradient to simulate groundwater seepage. Results show that SAP can re-swell and seal cracks, for example in the case of 0.3 mm cracks reducing peak flow rate and total flow by 85% and 98% respectively. Increasing SAP dosage accelerates sealing, but imparts a strength penalty and this limits practical applications. Modelling suggests that the effectiveness of SAP can be enhanced by increasing its re-swelling ratio and particle size, and depressing its initial swelling. These variables increase the SAP exposed in a crack and the gel volume available to seal it.     

Influence of amorphous silica on the hydration in ultra-high performance concrete

Amorphous silica particles (silica) are used in ultra-high performance concretes to densify the microstructure and accelerate the clinker hydration. It is still unclear whether silica predominantly increases the surface for the nucleation of C–S–H phases or dissolves and reacts pozzolanically. Furthermore, varying types of silica may have different and time dependent effects on the clinker hydration. The effects of different silica types were compared in this study by calorimetric analysis, scanning and transmission electron microscopy, in situ X-ray diffraction and compressive strength measurements. The silica component was silica fume, pyrogenic silica or silica synthesized by a wet-chemical route (Stoeber particles). Water-to-cement ratios were 0.23. Differences are observed between the silica for short reaction times (up to 3 days). Results indicate that silica fume and pyrogenic silica accelerate alite hydration by increasing the surface for nucleation of C–S–H phases whereas Stoeber particles show no accelerating effect.     

Numerical modelling of porous cement-based materials by superabsorbent polymers

The development of new cementitious materials raises new challenges with regard to structural design. One of the potential applications of superabsorbent polymers (SAP) is to deliver well-defined porosity to cement systems. This is particularly interesting for the development of porous cement-based materials with high technical performance. In this paper, a numerical approach to model the basic mechanical properties of the porous cement with SAP is developed. The modelling approach is based on computational micromechanics and uses a representative volume element that emulates the microstructure of the cement-based material to determine the overall material response and the local behaviour. Using this approach several sensitivity analyses are undertaken examining various parameters. The modelling approach is compared with experimental results showing reasonable correlation. The proposed approach provides faster and cheaper tool to design porous materials due to a reduction in the required experimental effort.     

Effect of temperature on mechanical properties of ultra-high performance concrete

High temperature mechanical property data are needed for evaluating fire resistance of structural members. Being a relatively new construction material, there is a lack of temperature-dependent mechanical property data on ultra-high performance concrete (UHPC). To address this knowledge gap, this paper presents results from an experimental study on the effect of temperature on mechanical properties of UHPC. Specimens made of two UHPC mixes: one with only steel fibers (UHPC-S) and the other with hybrid fibers, that is, both steel and polypropylene (UHPC-H), were tested under different heating conditions in 20 to 750°C temperature range. Compressive strength, tensile strength, stress-strain response, and elastic modulus of UHPC were evaluated at various temperatures. Results generated from these property tests on UHPC were compared with property relations specified in design codes for conventional normal strength concrete (NSC) and high strength concrete (HSC). The comparisons show that UHPC experiences faster degradation in compressive strength and elastic modulus as compared to conventional concrete. However, UHPC exhibits slower degradation in tensile strength and ductility at elevated temperatures due to the presence of steel fibers. Data generated from these property tests were utilized to propose relations for expressing the mechanical properties of UHPC as a function of temperature and these relations can be used as input to numerical models for evaluating fire resistance of structures made of UHPC.     

隧洞洞渣制超高性能混凝土的耐久性能研究

研究隧洞洞渣料单料掺入和综合利用于超高性能混凝土（UHPC）中时,对UHPC的耐久性能影响,检测隧洞洞渣骨料的碱活性,结果表明,隧洞洞渣骨料为非活性材料,不会产生膨胀应力,导致膨胀裂缝的产生;隧洞洞渣掺入UHPC中时,UHPC内部未发生碳化反应,具有良好的抗碳化性能;UHPC表面未产生裂缝,具有良好的抗早期开裂性能。     

Design and synthesis of superabsorbent polymers

A series of novel copolymer superabsorbents based on the monomers acrylamide (AM), acrylic acid (AA), acrylonitrile, methacrylic acid, sodium acrylate (SA), and 2‐hydroxyethyl methacrylate (HEMA) were prepared by copolymerization using ammonium persulfate as an initiator and N,N‐methylenebisacrylamide as a crosslinking agent. The experimental results of superabsorbent polymers (SAPs) show that the absorbency in water and NaCl solutions is maximum for AM, SA, HEMA and AM, AA, SA combinations. The copolymers were characterized by IR spectroscopy. The water retention of soil was also enhanced using the above superabsorbents. Use of SAPs for the growth of the croton plant was also investigated. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2635–2639, 2001     

Size-exclusion chromatography of cross-linked superabsorbent polymers

Superabsorbent polymers are cross-linked poly(acrylic acid) with some degree of neutralization. These polymers have found extensive use as constituents in personal care products such as sanitary napkins, incontinent devices, and disposable baby diapers that absorb body fluids. Polymer networks are formed, which makes the chromatography of these materials impossible. In this study, hydrolysis under heat and high pH was used to break the cross-links, which allows for traditional size-exclusion chromatography to determine molecular weight and molecular weight distribution of the primary chains. For the chromatography, TSK-GEL®?PW_XL columns and TSK-GEL PW columns were used. The mobile phase was buffered with sodium chloride and sodium phosphate salts to eliminate ionic interactions. Detection was by refractive index. © 1995 John Wiley & Sons, Inc.     

Synthesis of superabsorbent polymers by irradiation and their applications in agriculture

The superabsorbent polymers (SAPs) were synthesized by grafting copolymerization of acrylic acid and acrylamide onto starch by using γ‐ray radiation technique and poly(ethylene glycol) (PEG) as a crosslinker. The samples were characterized by IR spectroscopy. The effect of various synthetic parameters such as irradiation dose, irradiation dose rate, monomer concentration, monomer/starch ratio, and PEG content were studied. The effects of different drying methods on water absorbency of the SAPs were also studied. The experimental results showed that the water absorbency of the SAPs depends largely on the specific conductance of water when the specific conductance is below 500 μs/cm. The water retention of sand and soil was enhanced by using the SAPs. Effects of the SAPs on the germination of seeds and growth of young plants were investigated. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1748–1755, 2004     

Synthesis of nanocomposite polymers by UV-radiation curing

Clay-based nanocomposite polymers have been synthesized by photoinitiated crosslinking polymerization of acrylate and epoxy functionalized oligomers. The solvent-free resin containing a small amount (3 wt%) of organophilic clay was cured within seconds upon UV irradiation at ambient temperature. The polymerization reaction was followed by infrared spectroscopy and shown to proceed at a great extent in thick samples (2 mm), due to the higher exotherm and the resulting rise in temperature. The organoclay was shown to have no slowing down effect on the photopolymerization of both acrylate and epoxy resins. The addition of clay nanoparticles was found to reduce the gloss of UV-cured coatings due to an enhanced surface roughness. The viscoelastic and tensile properties of the nanocomposite photopolymer were comparable to those of the neat UV-cured polymer, while their resistance to moisture was significantly increased.     

超高性能混凝土制备及性能研究

为了探索超高性能混凝土的配合比及制备工艺,本次试验通过最紧密堆积理论确定各个粒级骨料的相对比例,在初始配合比的基础上,分别研究水胶比、砂胶比、钢纤维掺量及养护方式对混凝土性能的影响。结果表明：本次试验UHPC的最佳水胶比为0.19,砂胶比为1.56,钢纤维最佳掺量为2%,养护方式宜采用先标准养护后高温养护。     

Internal curing of blended cement pastes with ultra-low water-to-cement ratio: Absorption/desorption kinetics of superabsorbent polymer

Internal curing by superabsorbent polymer (SAP) is an effective method to mitigate the autogenous shrinkage of cement-based materials with low water-to-cement ratio (w/c). In this study, the water absorption/desorption kinetics of SAP were studied quantitatively in blended cement pastes with ultra-low w/c. An absorption process at a rate of 0 to 6 g/(g h) was calculated at early ages. After that, SAPs showed mainly two distinct water desorption behaviors with a rate of 0 to 1.1 g/(g h), which was mainly governed by the osmotic pressure and capillary pressure triggered by the drop of internal relative humidity (IRH). The size and amount of SAP played a predominant role in controlling its absorption and desorption kinetics in the cement paste. Compared with ordinary Portland cement, a different desorption process with a higher release rate was noticed in binary and ternary cement pastes, primarily due to the changes in osmotic pressure resulting from the acceleration of cement hydration by silica fume at early ages. Overall, the mitigation of autogenous shrinkage is found to be highly dependent on SAP's absorption and desorption kinetics.     

An efficient preparation method for superabsorbent polymers

High water‐absorbent polyacrylates could be prepared by concentrated aqueous solution polymerization within a short time period, heating the monomer mixture by water bath. The influences of initiator content, bath temperature, crosslinker content (Cc), and degree of neutralization on water absorbency (Q) were investigated. The temperature changing of the monomer mixture during polymerization was thoroughly analyzed. The increase of initiator content as well as the increase of bath temperature results in the increase of water absorbency and reduces the crosslinking efficiency. The results are in conformity with Flory's network theory and also illustrate that lower crosslinking efficiency occurs when the polymerization rate is faster. An empirical relation of Q = 1.40 Cc^−0.665 is obtained and its exponent is very close to −0.6 in a derived relation. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 119–124, 1999     

Characterization and biodegradability of amphoteric superabsorbent polymers

Novel amphoteric superabsorbent polymers (SAPs) based on carboxymethyl cellulose, acrylic acid, acrylamide, and [2‐(methylacryloyloxy)ethyl]trimethylammonium chloride were prepared by inverse suspension copolymerization. The SAPs were characterized by Fourier transforms infrared spectroscopy, thermogravimetric analysis, and scanning electron microscopy. The absorbency or swelling behaviors in deionized water, saline solutions, and pH value solutions were investigated. Experimental results indicate that the absorbency in various solutions decreases with an increase of the ion concentration, which was attributed to osmotic pressure of water and ions between the polymeric gel and the external solutions. Biodegradability of the amphoteric SAPs was also reported. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Preparation of superabsorbent polymers by crosslinking acrylic acid and acrylamide copolymers

High water-absorbent copolymers comprising acrylic acid (AA) and acrylamide (AM) were prepared in the presence of a crosslinking agent, monofunctional aldehyde, by a solution polymerization technique using a redox initiation system. Such copolymers have very high water absorbency and absorbing kinetics to the distilled water. The copolymer formed which absorbed about 900 g water/g dry copolymer was used to study the influence of sodium chloride on the absorption capacity at 24°C. The swelling of this copolymer was studied in alcohol/water mixtures of increasing alcohol content at 294, 304, and 314 K. The main transition for ethanol/water and methanol/water mixtures is a rapid decrease of the retention capacity of the copolymer at 50–60 vol % ethanol and 55–65 vol % methanol, respectively. Swelling in distilled water at different temperatures (T) and the effect of solvent composition were also studied. Among the variables examined were initiator concentration, polymerization temperature, and amount of AM in the copolymer. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1345–1353, 1997     

纤维混杂对超高性能混凝土性能影响试验研究

利用钢纤维、碳纤维及膨胀剂等制备超高性能混凝土,测试评估材料的工作性、力学性能、收缩和耐久性能及微观结构。研究表明,碳纤维替代钢纤维量1.0‰时工作性及力学性能最佳,28 d抗压与抗折强度可以达到182.9 MPa与60.9 MPa;膨胀剂4.0%掺量下效果较好,90 d膨胀率基本稳定在2?左右;水化产物微观结构致密,120 d碳化满足T-Ⅳ等级,抗硫酸盐等级大于KS60,动弹模量54.21 GPa,耐久性好。     

Deswelling stresses and reduced swelling of superabsorbent polymer in composites of fiber and superabsorbent polymers

The desired performance characteristics of absorbent products such as diapers include a low number of leaks, adequate liquid absorption capacity, and high skin dryness. By measuring the absorption behavior of mixtures of cellulose fluff pulp and superabsorbent polymers in nonimmersed conditions, we identify four major parameters that affect the swelling of the polymer in a composite: the Donnan ion‐exclusion effects of limiting the amount of liquid in contact with the superabsorbent polymer, the restriction of swelling by the physical constraints of the fibrous network that surrounds the polymer particles, the restriction of swelling by the capillary tension provided by the pores between the cellulose fibers, and the osmotic pressure of extracted polyelectrolytes. Swelling of the superabsorbent polymer in composites is always smaller than the value measured by immersion of the polymer followed by centrifugation. Donnan exclusion of ions from the gel phase results in about 16% less swelling than when the polymers are swollen in excess liquid. The swelling restriction imposed by the fiber network, which surrounds the granules, reduces swelling a further 10%. In addition, the presence of small pores between the fibers exerts capillary tension on the polymer and lowers the swelling by another 10%. The external compression of the composite pads does not reduce the extent of swelling of the superabsorbent polymer because the stress is supported by the fibers. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2493–2507, 2005     

Effect of internal curing on durability-related properties of high performance concrete

Internal curing of high performance concrete (HPC) by pre-saturated lightweight aggregates is a well-established method of counteracting self-desiccation and autogenous shrinkage. However, by introducing the internal water reservoirs strength and durability properties can be injured. Tests by the widely accepted methods of durability assessment, such as resistance to chloride penetration, air permeability, water absorption, autogenous and drying shrinkage and mass loss, were conducted on HPC mixes made at water to cement ratios in the range of 0.21–0.33. The effect of internal curing on the durability related properties of high-performance concretes as a function of water to cement ratio is reported.     

生物炭复合高吸水树脂吸附亚甲基蓝性能研究

研究了pH值、MB初始浓度、温度及吸附时间对生物炭接枝聚丙烯酸/丙烯酰胺复合树脂(BC-SA)亚甲基蓝(MB)吸附量的影响,对MB吸附过程进行了吸附等温线和动力学方程拟合,并采用SEM和FTIR对吸附MB前后的BC-SA进行了表征,探讨了BC-SA吸附MB机理。结果表明,BC-SA吸附MB的适用pH范围较宽,为3～10;MB的初始浓度为2 500 mg/L时,吸附量最大,可达1 748.5 mg/g; 30～50℃范围内,温度对吸附量影响不大;BC-SA吸附MB符合Langmuir方程和准二级动力学方程;BC-SA孔隙结构丰富,含有大量的羟基、羧基和酰胺基等官能团结构,可通过氢键、静电作用及离子交换等方式对MB进行吸附。     

A model for the swelling of superabsorbent polymers

When dry superabsorbent polymers are immersed in water they swell by two or three orders of magnitude. Both the ultimate degree of absorption and rate of absorption are important in practical applications. Experimentally they depend on the type of polymerisation, e.g. suspension or solution; the monomer composition, e.g. proportions of acrylic acid, sodium acrylate and acrylamide; and on the type of cross-linker, e.g. water or oil-soluble. In this paper relationships between the various parameters are investigated in order to improve understanding and to identify the basic limiting factors. Plots of swelling against time have been found to fit mathematical relationships which are based on spring and dashpot models and are applied in the polymer field to viscoelastic systems. By associating the spring element with resistance to expansion by the absorbent and the dashpot element with resistance to permeation and by relating the behaviour of the elements to the polymerisation system, underlying molecular factors influencing absorption behaviour could be identified. Relationships were also established between the ultimate degree of swelling and the ratio of cross-linker to monomer for the different polymerisation systems.