Introduction

concrete pouring.jpg
Concrete pouring
Concrete is the most widely used man-made material material in the world, ranging from use in sidewalks, to skyscrapers, to dams, to roadways, and much more.[1] There are several things that make concrete so versatile, such as how it can be made from many kinds of aggregate, how it is fairly inexpensive to use, and how it has a high strength after curing while having a high amount of workability before it cures. A concrete mix is made from a mixture of aggregate, portland cement, and water. The ratios of this mix can be changed to fit the type of application, modifying the strength, workability, and more. Additives can also be placed into a concrete mix to give it specific qualities such as faster curing, higher air entrainment, low-temperature curing, and etc. The finished product of concrete has a very low inherent energy requirement, is produced to order as needed with very little waste, is made from some of the most plentiful resources on earth, has very high thermal mass, can be made with recycled materials, and is completely recyclable.[2] However, while the finished product of concrete is a sustainable material, portland cement is not a sustainable material, therefore making concrete a non-sustainable material. Until concrete can be made without the use of portland cement, or until the process of making portland cement is greatly improved, concrete will not be a sustainable material. As a result, a number of intended improvements and replacements for portland cement are currently being propo
sed, developed, researched, and implemented.

Current Issues and Areas of Ongoing Improvement

As previously mentioned, cement production is the largest part of what makes concrete a non-sustainable material. Portland cement is made by crushing clay, limestone, and other materials into a fine powder. This powder is fed into a kiln (fueled by coal, oil, recycled materials, and more) and is heated until it is purified and combined into what is known as clinker. This clinker material is then ground up and combined again with limestone and other minerals to create what is known as Portland Cement. The process of making cement is very energy intensive. In fact, cement is among the most energy intensive materials used in the construction industry and is a major contributor of green house gases in the atmosphere.[3]

CO2 emissions

Through the process of making cement, CO2 is released into the atmosphere making nearly 5% of annual global carbon dioxide production.[4] Current estimates predict that with anticipated rises in cement production the consequential carbon dioxide emissions are likely to rise by nearly 100%.[5]

NOx emissions

Other gases that are released during the process of making cement are nitric oxide and nitrogen dioxide, otherwise known as NOx. For each metric ton of Portland cement clinker, 1.5 to 10 kg of NOx is released. In 2005, clinker production was about 85 million tonnes. This translates to 125 to 850 thousand tonnes of NOx produced.[6]

Limestone Availability

Limestone quarry
Limestone quarry
Limestone is used to make Portland cement, but can also be used as coarse aggregate in concrete. In the United States, limestone is common in some areas while nonexistent in others. The lack of areas suitable for limestone quarries has, in the past, caused a shortage in Portland cement, and establishing new quarries is a very time-intensive process. Therefore, the United States is not self-sufficient in its production of limestone, and consequently its production of cement. Current solutions to this problem include importing limestone and cement from other countries.[7] However, when shortages occur there is often a significant increase in the price of the overall product, which causes consumers to seek out substitutes. Current research is being done to explore possible alternatives for limestone in the making of cement. Some current alternatives include waste by-products and industrial by-products, though they do not completely replace the need for limestone.[8] Other alternatives include supplementary cementitious materials, described later in this article.

Recycling

Post-consumer wastes, industrial by-products, and even old concrete can be incorporated in concrete production as aggregates, fuel for making cement, and can be used as SCMs. Current efforts are being made to explore the uses of recycled materials in the production of concrete. While concrete itself can also be recycled and reused as coarse aggregate in other concrete mixes, special attention must be taken into consideration as recycled concrete as aggregate can change the way typical concrete would behave. If the recycled concrete was used on roadways where it came into contact with high amounts of salt, it may be completely unusable as it may create undesirable chemical reactions as the concrete is curing. Further research is needed to explore recycling concrete to be used as aggregate in other concrete mixes.[9]

Water Conservation

Post-consumer wastes, industrial by-products, and even old concrete can be incorporated in concrete production as aggregates. Potable water could serve a better purpose instead of being used for concrete purposes.


History

The following table demonstrates that concrete and cement have been around for a long time. Early on, humans realized the value of the material and incorporated it as a building material.

12,000,000 BC
Reactions between limestone and oil shale during spontaneous combustion occurred in Israel to form a natural deposit of cement compounds. The deposits were characterized by Israeli geologists in the 1960's and 70's.
3000 BC

Egyptians
Used mud mixed with straw to bind dried bricks. They also used gypsum mortars and mortars of lime in the pyramids.
Chinese
Used cementitious materials to hold bamboo together in their boats and in the Great Wall.
800 BC

Greeks, Crete & Cyprus
Used lime mortars which were much harder than later Roman mortars.
300 BC

Babylonians & As Syrians
Used bitumen to bind stones and bricks.
300 BC - 476 AD Romans
Used pozzolana cement from Pozzuoli, Italy near Mt. Vesuvius to build the Appian Way, Roman baths, the Coliseum and Pantheon in Rome, and the Pont du Gard aqueduct in south France. They used lime as a cementitious material. Pliny reported a mortar mixture of 1 part lime to 4 parts sand. Vitruvius reported a 2 parts pozzolana to 1 part lime. Animal fat, milk, and blood were used as admixtures (substances added to cement to increase the properties.) These structures still exist today!
1200 - 1500

The Middle

Ages
The quality of cementing materials deteriorated. The use of burning lime and pozzolan (admixture) was lost, but reintroduced in the 1300's.
1678
Joseph Moxon wrote about a hidden fire in heated lime that appears upon the addition of water.
1779
Bry Higgins was issued a patent for hydraulic cement (stucco) for exterior plastering use.
1780
Bry Higgins published ”Experiments and Observations Made With the View of Improving the Art of Composing and Applying Calcereous Cements and of Preparing Quicklime.”
1793
John Smeaton found that the calcination of limestone containing clay gave a lime which hardened under water (hydraulic lime). He used hydraulic lime to rebuild Eddystone Lighthouse in Cornwall, England which he had been commissioned to build in 1756, but had to first invent a material that would not be affected by water. He wrote a book about his work.
1796
James Parker from England patented a natural hydraulic cement by calcining nodules of impure limestone containing clay, called Parker's Cement or Roman Cement.
1802
In France, a similar Roman Cement process was used.
1810
Edgar Dobbs received a patent for hydraulic mortars, stucco, and plaster, although they were of poor quality due to lack of kiln precautions.
1812 -1813
Louis Vicat of France prepared artificial hydraulic lime by calcining synthetic mixtures of limestone and clay.
1818
Maurice St. Leger was issued patents for hydraulic cement. Natural Cement was produced in the USA. Natural cement is limestone that naturally has the appropriate amounts of clay to make the same type of concrete as John Smeaton discovered.
1820 - 1821
John Tickell and Abraham Chambers were issued more hydraulic cement patents.
1822
James Frost of England prepared artificial hydraulic lime like Vicat's and called it British Cement.
1824
Joseph Aspdin of England invented portland cement by burning finely ground chalk with finely divided clay in a lime kiln until carbon dioxide was driven off. The sintered product was then ground and he called it portland cement named after the high quality building stones quarried at Portland, England.
1828
I. K. Brunel is credited with the first engineering application of portland cement, which was used to fill a breach in the Thames Tunnel.
1830
The first production of lime and hydraulic cement took place in Canada.
1836
The first systematic tests of tensile and compressive strength took place in Germany.
1843
J. M. Mauder, Son & Co. were licensed to produce patented portland cement.
1845
Isaac Johnson claims to have burned the raw materials of portland cement to clinkering temperatures.
1849
Pettenkofer & Fuches performed the first accurate chemical analysis of portland cement.
1860
The beginning of the era of portland cements of modern composition.
1862
Blake Stonebreaker of England introduced the jaw breakers to crush clinkers.
1867
Joseph Monier of France reinforced William Wand's (USA) flower pots with wire ushering in the idea of iron reinforcing bars (re-bar).
1871
David Saylor was issued the first American patent for portland cement. He showed the importance of true clinkering.
1880
J. Grant of England show the importance of using the hardest and densest portions of the clinker. Key ingredients were being chemically analyzed.
1886
The first rotary kiln was introduced in England to replace the vertical shaft kilns.
1887
Henri Le Chatelier of France established oxide ratios to prepare the proper amount of lime to produce portland cement. He named the components: Alite (tricalcium silicate), Belite (dicalcium silicate), and Celite (tetracalcium aluminoferrite). He proposed that hardening is caused by the formation of crystalline products of the reaction between cement and water.
1889
The first concrete reinforced bridge is built.
1890
The addition of gypsum when grinding clinker to act as a retardant to the setting of concrete was introduced in the USA. Vertical shaft kilns were replaced with rotary kilns and ball mills were used for grinding cement.
1891
George Bartholomew placed the first concrete street in the USA in Bellefontaine, OH. It still exists today!
1893
William Michaelis claimed that hydrated metasilicates form a gelatinous mass (gel) that dehydrates over time to harden.
1900
Basic cement tests were standardized.
1903
The first concrete high rise was built in Cincinnati, OH.
1908
Thomas Edison built cheap, cozy concrete houses in Union, NJ. They still exist today!
1909
Thomas Edison was issued a patent for rotary kilns.
1929
Dr. Linus Pauling of the USA formulated a set of principles for the structures of complex silicates.
1930
Air entraining agents were introduced to improve concrete's resistance to freeze/thaw damage.
1936
The first major concrete dams, Hoover Dam and Grand Coulee Dam, were built. They still exist today!
1956
U.S. Congress annexed the Federal Interstate Highway Act.
1967
First concrete domed sport structure, the Assembly Hall, was constructed at The University of Illinois, at Urbana-Champaign.
1970
Fiber reinforcement in concrete was introduced.
1975
CN Tower in Toronto, Canada, the tallest slip-form building, was constructed. Water Tower Place in Chicago, Illinois, the tallest building was constructed.
1980
Superplasticizers were introduced as admixtures.
1985
Silica fume was introduced as a pozzolanic additive. The "highest strength" concrete was used in building the Union Plaza constructed in Seattle, Washington.
1992
The tallest reinforced concrete building in the world was constructed at 311 S. Wacker Dr., Chicago, Illinois.
[10]

Variations of Possible Solutions

Use of Supplementary Cementitious Materials (SCMs)

SCMs.jpg
A variety of SCMs
Supplementary cementitious materials (SCMs) are materials that take the place of cement in a concrete mix. These materials "contribute to the properties of hardened concrete through hydraulic or pozzolanic activity." Examples of these materials are fly ash, slag cement, and silica fume.
[11] In order to reduce greenhouse emissions and conserve limestone, cement can be replaced to an extent. Slag, from blast furnaces, and fly ash, from coal-fired power stations, are used as supplementary cementitious materials (SCMs). Portland cement cannot be entirely replaced. Instead, the previously mentioned materials are used in order to reduce the content of Portland cement.

SCMs can partially replace cement in the concrete mixture, or can be used to produce blended cement while the cement is being produced. In that case, the SCMs are initially added into the kiln mixture that produces cement clinkers. Advantages of blended cement are reduced energy usage and can also lead to higher QC/QA for SCMs used in cement and/or concrete mixes.[12] Portland cement replacement is limited because in doing so, alkali would is needed to activate the replacement material. This can become a problem since alkali tends to attack and break down the aggregates. A notable disadvantage is also slower early strength development. Replacing half of the cement clinker with slag or fly-ash drastically affects early strength.[13] Furthermore, this type of mix does not do well in colder temperatures as it gives off less heat and is more prone to freezing.

Overall, by reducing the amount of cement needed, the need for limestone is reduced and the sustainability of concrete improves as greenhouse gas emissions are decreased.

Use Less Concrete

Another way to reduce emissions, conserve limestone, and save money is to use less concrete. Creating ultra-strong concrete uses less concrete to do the same job, but at the cost of much more expensive concrete.[14] Fine tuning the concrete design mix can also be a tough task since a finite amount of aggregates is needed to produce the needed workability. This is where admixtures, such as plasticizers, can be used to offset these drawbacks. Plasticizers produce a low water/cement ratio resulting in high performance strength and maintaining workable concrete.[15]

Admixtures

Admixtures.jpg
Examples of what admixtures can look like
The lifespan of concrete can be directly related to its sustainability. While Portland cement has been the dominating cement ingredient in the modern concrete mixture for decades, advancements in technology have allowed the use of chemical admixtures which distinctively improve certain characteristics of the material. These chemicals are separate from the typical cement, aggregate, and water that is mixed in.[16] Admixtures are primarily used to reduce the cost of concrete and to modify the strengthening properties of hardened concrete.[17] Admixtures can also be used to ensure the quality of the concrete mixture during mixing, transporting, placing, and curing. Currently there are five distinct classes of admixtures used, all of which improve the economic feasibility of the concrete in one way or another. As previously mentioned, economy is an important factor in sustainability. More directly, though, corrosion-inhibiting admixtures increase the lifetime of concrete. These chemicals are used to slow the corrosion process of reinforced steel.in concrete. While these additives may be more expensive, they are more economical in the long run for specific projects such as marine facilities, bridges, and parking garages exposed to salt. [18] The major disadvantage to admixtures is largely the unavailability of the chemicals to most parts of the world. The admixtures may promise to save money in the long run, but they are undoubtedly more expensive up front.

Recycling

Ghidorzi Concrete Recycling IMG_2129.jpg
Typical site for concrete recycling
Foundries in the United States produce 7 million tonnes of by-products. Most of these products are landfilled. Landfilling is not a desirable option because it not only causes a huge financial burden to foundries but also increases liability for future environmental costs, as well as restrictions associated with landfilling.[19] Instead, these by-products along with post-consumer wastes such as glass, plastics, tires, and wood fibers can be used as aggregates and fuel to make cement.

Glass can be used as a partial replacement of fine aggregate in concrete. Wood ash can be used to make structural-grade concrete, blended cements, and other cement-based materials. Structural-grade concrete can be made with pulp and paper mill residual solids.[20] Also, concrete from demolitions can be recycled and added to new batches.

As quality aggregate becomes more costly to acquire, new and more economical approaches are being made in efforts to encourage recycling. Not only can a used building serve as material for a newer project, concrete recycling also proves itself to be a useful method in removing rubble from demolition sites. Before this method was utilized, concrete was routinely hauled to landfills. Since then, environmental awareness has gained more of a following and thus contributed to the formation of better practices. Uncontaminated concrete collected from demolition sites is put through a series of crushers. Reinforced concrete including metal is also accepted at these sites.[21] Once the aggregates and fine particles are separated, the material is used again in newer mixtures.

A disadvantage with recycling concrete is that if the recycled concrete belongs to a strength class lower than that of the new concrete, mechanical strength loss can be expected. However, the loss can be eliminated if the recycled concrete belongs to an equal or higher strength class.[22]

Further recycling can be done in terms of the water used in the production of concrete. Water resources are being depleted by several means, one of which is the production of concrete. In an aim to help reduce the use of potable water for the formation of concrete, rainwater, surface runoff, and graywater can be recycled to serve the same purposes that the potable water was being used for. By using nonpotable water, a significant amount of money can be saved by avoiding or reducing potable water purchases and sewerage costs.[23] One big disadvantage to water recycling is the challenge associated with cleansing polluted materials out of the water. If these materials are not removed from the water, they can harm the concrete mix and change it in unexpected ways.[24]

Pervious Concrete

Pervious concrete allows water from precipitation and other sources to pass through it. Pervious concrete is permeable because the design intentionally lacks or does not include fine aggregate. However, pervious concrete is not a new concept. Original applications have been dated back to 1852.[25] Advantages of pervious concrete are the considerable potential to manage runoff from urban landscapes, allowing the runoff to pass through the concrete and not drain off the site. It can also treat the water through natural biological processes like filtration. Other advantages include a better ability to manage heat and facilitating the growth of trees and other plants on the site. Disadvantages include the need to design the pavement, mix and construction process well to ensure maximum strength and bonding while eliminating the fine aggregate. Also, because pervious concrete can tend to have a lower strength compared with normal concrete, it is advisable to limit the use of recycled aggregates in these mix designs.[26] Further research is needed to verify how well pervious concrete can handle clogging situations.


Construction Application

Within the US specifically, the rapidly deteriorating and highly unsustainable concrete infrastructure system currently in use is reaching the end of service life, but remains essential in maintaining present living standards and economic vitality. However, replacing the system cannot be done with outdated techniques. Instead, infrastructure systems should be designed, built, operated, maintained, reconfigured, and recycled with sustainability in mind.[27] By implementing sustainable methods, construction crews can extend the durability of structures by using modified environmentally-safe materials that contain less mass but are still capable of producing the desired specifications. Along with new materials, improved methods can also reduce the build time necessary for projects to be completed. Recycling materials saves resources for future generations, preserves energy that would have been used to produce new materials, and creates a sustainable construction method that can be used indefinitely.

For more information on construction applications, please see both the "Variations of Possible Solutions" section, as well as the "Completed Sustainable Concrete Construction Projects" section of this page.


Financial Considerations

One of the highest cost benefits of sustainable concrete, or "green concrete," is the money saved by using blended cement, since the energy needed by the kiln to is reduced.[28] Other areas of benefit include the use of precast panels which can be cost competitive with cast in place concrete, while at the same time providing some thermal resistance (leading to a decrease in heating/cooling costs in buildings). A typical price range can be expected to be within $55 - $65 per linear foot of wall. Precast systems are competitive with other foundation walls, particularly when costs are examined as an assembly that includes footings and sub-slab drainage. Precast walls can also be installed quickly in any weather. Due to the concrete being cured in the factory, precast foundations can also be backfilled as soon as the slab is placed and first floor bracing is in place, enhancing job site safety and site accessibility. Not to mention, green concrete tends to lead to a longer service life of a structure and a decrease in maintenance costs.[29]
Quad-Lock Home.jpg

There are many companies that specialize in the use of green concrete. One of those companies is Quad-Lock, who specialize in building sustainable concrete homes. Quad-Lock has argued that when comparing the cost of a traditional home and mortgage to a Quad-Lock Home and Energy-Efficient Mortgage, buyers/homeowners can expect a net monthly saving of nearly $100. Though this doesn't seem like much, that amounts to over $1000 per year savings! In addition to these initial savings, many insurance companies will give credits for sustainable initiatives, leading to further savings. In the U.S., many state and local programs may also give tax breaks, rebates, and etc. for using sustainable materials and having a sustainable and environmentally friendly home.[30]

One important aspect in considering the sustainability and cost-effectiveness of sustainable concrete construction is the offset of initial vs. long term costs. As in many projects, including the Quad-Lock project, the initial cost can be higher than when considering using normal construction methods and materials. That being said, it is always good to look at the long-term value and savings because that is most often where you will see your benefits. Sustainable concrete construction can lead to savings by using local resources, minimizing environmental impact and fines, increase efficiency of the building being built, provide money for the local economy, and much more.[31]

Completed Sustainable Concrete Construction Projects

Phoenix Sky Train

130 pre-stressed, green concrete U-beams were used on the project. The U-beams were produced with an environmentally friendly concrete mix design, using EF Technology®, which can have a carbon footprint 30% lower than standard concrete mix designs. This proprietary technology not only provides enhanced performance of the concrete products and a reduced environmental footprint, but can also provide credits toward LEED certifications.[32]
Phoenix Sky Train.jpg
Phoenix sky train construction


Morrow Royal Pavilion

morrow_main_hires.jpg
Morrow Royal Pavilion, Henderson, NV

The Morrow Royal Pavilion was created with special concrete that utilized recycled bottles. The bottles were crushed and formed into a composite material called GreenStone. 500,000 bottles were used in the construction, saving an estimated 400,000 cubic yards of landfill space.[33]
GreenStone.jpg
Up close picture of GreenStone


ekkomaxx(TM) Green Concrete Projects

One significant advance in green concrete construction is the use of ekkomaxx(TM) carbon neutral concrete. This concrete is a high strength concrete that has a non-portland, activated fly ash system, that increases its sustainability and durability. This specific concrete has increased crack resistance, durability, corrosion resistance, high early strengths, and more. Currently, ekkomaxx is being used for commercial construction projects, highways and bridges, military applications, and industrial applications as well. Below you can see some pictures of completed smaller-scale projects utilizing ekkomaxx(TM) green concrete.

ekkomaxx livestock slab.png
Agricultural Livestock Facility Slab - this facility chose ekkomaxx because of its high durability and resistance to acidic cleaning agents. After pouring they applied a roller finish to minimize cattle slipping on the pavement.


green concrete gantry crane runway.png
Mobile Gantry Crane Runways - the Georgia Port Authority and Port of Savannah use ekkomaxx due to high strengths and sustainability characteristics. This is the first major cast in place project that completely replaces portland cement with an alternative binder.


zero carbon car wash concrete.png
Advanced Auto Wash Facility - this chemical resistant slab is being used for both the sustainability, stewardship of materials, chemical resistance to harsh cleaning agents, and long term durability.


Recent Research

Permeable Concrete

Permeable concrete has the potential to make a positive contribution to sustainable road construction, and it can create huge benefits for reducing runoff and improving hydrological properties of the site it is installed on. One major issue that needs attention is the need to closely apply quality management to pavement and mix design, as well as to concrete placement. More research is required to better manage the disadvantages that can be caused by using permeable concrete, such as the possible potential to clog under certain circumstances, as well as to minimize binder material leaching into the environment.[34] That being said, permeable concrete has the potential to create dramatic benefits towards sustainability, especially in terms of environmental impact where it is installed.

For more on the subject, please read: https://eprints.usq.edu.au/18316/4/Thorpe_Zhuge_ARCOM_2010_PV.pdf

Engineered Cementitious Composites (ECC)

ECC is a highly ductile cement-based composite developed at Stanford as a replacement for concrete. Unlike concrete, ECC reveals an elastic-plastic stress-strain curve in tension, similar to that of ductile metal, and demonstrates a strain capacity 500-600 times greater than normal concrete. While, the properties differ, the constituents of ECC (cement, water, aggregates, admixtures) are roughly the same as concrete. The slight differences result in ECC being tailored in type, size, and content to act synergistically under load to suppress brittle fracture. Due to its ductility, ECC has shown to be more durable than traditional concrete, and has incorporated numerous industrial wastes such as fly ash, waste foundry sand, and cement kiln dust to improve material sustainability while not sacrificing performance.[35] ECC has been used in large scale applications, including a bridge deck in Michigan, that required 30 cubic meters of material.[36]

For more on the subject, please read: http://stanford.edu/~mlepech/research.projects/cife.09.2/cife.ecc.cifeproposal.pdf

Less Cement

Investigations have been conducted to determine the effects of using less cement in a concrete mix. Ravindra Dhir, director of the concrete technology group at the University of Dundee, UK, determined that 20 percent of cement content can be excluded while still retaining durability. Further research proved that reducing cement content can, in some cases, improve durability. Less cement, however, means a reduction in ultimate strength. Dhir advises that due to the decrease in ultimate strength, the tradeoff in decreased greenhouse gas emissions is not worth the risk of weaker concrete.[37]

For more on the subject, please read: http://www.rsc.org/images/Construction_tcm18-114530.pdf

Recycling Concrete

Currently, concrete accounts for nearly 50% of total waste in landfills. As such, there is a great demand for research into both how concrete can be recycled, and whether or not it is cost-effective and beneficial. In a study comparing concrete made with new materials, and concrete made with used materials, it was estimated that nearly $31M could be saved by using recycled concrete. Current recommendations suggest research and further work to increase the quality of recycled materials, as well as designing programs that encourage the use of recycled concrete and teach how to work with this concrete. Many people are hesitant to use recycled concrete as it requires a different design and special attention to the aggregate material quality, yet with the right training it would be easy to reap the benefits both financially and environmentally.[38]

For more on the subject, please read: http://www.sciencedirect.com/science/article/pii/S0921344907002248


Further Reading

The following links provide more information on the topic of concrete sustainability.

http://www.cement.org/

http://www.wbcsdcement.org/index.php

http://www.sustainableconcrete.org/

http://www.us-concrete.com/index.asp

http://www.nrmca.org/sustainability/index.asp

http://www.lafarge-na.com/wps/portal/na/en/6-Sustainability

http://www.rmc-foundation.org/MIT_CSH.htm
  1. ^ Cement Sustainability Initiative. (2015). "Sustainability with Concrete." <http://www.wbcsdcement.org/index.php/en/key-issues/sustainability-with-concrete> (Dec. 2, 2015).
  2. ^ Naik, T.R., and Moriconi, G. (2006). "Environmental-friendly durable concrete made with recycled materials for sustainable concrete construction." <https://www4.uwm.edu/cbu/Coventry/Naiefd.pdf>
  3. ^ Babor, D., Plian, D., and Judele, L. (2009). "ENVIRONMENTAL IMPACT OF CONCRETE." <http://www.ce.tuiasi.ro/~bipcons/Archive/161.pdf>
  4. ^ Crow, J.M. (2008). "The concrete conundrum." Chemistry World., 63.
  5. ^ Naik, T.R.(2008). "Sustainability of Concrete Construction." Practice Periodical On Structural Design and Construction, ASCE, 99.
  6. ^ Naik, T.R.(2008). "Sustainability of Concrete Construction." Practice Periodical On Structural Design and Construction, ASCE, 99.
  7. ^ U.S. Geological Survey. (2012). "Limestone—A Crucial and Versatile Industrial Mineral Commodity." <http://pubs.usgs.gov/fs/2008/3089/fs2008-3089.pdf>
  8. ^ Cembureau. (2015). "Raw Material Substitution." <http://lowcarboneconomy.cembureau.eu/index.php?page=raw-material-substitution> (Dec. 2, 2015).
  9. ^ Adrianne Jeffries. (2009). "Is It Green?: Concrete." <http://inhabitat.com/is-it-green-concrete/> (Dec. 2, 2015).
  10. ^ CEMEX. (2014). "History of Concrete & Cement." <http://www.cemexusa.com/ProductsServices/ReadyMixConcreteHistoryFacts.aspx>
  11. ^ Portland Cement Association. (2015). "Supplementary Cementing Materials." <http://www.cement.org/cement-concrete-basics/concrete-materials/supplementary-cementing-materials> (Dec. 2, 2015).
  12. ^ Naik, T.R., and Moriconi, G. (2006). "Environmental-friendly durable concrete made with recycled materials for sustainable concrete construction." <https://www4.uwm.edu/cbu/Coventry/Naiefd.pdf>
  13. ^ Crow, J.M. (2008). "The concrete conundrum." Chemistry World., 65.
  14. ^ Crow, J.M. (2008). "The concrete conundrum." Chemistry World., 64.
  15. ^ ConcreteNetwork.com. "WHAT SPECIAL PERFORMANCE DO YOU NEED? <http://www.concretenetwork.com/concrete/concrete_admixtures/special-performance.html> (2014).
  16. ^ United States Department of Transportation - Federal Highway Administration (2011) "Admixtures" <http://www.fhwa.dot.gov/infrastructure/materialsgrp/admixture.html> (Dec. 3, 2013)
  17. ^ Portland Cement Association (2013) "Cement and Concrete Basics" <http://www.cement.org/basics/concretebasics_chemical.asp> (Dec. 3, 2013)
  18. ^ Portland Cement Association (2013) "Concrete Technology" <http://www.cement.org/tech/cct_concrete_prod_asp> (Dec. 3, 2013)
  19. ^ Naik, T.R.(2008). "Sustainability of Concrete Construction." Practice Periodical On Structural Design and Construction, ASCE, 100.
  20. ^ Naik, T.R., and Moriconi, G. (2006). "Environmental-friendly durable concrete made with recycled materials for sustainable concrete construction." <https://www4.uwm.edu/cbu/Coventry/Naiefd.pdf>
  21. ^ (2006) "Good Economic Sense" <http://www.concreterecycling.org> , (Dec. 3, 2013)
  22. ^ Naik, T.R., and Moriconi, G. (2006). "Environmental-friendly durable concrete made with recycled materials for sustainable concrete construction." <https://www4.uwm.edu/cbu/Coventry/Naiefd.pdf>
  23. ^ Naik, T.R.(2008). "Sustainability of Concrete Construction." Practice Periodical On Structural Design and Construction, ASCE, 101.
  24. ^ Lauritzen, E.K.(2005). "RECYCLING CONCRETE - AN OVERVIEW OF DEVELOPMENT AND CHALLENGES." DEMEX Consulting Engineers A/S, Denmark <http://mmsconferencing.com/nanoc/pdf/034-ID_193.pdf>
  25. ^ Obla, K. "Pervious Concrete for Sustainable Development." Recent Advances in Concrete Technology, Sep. 2007, Washington DC
  26. ^ Thorpe, D and Zhuge, Y (2010). "Advantages and disadvantages in using permeable concrete pavement as a pavement construction material." In: Egbu, C. (Ed) Procs 26th Annual ARCOM Conference, September 2010, Leeds, UK.
  27. ^ Lepech. (2007). "Sustainable Design and Manufacturing of Prefabricated Durable Infrastructure." <http://stanford.edu/~mlepech/research.projects/cife.09.2/cife.ecc.cifeproposal.pdf>
  28. ^ Naik, T.R.(2008). "Sustainability of Concrete Construction." Practice Periodical On Structural Design and Construction, ASCE, 100.
  29. ^ Home Innovation Research Labs. "Precast Concrete Foundation and Wall Panels." <http://www.toolbase.org/Building-Systems/Foundations/precast-concrete-panels>
  30. ^ Quad-Lock. (2015). "Financial Analysis & Incentives for Insulated Concrete Forms." <http://www.quadlock.com/concrete_forms/insulated_concrete_forms_incentives.htm> (Dec. 2, 2015).
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