NANO TECHNOLOGY IN CIVIL ENGINEERING

NANO TECHNOLOGY IN CIVIL ENGINEERING

INTRODUCTION

As people involved in construction, we are very familiar with the concept of getting raw

materials, bringing them together in an organized way and then putting them together into a recognizable form. The finished product is a passive machine that does not change or adapt to the surroundings or environment. It works and slowly decays as it is used and abused by the environment and the owners of the project. It gets periodic maintenance but its main goal is to survive the demands made of it until it becomes obsolete and then it is dismantled and discarded to make way for something new. This is our role in society and we have performed it well for hundreds or thousands of years. Construction then is definitely not a new science or technology and yet it has undergone great changes over its history. The industry we see today is the result of a progression in science, technology, process and business.

In the same vein, nanotechnology is not a new science and it is not a new technology either. It is rather an extension of the sciences and technologies that have already been in

development for many years and it is the logical progression of the work that has been done to examine the nature of our world at ever smaller and smaller scale.

NANO TECHNOLOGY

Nanotechnology is the use of very small pieces of material by themselves or their

manipulation to create new large scale materials.

The size of the particles is the critical factor. At the nanoscale (anything from one hundred or more down to a few nanometres, or 10-9m) material properties are altered from that of larger scales (the exact point at which this occurs depends on the material). The size boundary is kind of like the difference between standing on the edge of the Grand Canyon and taking a further step. There is a dramatic change in situation and this is what happens at the scale of nanotechnology. On one side, above the boundary, the world is pretty much as we experience it everyday, the laws, effects and consequences that are apparent and important to us at our size are still important and determine the nature of things. On the other side things are quite different and we cannot simply reduce the size of our tools or machines to cope. Different things start to happen below the boundary e.g. gravity becomes unimportant, electrostatic forces take over and quantum effects kick in. Another important aspect is that, as particles become nano-sized, the proportion of atoms on the surface increases relative to those inside and this leads to novel properties. It is these “nano-effects”, however, that ultimately determine all the properties that we are familiar with at our “macro-scale” and this is where the power of nanotechnology comes in – if we can manipulate elements at the nanoscale we can affect the macro-properties and produce significantly new materials and processes.

“Nanotechnology is an enabling technology that allows us to develop

materials with improved or totally new properties”

Nanotechnology and Concrete

             .                                                             Nanolayered calcium aluminate particles

        Concrete is probably unique in construction in that it is the only material exclusive to the business and therefore is the beneficiary of a fair proportion of the research and development money from industry. The following section describes some of the most promising applications of nanotechnology in construction that are being developed or are even available today. More details are available on concrete than the other materials because much of the research described is performed in universities and research institutes and therefore is in the public domain. At the basic science level, much analysis of concrete is being done at the nano-level in order to understand its structure using the various techniques developed for study at that scale such as Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and Focused Ion Beam (FIB) (box 10, p29). This has come about as a side benefit of the development of these instruments to study the nanoscale in general, but the understanding of the structure and behaviour of concrete at the fundamental level is an important and very appropriate use of  Nanotechnology. One of the fundamental aspects of nanotechnology is its interdisciplinary nature and there has already been cross over research between the mechanical modeling of bones for medical engineering to that of concrete which has enabled the study of chloride diffusion in concrete (which causes corrosion of reinforcement). Concrete is, after all, a macro-material strongly influenced by its nano-properties and understanding it at this new level is yielding new avenues for  improvement of strength, durability and monitoring as outlined in the following paragraphs Silica (SiO2) is present in conventional concrete as part of the normal mix. However, one of the advancements made by the study of concrete at the nanoscale is that particle packing in concrete can be improved by using nano-silica which leads to a densifying of the micro and nanostructure resulting in improved mechanical properties. Nano-silica addition to cement based materials can also control the degradation of the fundamental C-S-H (calcium-silicatehydrate) reaction of concrete caused by calcium leaching in water as well as block water penetration and therefore lead to improvements in durability. Related to improved particle packing, high energy milling of ordinary portland cement (OPC) clinker and standard sand, produces a greater particle size diminution with respect to conventional OPC and, as a result,the compressive strength of the refined material is also 3 to 6 times higher.

Nanotechnology and Steel

Steel has been widely available since the second industrial revolution in the late part of the 19th and early part of the 20th Century and has played a major part in the construction industry since that time. A total of 185m tonnes of steel are produced per year in the EU and steel benefits from its wide use in industries which neighbour construction (e.g. automotive) and therefore enjoys a healthy allocation of research funding. The construction industry can benefit from the application of nanotechnology to steel and some of the promising areas currently under investigation or even available today are explored in the following paragraphs. Fatigue is a significant issue that can lead to the structural failure of steel subject to cyclic loading, such as in bridges or towers. This can happen at stresses significantly lower than the yield stress of the material and lead to a significant shortening of useful life of the structure. The current design philosophy entails one or more of three limiting measures: a design based on a dramatic reduction in the allowable stress, a shortened allowable service life or the need for a regular inspection regime. This has a significant impact on the life-cycle costs of structures and limits the effective use of resources and it is therefore a sustainability as well as a safety issue. Stress risers are responsible for initiating cracks from which fatigue failure results and research has shown that the addition of copper nanoparticles reduces the surface unevenness of steel which then limits the number of stress risers and hence fatigue cracking. Advancements in this technology would lead to increased safety, less need for monitoring and more efficient materials use in construction prone to fatigue issues.

 

Current research into the refinement of the cementite phase of steel to a nano-size has

produced stronger cables. High strength steel cables, as well as being used in car tyres, are used in bridge construction and in pre-cast concrete tensioning and a stronger cable material would reduce the costs and period of construction, especially in suspension bridges as the cables are run from end to end of the span. Sustainabilty is also enhanced by the use of higher cable strength as this leads to a more efficient use of materials.

High rise structures require high strength joints and this in turn leads to the need for high

strength bolts. The capacity of high strength bolts is realized generally through quenching and tempering and the microstructures of such products consist of tempered martensite.

When the tensile strength of tempered martensite steel exceeds 1,200 MPa even a very small amount of hydrogen embrittles the grain boundaries and the steel material may fail during use. This phenomenon, which is known as delayed fracture, has hindered the further strengthening of steel bolts and their highest strength has long been limited to somewhere around 1,000 to 1,200 MPa. Research work on vanadium and molybdenum nanoparticles has shown that they improve the delayed fracture problems associated with high strength bolts. This is the result of the nanoparticles reducing the effects of hydrogen embrittlement and improving the steel micro-structure through reducing the effects of the inter-granular cementite phase.

Nanotechnology and Wood

Further characteristics include:

·           Breathing activity remains in tact

·           Direct applicability

·           Dirt, grime, grease oil and soiling easy to remove with water spray

·           Easy to apply

·           Fast drying

·           Fire damp resistant

·           Long lasting protection of surfaces against grease, oil, dirt, grime, water, etc.

·           Nanoformed open-porous surfaces

·           Prevents weathering or decomposition of wood surfaces

·           Removes the fertile foundation for the growth of greens such as algae and moss

·           UV-stabile

·           Weather resistant

New Nanopaper is Stronger Than Iron, Still Made of Wood

        Carbon nanotubes (box 3, p8) are a new discovery, whereas wood is an ancient material which has been used since the dawn of civilization. However, perhaps not surprisingly given nature’s evolutionary process, wood is also composed of nanotubes or “nanofibrils”; namely,lignocellulosic (woody tissue) elements which are twice as strong as steel. Harvesting these nanofibrils would lead to a new paradigm in sustainable construction as both the production and use would be part of a renewable cycle. Some developers have speculated that building functionality onto lignocellulosic surfaces at the nanoscale could open new opportunities for such things as self-sterilizing surfaces, internal self-repair, and electronic lignocellulosic devices. These non-obtrusive active or passive nanoscale sensors would provide feedback on product performance and environmental conditions during service by monitoring structural loads, temperatures, moisture content, decay fungi, heat losses or gains, and loss of conditioned air. Currently, however, research in these areas appears limited. Due to its natural origins, wood is leading the way in cross-disciplinary research and modelling techniques which have already borne fruit in at least two areas. Firstly, BASF have developed a highly water repellent coating based on the actions of the lotus leaf as a result of the incorporation of silica and alumina nanoparticles and hydrophobic polymers. And, secondly, mechanical studies of bones have been adapted to model wood, for instance in the drying process.

In the broader sense, nanotechnology represents a major opportunity for the wood industry to develop new products, substantially reduce processing costs, and open new markets for biobased materials.

Nanotechnology and Glass

An artist's representation shows how a cost-effective solar concentrator could help make existing solar panels more efficient. The dye-based luminescent solar concentrator functions without the use of tracking or cooling systems, greatly reducing the overall cost compared to other concentrator technology. Dye molecules coated on glass absorb sunlight, and re-emit it at a different wavelengths. The light is trapped and transported within the glass until it is captured by solar cells at the edge. Some light passes through the concentrator, and is absorbed by lower voltage solar cells underneath.

The European glazing market, which represents 45% of the worldwide market, reached a volume of 80,000 units in 2001, at a sales volume of €18bn. The current state of the art in cladding is an active system which tracks sun, wind and rain in order to control the building environment and contribute to sustainability, but this is unreliable and difficult to calibrate and maintain. Consequently, there is a lot of research being carried out on the application of nanotechnology to glass and some of the most promising areas are outlined below as well as some products that are already available.

Titanium dioxide (TiO2) (box 2, p7) is used in nanoparticle form to coat glazing since it has sterilizing and anti-fouling properties. The particles catalyze powerful reactions which breakdown organic pollutants, volatile organic compounds and bacterial membranes. In addition, TiO2 is hydrophilic (box 4, p9) and this attraction to water forms sheets out of rain drops which then wash off the dirt particles broken down in the previous process. Glass incorporating this self cleaning technology is available on the market today.

Fire-protective glass is another application of nanotechnology. This is achieved by using a clear intumescent layer sandwiched between glass panels (an interlayer) formed of fumed silica (SiO2) nanoparticles which turns into a rigid and opaque fire shield when heated.

Most of glass in construction is, of course, on the exterior surface of buildings and the control of light and heat entering through building glazing is a major sustainability issue. Research into nanotechnological solutions to this centres around four different strategies to block light and heat coming in through windows. Firstly, thin film coatings are being developed which are spectrally sensitive surface applications for window glass. These have the potential to filter out unwanted infrared frequencies of light (which heat up a room) and reduce the heat gain in buildings, however, these are effectively a passive solution. As an active solution, thermochromic technologies are being studied which react to temperature and providethermal insulation to give protection from heating whilst maintaining adequate lighting. A third strategy, that produces a similar outcome by a different process, involves photochromic technologies which are being studied to react to changes in light intensity by increasing absorption. And finally, electrochromic coatings are being developed that react to changes in applied voltage by using a tungsten oxide layer; thereby becoming more opaque at the touch of a button. All these applications are intended to reduce energy use in cooling buildings and could make a major dent in the huge amounts used in the built environment. Further details on this area are covered in the section on Sustainability and the Environment.

Nanotechnology and Coatings

Coatings is an area of significant research in nanotechnology and work is being carried out on concrete and glass (see sections above) as well as steel. Much of the work involves Chemical Vapour Deposition (CVD), Dip, Meniscus, Spray and Plasma Coating in order to produce a layer which is bound to the base material to produce a surface of the desired protective or functional properties. Research is being carried out through experiment and modelling of coatings and the one of the goals is the endowment of self healing capabilities through a process of “self-assembly” (box 6, p15).

Nanotechnology is being applied to paints and insulating properties, produced by the addition of nano-sized cells, pores and particles, giving very limited paths for thermal conduction (R values are double those for insulating foam), are currently available. This type of paint is used, at present, for corrosion protection under insulation since it is hydrophobic and repels water from the metal pipe and can also protect metal from salt water attack.

As well as the applications for concrete detailed in the section above on Nanotechnology and Concrete, there are also potential uses in stone based materials. In these materials it is common to use resins for reinforcing purposes in order to avoid breakage problems, however, these resin treatments can affect the aesthetics and the adhesion to substrates. Nanoparticle based systems can provide better adhesion and transparency than conventional techniques.

In addition to the self-cleaning coatings mentioned above for glazing, the remarkable

properties of TiO2 (box 2, p7) nanoparticles are being put to use as a coating material on

roadways in tests around the world. The TiO2 coating captures and breaks down organic and inorganic air pollutants by a photocatalytic process (a coating of 7000m2 of road in Milan gave a 60% reduction in nitrous oxides). This research opens up the intriguing

possibility of putting roads to good environmental use.

Nanotechnology and Fire Protection and Detection

Fire resistance of steel structures is often provided by a coating produced by a spray-on

cementitious process. Current portland cement based coatings are not popular because they need to be thick, tend to be brittle and polymer additions are needed to improve adhesion.

However, research into nano-cement (made of nano-sized particles) has the potential to create a new paradigm in this area of application because the resulting material can be used as a tough, durable, high temperature coating. This is achieved by the mixing of carbon nanotubes (CNT’s) with the cementious material to fabricate fibre composites that can inherit some of the outstanding properties of the nanotubes (box 3, p8) such as strength. Polypropylene fibres also are being considered as a method of increasing fire resistance and this is a cheaper option than conventional insulation.

The use of processors in fire detection systems which are built into each detector head is

fairly well established today. These improve reliability allowing better addressability and the ability to identify false alarms. The use of nanotechnology in the future through the

development of nano-electromechanical systems (NEMS) (box 7, p16) could see whole

buildings become networked detectors, as such devices are embedded either into elements or surfaces.

Nanotechnology in Sustainability and the Environment

Sustainability is defined as “the ability to provide for the needs of the world's current

population without damaging the ability of future generations to provide for themselves”. A key aspect of sustainability is conservation through the efficient use of the resources that are tied up in the already built environment. As existing stock increases so will the need for effective maintenance and significant benefits will be offered by a realistic assessment of material lifetimes. Materials scientists have quantitative models which go from nanometres to millimetres and cover 6 length scales (e.g. pore network models to study the permeability of concrete). Engineers have models that go from tenths of millimetres to tens of metres and therefore cover about 6 length scales (e.g. structural analysis). Together they can, theoretically, cover 12 scale lengths and a model covering such a scale would be a powerful tool for service life predictions. This is one of the research areas currently under investigation and part of its advancement depends on the development of computing power which itself is dependent on advances in nanotechnology in the electronics field.

Another key aspect of sustainability is the efficient use of energy. In the EU, over 40% of

total energy produced is consumed by buildings. Insulation is an obvious solution to reduce some of this energy use, however, limited space for installation is a major problem for building renovation. Micro and nanoporous aerogel (box 8, p17) materials are very good candidates for being core materials of vacuum insulation panels but they are sensitive to moisture. This risk is not acceptable for high performance thermal insulation and the next challenge is to develop a totally airtight wrapping, taking into account the foil and the welding. As a possible remedy, work by Aspen Aerogels has produced an ultra-thin wall insulation which uses a nanoporous aerogel structure which is hydrophobic (box 4, p9) and repels water so it is mould free. Another intriguing application of aerogels is silica based products for transparent insulation, which leads to the possibility of super-insulating windows. Micro or Nano Electomechanical Systems (MEMS or NEMS) also offer the possibility of monitoring and controlling the internal environment of buildings (through a potentially integrated network). This could lead to energy savings much in the way that current motion detectors switch on light only when needed.

A lot of energy is required to grind clinker into cement and stearic acid (a natural, saturated vegetable oil) can be added to the grinding process to reduce this energy use. This increases cement fineness without a loss of strength, by retarding the caking and agglomeration of the cement during grinding. Saturated oil acid helps grinding without a loss of strength whereas unsaturated oil reduces strength and this is probably due to the oxidation of double carbon bonds in the unsaturated fats by available water. Research has also shown that layered double hydroxide materials (basically stacked molecules which have zones within them for ion exchange) can be used for slow release of admixtures in concrete and therefore increase workability with time. This is a sustainability issue, as the timed release of admixtures can reduce cement usage.

A lot of energy is required to grind clinker into cement and stearic acid (a natural, saturated vegetable oil) can be added to the grinding process to reduce this energy use. This increases cement fineness without a loss of strength, by retarding the caking and agglomeration of the cement during grinding. Saturated oil acid helps grinding without a loss of strength whereas unsaturated oil reduces strength and this is probably due to the oxidation of double carbon bonds in the unsaturated fats by available water. Research has also shown that layered double hydroxide materials (basically stacked molecules which have zones within them for ion exchange) can be used for slow release of admixtures in concrete and therefore increase workability with time. This is a sustainability issue, as the timed release of admixtures can reduce cement usage.

“Nanotechnology is an enabling technology that is opening a new world of

materials functionalities, and performances. But it is also opening new

possibilities in construction sustainability. On one hand it could lead to a

better use of natural resources, obtaining a specific characteristic or propertywith minor material use. It can (also) help to solve some problems related toenergy in building (consumption and generation), or water treatment tomention only a few matters”

Sustainability and environmental concerns are closely linked and clean water is a key

sustainable resource. Clean water has been one of the great leaps forward in public health

provided by civil engineering and nanotechnology is being used to further this advance. In particular, iron nanoparticles, which have a high surface area and high reactivity are being used to transform and detoxify chlorinated hydrocarbons (some of which are carcinogens) in groundwater. These nano-materials also have the potential to transform heavy metals such as soluble lead and mercury to insoluble forms, thus limiting their transport and contamination. In addition, dendrimers (a regularly branched molecule which resembles a nano “sponge”) are capable of enhancing environmental clean-up as they can trap metal ions in their “pores”, which can be subsequently filtered out of the water by ultra-filtration. In addition, nano-sized filters for water treatment have been produced which could possibly be applied to geoenvironmental remediation though barriers and cut-off walls. These filters can work on both metallic and organic contaminant ions because they have a charged membrane and both the Steric (physical filtration based on its size of openings) and Donnan (filtration based on electrical charge) effects are exploited to filter and collect unwanted contaminants from the system.

CONCLUSION

In conclusion, nanotechnology is disruptive and offers the possibility of great advances

whereas conventional approaches, at best, offer only incremental improvements.

Nanotechnology is not exactly a new technology, rather it is an extrapolation of current ones to a new scale and at that scale the conventional tools and rules no longer apply.

Nanotechnology is therefore the opposite of the traditional top-down process of construction, or indeed any production technique, and it offers the ability to work from the “bottom” of materials design to the “top” of the built environment. However, many of the advances offered by nanotechnology, be they for economic (carbon nanotubes cost 20-1000€/gram) or technical reasons, are years away from practical application, especially in the conservative and fragmented construction business.