Concrete+Performance

//**Ramsey Nasser, M.S Architectural Engineering (Structures), Penn State University**//
 * Concrete Performance and Durability **

=Introduction =

toc Structural concrete plays the role of being the main contributing component to the strength of a structure, and it is widely perceived that sound concrete is synonymous to strong concrete. While this statement holds true in terms of the structural aspect of concrete, the performance of concrete, in the form of its durability, is also an indispensable factor to the maintenance of strength and serviceability of the structure.

From a structural standpoint, buildings are designed to stay erect for scores of years without any notable failures, and concrete is known to maintain its strength over time. For this, the durability of concrete, which is independent from achieving strength in the batching plant, is an important factor in building for performance, and not just strength, and is therefore key to study and take into account in any design process.

In addition to the prevalent environmental conditions and exposure of the construction site, the factors affecting the durability of concrete are dependent on all the elements that constitute the final concrete mix, as well as the how the concrete is cast, finished, and cured on site. The aggregates used and their gradation, the gradation and fineness of sand used, the type of mixing water and the water cement ratio obtained, as well as all components of the casting, finishing, and the curing process, therefore all affect how durable the concrete will be.

This study takes two issues of primordial importance to concrete performance into account: Corrosion, and Alkali-Silica Reactions (ASR). The former is primarily caused by the environmental conditions in which the concrete resides, whereas the latter is caused by constituents of the concrete mix itself (aggregates). Other factors also greatly affect the durability and serviceability of concrete, but are not examined in this particular study. These factors include damage caused by freeze thaw cycles of concrete, the concrete mix itself, and chemical attacks to concrete. Corrosion is the most common problem pertaining to concrete durability and is caused primarily by environmental factors, and ASR is caused primarily by the very constituents of the concrete mix, the aggregates. For these reasons, in addition to showing how two completely independent issues can trigger and accelerate the rate of occurrence of each other, these two phenomena will be the ones on which this study focuses. A shift in the paradigm of how concrete performance is perceived in the industry, and how to achieve sound performance is also explored in the study.

=Alkali-Silica Reaction=

Alkali-Silica reactions, as their name suggests, are reactions that occur within the concrete, between bi-products of the alkaline environment within the concrete matrix, and reactive silica found in some types of aggregates. = =

Causes and Mechanism
The origin to alkali-silica reactions lies within the cement/concrete matrix itself. The cement paste in concrete has alkali hydroxide, which is a main contributor to the highly alkaline pH of concrete, in the form of H-OH molecules. Reactive silica exists in the petrographic configuration of some types of aggregates. While some amount of silica is almost always found in most aggregates, if the amount is beneath a certain threshold, the reaction does not occur, or occurs on a very limited scale. When reactive silica is present in sufficient amounts in the aggregate, and in the presence of water/moisture (from the cement paste), the alkali-silica reaction takes place in three stages, with the end product being the expansive white gel that can be observed in extensive alkali-silica reactions. (Portland Cement Association, 2012) = = The three stages of the reaction are shown below. (Monteiro, 2012)

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//Alkaline pH promotes hydrolysis of silica// **Si-O-Si + H-OH --> Si-OH+ Si-OH** = = //Si-OH react with the paste to form// **Si-O** = = //Si-O adsorbs Na (Sodium), K (Potassium), and Ca (Calcium) to form a// **Swelling Gel (Calcium Silicate Hydrate)**

= = The gel formed is made of calcium silicate hydrate (CSH), and has the property of swelling, thereby causing cracks and at a later stage, spalling of the concrete. (Micro Analysis Consultants, 2005) = =

Identification and Mitigation
Identification of ASR problems through visual inspection is possible because of the map cracking, which is a distinct symptom of the occurrence of alkali-silica reactions within the concrete mix. This is shown below in Figure 2. When damage is extensive, larger cracks are observed, and eventually, there is spalling of the concrete, and the gel is sometimes sufficiently abundant and can be seen on the outside surface of the concrete. FIgure 1 below shows an extreme case of cracking within the concrete, due to ASR. = =





When dealing with a problem that is inherent to the very components of the concrete mix, prevention is possibly the best mitigation measure to be taken when dealing with alkali-silica reactions. Choosing an aggregate with the minimum amount of reactive silica possible can eliminate, or minimize the effect of alkali-silica reactions. Once the aggregate with reactive silica is actually within the concrete mix, ASR reactions will take place. Prevention of ASR is both easy and cheap, therefore making it the optimal mitigation measure. Having a credible track record of the rock quarry from which the aggregate is chosen is essential; quarries with aggregates with a history of containing reactive silica, or that have been used in concrete mixes that manifested alkali-silica reactions are most likely to produce them when used in a concrete mix. If a track record or field performance history of the quarry is not available, petrographic analysis testing, as per ASTM C295, can decisively reveal the presence and amount of reactive silica in the aggregates. (Portland Cement Association, 2012)

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It is important to note that map cracking, and the spalling of the concrete both contribute to exposing the concrete and the reinforcing steel. Corrosion could therefore be triggered or accelerated by ASR. Conversely, the fact that alkali-silica reactions occur in the presence of moisture, implies that corrosion, and exposing the concrete to the environment could accelerate the ASR process from the penetration of moisture into the cement paste. What this demonstrates is that achieving concrete performance requires a holistic analysis, and that the factors that influence concrete performance can be cumulative in the degree and rate of damage they cause to concrete. Cracking that leaves the concrete exposed could lead to more cracking, at an accrued rate, from the same and other factors. For this, adopting a thorough and rigorous approach to repairing damaged concrete, as opposed to short-term cheaper patch-up solutions, comes out to be the most economical. The increasing rate at which corrosion occurs, examined in the next section, provides a clearer image of this notion. = =

= = =Corrosion=

Corrosion is the leading cause of concrete deterioration and is a widespread durability issue that can inflict considerable damage and incur high cost on building owners, as it spreads at an exponentially increasing pace. Practically speaking, corrosion is the electrochemical process by which the steel bars in the reinforced concrete matrix get covered with rust, a yellowish-brown compound. Corroded steel, or steel covered with rust, has a greater volume than that of non-corroded steel, and therefore causes the concrete to crack. (Portland Cement Association, 2012) = =

Mechanism
The highly alkaline environment (pH 12-13) in reinforced concrete provides protection for the steel reinforcement, and is at the origin of a protective film, referred to as the passive layer. The passive layer enwraps and protects steel from reacting with its surroundings and thereby from corroding. Having one or more of the triggering factors that cause the thin film to be penetrated, and in the presence of an electrolyte solution such as water, and in the presence of oxygen, corrosion can take place. = =

Corrosion occurs by an electrochemical process involving the flow of charges, mostly in the form of electrons. The reaction by which steel gives up electrons is an Oxidation reaction, and is consistent with an Anodic Half-cell reaction, with the anode, or the steel in this case, being the electron donor. Two subsequent reactions that lead to the formation of rust follow the oxidation reaction of steel. (Portland Cement Association, 2012). = =

//Oxidation of Steel// **2Fe → 2Fe** 2+ **+ 4e** - = =

= = //Reduction Reaction // **2H** 2 **O + O** 2 **+ 4e** - **→ 4OH** - = =

= =

//Formation of Rust// **2Fe** 2+ **+ 4OH** - **→** 2Fe(OH) 2 = =

Steel is not a naturally occurring metal, and is produced from iron ore, a more noble metal. A more noble metal has a lesser tendency to give up electrons than a less noble metal would have, through oxidation reactions. For this, in addition to the fact that steel is thermodynamically unstable, steel has a tendency to give up the energy it has acquired in its production, and therefore gives up electrons more easily. (Portland Cement Association, 2012)

= = The reactions shown above are illustrated in Figure 3 below, and the electrical flow and movement of charges is shown.

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Once rust begins to form, and the protective film on the reinforcing steel is penetrated and broken, the reinforcing steel of the concrete becomes exposed to its surroundings, thereby making it prone to further reactions, leading to more rust, and making it vulnerable to other triggering mechanisms of corrosion. This slippery slope type of effect is discussed in the next section. = = Figure 4 below shows the volumetric difference between steel atoms, and the final products of rust, as a result of the corrosion process. The final products have a volume of the order of 6 times greater than that of steel atoms. Fe(OH)2 for example, which is the initial rust formed, is only 3.5 times the initial volume of steel atoms. This demonstrates that corrosion is a problem that, when left neglected, worsens considerably, and incurs increasingly more repair cost and damage. (Monteiro, 2012) = =



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Causes, Triggering Mechanisms, and Amplifying Factors
The penetration of the protective film of reinforcing steel bars discussed above is problematic for more than one reason; the loss of the steel’s protective film facilitates further corrosion, which in turn causes more extensive loss of film, which will expose the steel further, and therefore constantly increase the pace at which corrosion occurs. Moreover, when rust forms, the concrete cover is gradually lost and the concrete eventually spalls, therefore making it even more exposed to the environment, and making the steel increasingly more prone to corrosion.

It is of high importance to note that when the concrete cover is lost, and the concrete spalls, the bond between the reinforcing steel bars and the concrete is lost, therefore leading to a loss of strength in the concrete lose strength, and more importantly, ductility in the tension zone. = = Several factors may trigger or cause corrosion, with some also contributing to its amplification. These include the penetration of chloride ions into the concrete matrix, the penetration of carbon dioxide through the concrete, also knows as carbonation, and the concrete being exposed to environmental conditions and air in general, as discussed above. Chloride attacks and carbonation are discussed further below. = =

Chloride Ions
Chloride ions from seawater or deicing salts can seep through the surface of sound concrete, and while it has not been conclusively determined how chlorides trigger corrosion, it is widely accepted that chlorides penetrate the passive layer of the reinforcing steel more easily than other ions do, thus making chloride attacks considerably harmful. By breaking through the passive layer, chloride ions therefore trigger the corrosion process, but do not directly contribute to the electrochemical process of corrosion.The Federal Highway Agency (FHWA) determined that the threshold amount of chloride by weight of cement that causes corrosion is 0.20%. (Portland Cement Association, 2012)

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Carbonation
Carbonation occurs when carbon dioxide penetrates through concrete to react with the hydroxides present in the concrete, to form calcium carbonate. The impact of calcium carbonate is that it lowers the pH of concrete from around 12, to around 8.5, making it more acidic, and therefore breaking the passive layer that protects the reinforcing steel from corrosion. This phenomenon also causes corrosion, but does not contribute to the electrochemical process. For carbon dioxide to be reactive within the concrete matrix, relative humidity needs to be between 50% and 75%. Given that relative humidity is a function of the weather conditions, carbonation is more likely to occur when concrete is permeable, and when rain water drips down the façade of a building, manifested through yellowish rust, and at a later stage spalled concrete. (Portland Cement Association, 2012) = = The reaction for carbonation, leading up to the formation of the pH lowering (acidic) calcium carbonate, is shown below.

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//Carbonation Reaction// **Ca(OH)** 2 **+ CO** 2 **→ CaCO** 3 **+ H** 2 **O** = = Another important parameter to note, and that shows that achieving concrete performance necessitates a holistic approach, is that the threshold of the amount of chlorides to cause corrosion is 7000-8000 ppm at a concrete pH of 12-13, whereas it is a mere 100 ppm at a pH of 10-11. Given the fact that carbonation reduces the pH of the concrete, it can therefore be stated that carbonation therefore triggers, and also could accelerate the rate of corrosion. (Portland Cement Association, 2012)

Identification and Mitigation
The signs of corrosion are characterized by cracks in and spalling of the concrete, the appearance of rust at the surface of the cracked concrete (dark yellowish color), or the exposing of the steel reinforcement. Figures 6 and 7 below show the manifestation of corrosion at the surface of the concrete.





In mitigating for corrosion, it is essential to bear in mind that corrosion occurs at an increasingly sharp rate, and that it is the more economical solution to repair corroded reinforced concrete when the problem is still localized. Preventing corrosion from occurring to start with can be achieved through implementing several fundamental principles in concrete casting and mixing, or through applying coatings that enhance the concrete's impermeability. Having adequate concrete cover is the most important preventive measure to implement in order to protect the reinforcing steel. Given that pH, temperature, relative humidity, the amount of oxygen, and resistivity are the parameters that affect the occurrence and rate of corrosion, providing proper cover would ensure that the reinforcing steel is isolated from the external environment and maintain the protective passive layer that keeps it inert. Adequate concrete cover depends on several factors, namely the extent of exposure to chlorides, the anticipated deflection of the element in question, and how much weathering the concrete will potentially undergo. For cantilevered elements such as parapets or overhangs for example, where maximum deflection is anticipated to be larger than for other elements, a thicker concrete cover could be envisaged in order to minimize the risk of corrosion. Other measures such as epoxy films and coatings, or corrosion resistant admixtures, can be considered for the same purpose.

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=Concrete Mix Guidelines and Recommendations: The P2P Approach (Prescription to Performance)=

Achieving an ideal and universal concrete mix, that works for all types of structures, in all locations, and in all environments, is not a realistic objective in designing concrete for performance. Given that several issues affecting concrete performance are directly related to the prevalent environmental conditions, what may work in a set of conditions, may not work in another. Moreover, depending on the location of the element in question, and the type of structure in question, the requirements for achieving durable and performant concrete vary. For example, corrosion is more likely to occur in structures that are at proximity of salt water sources (presence of chlorides). Freeze thaw cycles are a main concern in colder climates. Carbonation is more critical where carbon dioxide concentrations are higher, in places such as car tunnels for example. For this, while a generalized and universal approach to casting concrete might be effective in achieving concrete strength, this does not necessarily hold true for concrete performance and durability. While design codes are prescriptive in their nature, and while maximizing concrete strength is also, to a certain degree, the product of a prescriptive approach, achieving durable and high performance concrete, is more the product of an optimization of a "checks and balances" process.

The prescription to performance initiative was launched by the National Ready Mixed Concrete Association (NRMCA), and aims at eventually developing performance based specifications, rather than mere prescriptive specifications. In a prescriptive approach, the constituents and amount of material are specified, and specifications are therefore satisfied through the means to obtaining the final concrete mix. In performance specifications, the desired performance is the only criteria, and the concrete mix can be optimized in any way as to achieve the end result of durable and performant concrete. (Bickley, Hooton, and Hover, "Performance Specifications for Durable Concrete", 2006)

The need for a shift in paradigm in the way concrete performance is achieved, towards performance oriented specifications is underlined by two factors that greatly influence designing the concrete mix for performance. First, the different risks from the various exposure conditions the concrete is subject to, makes the optimizing of concrete mixes relatively to the anticipated/desired performance for the given environment an imperative. Second, the fact that performance issues such as corrosion, or alkali-silica reactions can facilitate one another, and can increase the rate of one another despite the fact that opposite properties in the concrete mix are what govern them, imposes that a balanced concrete mix must be produced. Table 1 below, taken from the ACI 318, and formatted by the Portland Cement Association (2012), summarizes the exposure classes of concrete.

– Soil: SO4 <0.10% – Water: SO4 <150 ppm || – Soil: 0.10% ≤ SO4 < 0.20% – Water: 150 ppm ≤ SO4 <1500 ppm (and Seawater) || – Soil: 0.20% ≤ SO4 < 2.0% – Water: 1500 ppm ≤ SO4 <10,000 ppm || – Soil: SO4 > 2.0% – Water: SO4 >10,000 ppm ||
 * **F** ||  || **Freezing and Thawing** ||
 * [[image:http://www.cement.org/tech/images/exposure_F.jpg width="140" height="117"]] ||  || **F0** (Not applicable) – for concrete not exposed to cycles of freezing and thawing ||
 * ^  ||^   || **F1** (Moderate) – Concrete exposed to freezing and thawing cycles and occasional exposure to moisture (and no deicing chemicals) ||
 * ^  ||^   || **F2** (Severe) – Concrete exposed to freezing and thawing cycles and in continuous contact with moisture ||
 * ^  ||^   || **F3** (Very Severe) – Concrete exposed to freezing and thawing cycles that will be in continuous contact with moisture and exposure to deicing chemicals ||
 * S ||  || **Sulfates** ||
 * [[image:http://www.cement.org/tech/images/exposure_S.jpg width="140" height="104"]] ||  || **S0** (Not applicable)
 * ^  ||^   || **S1** (Moderate)
 * ^  ||^   || **S2** (Severe)
 * ^  ||^   || **S3** (Very severe)
 * **C** ||  || **Corrosion** ||
 * [[image:http://www.cement.org/tech/images/exposure_C.jpg width="140" height="89"]] ||  || **C0** (Not applicable) - Concrete that will be dry or protected from moisture in service ||
 * ^  ||^   || **C1** (Moderate) - Concrete exposed to moisture but not to an external source of chlorides in service ||
 * ^  ||^   || **C2** (Severe) - Concrete exposed to moisture and an external source of chlorides in service ||
 * **P** ||  || **Permeability** ||
 * [[image:http://www.cement.org/tech/images/exposure_P.jpg width="140" height="98"]] ||  || **P0** (Not applicable) - Concrete where low permeability to water is not required ||
 * ^  ||^   || **P1** - Concrete required to have low permeability to water ||

//Table 1: Exposure Classes to Various Factors Affecting Concrete Durability (Taken from ACI 318, formatted by Portland Cement Association)//

Permeability
Achieving the lowest possible permeability can be considered a quasi-universal recommendation. Low permeability is of primordial importance to any concrete mix, and can be generalized as a preventive measure for all concrete durability issues triggered or accelerated by the environment. Low permeability is achieved when the concrete mix is less porous, through using aggregates that have low permeability, and sand that is properly gradated as to fill out the voids in the cement paste. Low permeability concrete would greatly reduce the risk and rate of corrosion for example, as less oxygen would be available for the anodic reaction to take place. It would also reduce the rate of alkali-silica reactions, as less moisture would be available for the hydrolysis reaction. The table below shows the difference in permeability between some types of aggregates, and is an indication of the importance of using low permeability aggregates. Despite the fact that low permeability is the parameter that is the closest to being a universal recommendation, it can not be a prescriptive one, as reducing permeability of aggregates beyond 30-40% becomes less feasible. The balance between cost and performance is to be considered. (Monteiro, 2012) Table 2 below (Monteiro, 2012), shows a few types of aggregates and their permeability. // Table 2: Permeability of Different Types of Aggregates //
 * = //**Type of Rock**// ||= //**Permeability**// ||
 * = Dense trap ||= 2.47 x 10^-12 ||
 * = Quartz diorite ||= 8.24 x 10^-12 ||
 * = Marble ||= 2.39 x 10^-10 ||
 * = Granite ||= 5.35 x 10^-9 ||
 * = Sandstone ||= 1.23 x 10^-8 ||

Water/Cement Ratio
Reducing the water/cement ratio in a concrete mix not only increases the strength of the concrete, but also enhances its durability, as excessive water, in general, is not desirable to any concrete mix. For example, freeze thaw cycles would be restricted because of the relatively low amount of water, hydrolysis in alkali-silica reactions would be reduced, and any ions in the water that could trigger a chemical attack would be present in lesser quantities. However, water/cement ratio is not a prescriptive parameter: reducing the w/c ratio too much would result in reduced workability and sludge, and would therefore be an inadequate measure to take. Changing the w/c ratio must therefore be a balanced choice. (Cement, Concrete, and Aggregates Australia, 2004)

Several other parameters should be taken into account when optimizing a concrete mix, all whilst basing the performance to achieve on exposure conditions, and on considering that a balanced mix is probably the optimal one.

=Conclusion=

The need for a shift in the approach to concrete performance, from prescriptive standards, to performance standards, is quite well captured in a statement from a study conducted by Bickley, Hooton, and Hover, in 2006. “For example, the only functional requirement for an interior building column, where durability is rarely an issue, might be minimum compressive strength. The functional requirements for a bridge deck, however, might include minimum strength, low permeability, scaling resistance, a low amount of cracking, and other criteria related to durability" (Bickley, Hooton, and Hover, "Performance Specifications for Durable Concrete", 2006). This goes to show the importance of performance and durability in concrete in a structural element like a bridge deck, as the performance criteria stated are referred to as functional requirements, and not just serviceability requirements. With regards to the issues affecting concrete durability, discussed and listed in this study, prevention is the most effective, and economical solution. In practice, and as a first step, the standard of the desired performance should be set. This should be followed by an analysis of the prevalent environmental conditions at the site of construction, what the structure will be used for (in order to determine what type of wear and tear it will be subject to), and the critical elements requiring more stringent specifications (such as cantilevers and parapets on top of structures, with higher deflections and more critical environmental exposure). Engineering judgement is essential to achieving performant concrete. References such as the Portland Cement Association, ACI 318, the National Ready Mixed Concrete Association (NRMCA), and similar references, provide guidelines for quantitative and qualitative measures in achieving performant and durable concrete.

Relevant Studies Like in the case of ASR and corrosion, different types of concrete cracks are symptomatic of different problems, and therefore require different types of repair. Identifying the various types of cracks that occur in concrete is therefore of equal importance to understanding the mechanisms behind the cracks. A study conducted by Victoria Interval (B.A.E/M.A.E program at Penn State University) (2012), provides a comprehensive resource for identifying the various types of cracks that occur in concrete. Click here to access the wiki page on this topic. A specific study on failure associated with corrosion (Carlos, Fletcher, Valentino, and Donovan-Green, 2010) offers further insight into the importance of preventing and mitigating against corrosion. The study can be accessed by clicking here. =

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= Annotated Bibliography =


 * Portland Cement Association (PCA), " ****Concrete Technology and Construction, Durability", 2012 **
 * [|//http://www.cement.org/tech/cct_durability.asp//]
 * //This publication, by the PCA, exposes schematically and through some forensic investigative pictures (from the links under the mechanism rubric), the different mechanisms affecting the durability of concrete. Alkali-silica attacks, chemical attacks, corrosion, freeze thaw attacks, and miscellaneous mechanisms of attacks are shown, and an explanation is presented, more or less extensively depending on the mechanism mentioned. //


 * Monteiro, Paulo, Berkeley University, "Durability of Concrete" **
 * [|//http://www.ce.berkeley.edu/~paulmont/CE60New/durability.pdf//]
 * //This presentation on the durability of concrete, prepared by an instructor at Berkeley University discusses the different aspects affecting durability of concrete and the causes of some, and goes into the volumetric change, or effect, of some mechanisms that affect concrete durability (up to 7-8 times expansion during corrosion, and a 9% volumetric increase when water freezes, etc). The presentation also briefly goes into examining the processes on a microscopic level as well, which could be useful when discussing, in the final paper, which admixtures to use and how they affect the cement paste/concrete mix. This source is more useful in terms of providing information on the factors affecting durability and their mechanisms, rather than on providing mitigation from durability issues. //


 * Bickley, John, Hooton, Doug, Hover, Kenneth (September 2006), "Performance Specifications for Durable Concrete", Concrete International **
 * [|//http://www.nrmca.org/p2p/ci2809bickley.pdf//]
 * //This publication in Concrete international, issue of September 2006, presents a new, more flexible, way of approaching performance related issues in concrete, in contrast with what it describes as the prescriptive, and more restrictive approach stipulated in the ACI. The article presents a set of tests, of which some are closely related to performance prediction, and evaluation for concrete, in addition to tables that categorize the exposure of concrete, by the ACI and by the AS (Australian Standards). This publication would be useful for the laboratory diagnostic aspect of the final paper, which would supplement the on-site investigative techniques. //


 * Cement, Concrete, and Aggregates, Australia, (August 2004)"Concrete basics, a guide to concrete practices" **
 * [|//http://www.concrete.net.au/publications/pdf/concretebasics.pdf//]
 * //This publication on concrete basics of mix design extensively goes over the entire concreting process, from the ordering of the concrete, to its transportation and casting, to its mix design and the variability in its parameters, to the end result. The publication will be very useful in determining how the mix design could affect durability, and to better understand how to achieve a sound mix design. //

= Additional Readings and References =


 * //Articles on Concrete Durability//**
 * []
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 * Bickley, John, Hooton, Doug, Hover, Kenneth, (September 2006) "Preparation of a Performance-based Specification for Cast-in-Place Concrete" **
 * [|//http://www.nrmca.org/P2P/Phase%20I%20Report%20Final%20January%202006.pdf//]
 * //This specifications document presents a new approach to specifying criteria and tests for concrete performance, and presents performance criteria depending on the application and location and exposure of the concrete, as opposed to prescriptive, rigid, specifications. //


 * Pizhong Qiao PhD, McLean, David, PhD, Huajie Wen, PhD, Fanglian Cheng, (2011) " ****<span style="font-family: 'Times New Roman',serif; font-size: 16px;">Accelerated Degradation and Durability of Concrete in Cold Climates" **
 * [|//http://ine.uaf.edu/autc/files/2011/08/410029-Accelerated-Degradation-ppt.pdf//]
 * //<span style="font-family: 'Times New Roman',serif; font-size: 16px;">This presentation, from Washington State University, while not of core importance to the final paper serves as a good link between lab and field testing/investigation. The study tests and attempts to establish a probabilistic correlation between freeze thaw cycles and fracture of concrete from lab results (in comnpliance with the corresponding ASTM standards), and damage observed on site. These techniques can also serve to investigate aging concrete that is presenting its first signs of problematic issues in order to determine the extent of the problem. //


 * <span style="font-family: 'Times New Roman',serif; font-size: 16px;">Portland Cement Association, "Admixtures of Concrete, Chapter 6, Design and Control of Concrete Admixtures" **
 * [|//http://www.ce.memphis.edu/1101/notes/concrete/PCA_manual/Chap06.pdf//]
 * //<span style="font-family: 'Times New Roman',serif; font-size: 16px;">This chapter from the PCA tackles the various admixtures that can be added to concrete in order to enhance its performance and durability. These will be useful in the mitigation part of the final paper. //


 * <span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif;">Yingzi Yang, Michael D. Lepech, En-Hua Yang and Victor C. Li., (2009) " Autogenous healing of engineered cementitious composites under wet–dry cycles"<span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif;">. //<span style="background-color: #ffffff; font-family: Arial,Helvetica,sans-serif;">Cement and Concrete Research //**
 * [|//http://www.sciencedaily.com/releases/2009/04/090422175336.htm//]
 * <span style="font-family: 'Times New Roman',serif; font-size: 16px;">//This article from the Science daily presents a new technology that consists of self-healing concrete with regards to cracking thanks to a new substance that has been put together at the University of Michigan//.


 * <span style="font-family: 'Times New Roman',serif; font-size: 16px;">VanderWerf, Pieter, (2012),"Concrete for severe environments", Concrete Construction **
 * [|//http://www.concreteconstruction.net/cementitious-materials-and-pozzolans/cement-for-severe-environments.aspx//]
 * //<span style="font-family: 'Times New Roman',serif; font-size: 16px;">This article from Concrete Construction tackles the issue of producing concrete that would be durable in severe environmental conditions, such as the prevalent ones in the North East of the US. //


 * <span style="font-family: 'Times New Roman',serif; font-size: 16px;">Kulkarni, Vijay, (March 2009), "Exposure Classes for Designing Durable Concrete", //The Indian Concrete Journal// **
 * []
 * //Publication on the durability of concrete. Provides several informative tables.//


 * <span style="font-family: 'Times New Roman',serif; font-size: 16px;">MicroAnalysis Consultants, (2005), "Understanding Cement". **
 * []
 * //<span style="font-family: 'Times New Roman',serif; font-size: 16px;">Article on the hydration of cement and its products. The website of the company in general handles all issues related to cement, and provides some comprehensive information on the process of manufacturing cement. //