Impact resistance of concrete

After an absence of more than 12 months, due to some personal issues, I am back and ready to start bloging again. My first post will be about a new exciting research program we currently manage in Cyprus and is a collaboration between the two major technical universities of Cyprus, civil defence authority, construction companies and Queens University Belfast.  The project concerns the impact resistance of ultra high performance fibre reinforced cementitious composites.

The web page of the program is: http://www.i4-services.com/i4-services/impact/

Structural integrity cannot be assumed!

The above image comes directly from the first pages of Eurocode-2 (Design of concrete structures) and states a number of assumptions which, when fulfilled, make the Code a useful tool in the hands of civil and structural engineers.  However, to me it seems a bit ironic that the whole construction design philosophy depends on few vague assumptions which in many cases are not satisfied. Leaving aside the point concerning the quality of the materials used (assuming that this is done properly, although in many countries that is not the case), I am not quite sure that issues such as the quality control on site as well as the employment of experienced personnel are treated with the required attention.

We, as engineers, need to focus more on the small bits and pieces that make the difference. We need to pay serious attention on issues that strongly affect the integrity of the material as single element as well as holistically (as a series of elements that make a structural frame). We need to identify how and to what extent the workmanship (mixing, transporting, concreting, consolidation, finishing) affect the quality of the microstructure of concrete. Although such things in the past have been widely discussed in the literature, they need to be revisited and reevaluated since new construction techniques and new concrete materials have been introduced the last 25 years (high performance concretes, self-compacting concrete, fibre reinforced concretes, polymer concrete etc).

It is important not to overlook properties of the fresh state such as viscosity, yield stress and thixotropy as well as properties of the hardened state such as permeability, sorptivity, pore network and porosity. These properties coupled with the quality of workmanship in the first hours of life of concrete, will strongly affect its characteristics and define the fate of the structure.

The structural integrity of concrete structures cannot be founded on few bullet-pointed assumptions.

Recycled Concrete

Construction and demolition waste constitutes a major portion of all generated solid waste. Typically, such wastes are being disposed in landfills. However, this cannot be a viable solution for the future, since in many countries such disposal is not acceptable and moreover the landfill capacity is not expected to last for many years. The promotion of environmental management and the mission of sustainable development worldwide have exerted the pressure for the adoption of appropriate methods to promote effective introduction of recycled aggregates (RA) from construction and demolition rubbles into the production processes. This increases the life cycle of these materials, thereby reducing the amount of waste dumping and natural resource extraction. Besides, it diminishes the energy consumption for the production and transport of raw materials. In Greece and Cyprus the recycling of construction wastes is a critical issue, considering that at the present time recycling of waste materials is practically inexistent and the fact that landfill sites are becoming increasingly difficult to come by. This deficiency is getting sharper considering the by-laws and regulations for the operation of such disposal sites.

Within the European Union, the construction and demolition wastes come to at least 180 million tons per year and are estimated to reach 450 million tons in the near future (European Commission Report, 1999). Roughly 75% of the waste is disposed to landfill, despite its major recycling potential. Some Member States (in particular Denmark, The Netherlands and Belgium) investigated thoroughly the technical and economic feasibility of recycling, achieving recycling rates of more than 80%. On the other hand, the South European countries recycle very little of their construction wastes. The bulk constituents of demolition debris are concrete (50-55%) and masonry (30-40%) with only small percentages of other materials such as metals, glass and timber. In Greece and Cyprus, at present, recycling of waste materials is practically inexistent and almost the entire demolition waste products are disposed off in landfill sites, with all possible economic, technical and environmental impacts.

Several researchers studied the use of recycled aggregates (RA) to partially or globally replace natural aggregates in the production of concrete. Density and water absorption ratio are the properties having the greatest differences in comparison with natural aggregates. These differences are mainly attributed to the lower density of the adhered mortar in the recycled aggregate, as reported by many authors ( and have a negative impact on the concrete mixes. However, there are a few studies that prove that concrete made with coarse aggregates deriving from concrete recycling have mechanical properties similar to those of conventional concretes. On the other hand, there is skepticism in the use of the fine fraction of these recycled aggregates. Not many studies have been conducted using fine fractions due to the belief that their greater water absorption can jeopardize the final results, particularly for replacement ratios exceeding 30%. Moreover, recycled fine aggregates contain a larger amount of adhered mortar, which results in difficulties in procuring the required slumps, as well as a substantial increase in deformation, and sharp drops in the elasticity modulus and strengths. Some researchers examined the effect of the addition of silica fume on the basic properties of recycled concrete. However, very few projects have been able to provide conclusive evidence regarding the effect of mineral admixtures on the properties of recycled concrete. Finally, although a significant amount of work has been conducted regarding the mechanical behavior of concretes made with RA, very limited work can be found in the literature about their durability aspects. The replacement of natural aggregates with recycled coarse aggregates increases dramatically the water demand. Therefore, dispersing agents have been used by several researchers.

In 2002, the CEN/TC 154 Technical Committee drew the EN 12620 “Aggregates for concrete” in which artificial or RA are considered beside natural aggregates for use in concrete. At present, though, RA are mainly used for excavation filling, roadbeds or floor foundation. Very few researchers have studied the possibility of using RA to produce structural concrete. Therefore, the available international experience and knowledge on this topic is not extensive. The particular project aims, amongst others, to elucidate this scientific area and explore the limits of applications of recycled concrete. The accomplishment of the aforementioned task will also establish a specific working method (in terms of recycling, treatment and use of these materials in concrete mixes) in the local industry, aiming in recycling a significant amount of local material. Although extensive work has been conducted regarding the mechanical behavior of concretes made with RA, very limited work can be found in the literature about durability aspects.

Mechanical and fracture properties of cement-based bi-materials after thermal cycling

This paper just published in the latest issue of Cement and Concrete Research. I am just putting here some introductory points and the first page of the paper itself. You can download the full version from here.

So:

Based on their tensile stress-deformation response, most engineering materials can be categorized into; brittle, ductile and quasi-brittle. Plain and reinforced concrete belong to the last category. Structures made of concrete contain inherent flaws, such as water filled pores, air voids, shrinkage cracks, even prior to the application of load.  These flaws, and especially the small cracks (micro-cracks), grow stably under external loading and coalesce with existing or newly-formed micro-cracks until large fractures are formed which cause the collapse of the structure.  To date structures are designed without regard to either the propagation of large cracking zones through them or an energy failure criterion.  Fracture mechanics provides an energy based failure theory that could be used in designing cement-based structures against the consequences of crack initiation and propagation.

Much attention has been paid to the mechanical and fracture properties of concrete at room temperature, including strength, stiffness, toughness and brittleness. Information about these properties under high temperature environment, however, is very useful for designing concrete structures subject to various high temperature environments.  So far, much attention has been paid to the study of concrete strength and stiffness under various heating scenarios (heating rate, maximum heating temperature, exposure period, heat cycle). Some work has also been done on concrete toughness  and brittleness and fracture energy . Most of these studies have focused on concrete properties after fire. It was found that concrete exposed to elevated temperatures experiences severe deterioration. However, research on hardened concrete after the occurrence of climatic changes seems to be very limited.

A bi-material is by definition the combination of two materials which are perfectly bonded. Application of any repair material results in a bi-material interface between the repair composite and the existing material of the retrofitted structure. The bonding at the interface is important for safety and durability. Failure of repaired system relates to cracking along the interface or kinking out of the interface. In the interface cracking case, the interface is relatively weaker than the bordering materials, so that the interface crack will propagate exclusively along the path of least resistance, i.e. the interface. In the crack kinking case, the interface is relatively stronger than at least one of two adjoining materials. Quantitative evaluation of whether an interface crack will advance straight ahead or kink-out of the interface requires the knowledge of interfacial fracture toughness .  The research presented in this paper examines bi-material systems formed between a new class of ultra high-performance fibre-reinforced cementitious composite (UHPFRCC, denoted UHP hereinafter for brevity and for avoiding the use of its trade mark name CARDIFRC) and normal strength concrete (NSC) and high strength concrete (HSC).

Durability of Self-Compacting Concrete

In recent years, durability aspects of concrete structures have emerged as the most vital area of interest in concrete and structural mechanics, driven by the rapid decay of our built infrastructure. Durability aspects of concrete structures can be roughly broken down in three components: transport properties, degradation mechanisms and volumetric changes. In 1986 a new type of concrete was developed, that aimed to solve some of the problems associated with poor compaction. It was termed self compacting concrete (SCC), and its main function was the consolidation with no external mechanical vibration, simply by its own weight. SCC was gradually introduced to many construction sites all around the world, primarily in Japan and North America. In the last decade SCC is also being utilised in Europe. However, there is a lack of understanding in some critical areas regarding the use of SCC. Although, a lot of research has been done regarding its mix design, rheological and mechanical properties, not much is known as far as its durability is concerned and, more importantly, how this is compared with the durability of a similar, conventionally vibrated concrete (CVC).

The concept of a material that will be compacted on its own and that, at the same time, will exhibit better mechanical properties compared to conventional concrete was initially introduced in early 1980s in Japan. SCC was first completed in 1988 utilizing materials already in the construction market. It gained rapid attention and became the most innovative type of concrete. SCC offers fast concrete placement, with faster construction times and ease of flow around congested reinforcement. The fluidity and segregation resistance of SCC ensures a high level of homogeneity, minimal concrete voids and uniform concrete strength, providing the potential for better level of finish and durability to the structure. Worldwide many different mix designs and proportioning methods have been developed, that can produce mixtures that fulfil the EFNARC requirements in order to be characterized as SCC. In Cyprus, construction industry is eager to adopt SCC since it is considered to be the solution to the existing problems of poor compaction and finishing. To this extent, SCC mix designs with very good mechanical and rheological characteristics have been developed locally, since early 2006.

The different mix designs, the wide range of materials and the varying water to cement ratios, in conjunction with the absence of vibration, result in a very different pore network in SCC compared to traditionally vibrated concrete. In the literature, the work done on the transport properties and degradation mechanisms of SCC is very limited and either compares concretes (CVC and SCC) of the same strength (Persson B., 2001 – Assie et al., 2006) or examines only specific durability problems such as acid attack (Bassuoni and Nehdi, 2007) and the effect of replacement materials on chloride diffusion and sulfate attack (Nehdi et al., 2004). Another rather important issue that defines the durability of SCC is shrinkage. Although the phenomenon has been studied extensively in the case of CVC, it seems that very few researchers have addressed the issue of volumetric changes in SCC and how these can affect the durability of the material (Hammer, 2007 – Loser and Leemann, 2007). The combination of high powder content with high/low water to cement ratios makes SCC prone to exhibit drying/autogenous shrinkage, which can cause severe cracking and hence may significantly reduce the durability of the material. It is also vital to know the effects of restraints on shrinkage. Restrained shrinkage has been studied for CVC (Bazant and Wittmann, 1982 – Wiegrink et al., 1996) but very few results are reported in the literature regarding SCC (Hammer, 2007). Finally, the absence of analytical investigation of durability-associated mechanisms of SCC is noticeable in the literature and it has primarily to do with the lack of extended experimental data.

References

Assie S., Escadeillas G. and Waller V. “Estimates of self compacting concrete potential durability”, Construction and Building Materials, Vol. 21, 2006, pp. 1909-1917.

Bassuoni M.T. and Nehdi M.L. “Resistance of self consolidating concrete to sulfuric acid attack with consecutive pH reduction”, Cement and Concrete Research, Vol. 37, 2007, pp. 1070-1084.

Bazant Z.P. and Wittmann F.H., “Creep and shrinkage in concrete structures”, John Wiley & Sons, First Edition, 1982.

Hammer T.A., “The influence of some mix design parameters on drying shrinkage of SCC”, Proceedings of the 5th International RILEM Symposium – SCC 2007, edited by De Schutter G. and Boel V., RILEM Publications S.A.R.L., 2007, pp. 559-564.

Loser R. and Leemann A., “Effects of curing time and drying behavior of SCC in case of restrained shrinkage deformations”, Proceedings of the 5th International RILEM Symposium – SCC 2007, edited by De Schutter G. and Boel V., RILEM Publications S.A.R.L., 2007, pp. 539-544.

Nehdi M.L., Pardhan M. and Koshowski S. “Durability of self consolidating concrete incorporating high volume replacement composite cements”, Cement and Concrete Research, Vol. 34, 2004, pp. 2103-2112.

Wiegrink K., Marikunte S. and Shah SP., “Shrinkage cracking of high performance concrete”, ACI Materials Journal, Vol. 93, No. 5, 1996, pp 409-415.

Self-healing in cement based composites

For many concrete structures in the course of their lifetime, the assessment of durability is a key parameter needed in order to know whether safety is ensured or not. Cracks can occur during any stage of the life of a concrete structure. They can be due to the concrete material itself as in the case of restrained shrinkage, or due to external factors such as excessive loading, harsh environmental exposure, poor construction procedures, or design error. Cracks have negative effects on the mechanical performance and durability of concrete structures.

One of the first works on self-healing of cementitious materials has been published even more than 20 years ago. Self-healing of macrocracks has been studied quite intensively in the past decade in view of recovery of tightness of liquid retaining structures. Recently self-healing of microcracks has been suggested the reason why the diffusion coefficient of concrete in marine structures reduces with time. Thus far, however, the self-healing capacity of cement-based materials has been considered as something “extra”. It can be called passive self-healing, since it was not a designed feature of the material, but an inherent property of it. It would be worthwhile, however, if we could create self-healing properties of cement-based materials by design.

Corrosion

The greatest threat to the durability of a reinforced concrete structure is corrosion of the reinforcement. Corrosion of steel in concrete can be explained as an electrochemical process similar to the action that takes place in a battery. The transformation of metallic iron to rust is accompanied by an increase in volume, which depending on the state of oxidation may be as large as 600% of the original metal. Such volume increase causes severe damage on the reinforced concrete structural elements. Reinforcing steel in concrete is usually protected against corrosion by a highly alkaline environment (pH ≈ 12.5) provided by the surrounding Portland cement paste. Unfortunately, hardened Portland cement paste reacts with carbon dioxide (carbonation) reducing the natural alkalinity of concrete (pH ≈ 8). This process destroys the passivity of embedded reinforcement and initiates the corrosion reaction. Carbonation is a very slow process and will not typically penetrate the concrete cover. However, if the concrete contains cracks, the carbon dioxide can reach further into concrete and the depth of carbonation will increase.

The presence of strong chloride ions can also start or accelerate the corrosion process. Very small concentration of chlorides can destroy the protective oxide film of the embedded steel reinforcement. Small amounts of chlorides come from the mix ingredients: cement, potable water, admixtures, and aggregates. More important are the chlorides that introduced in the concrete by deicing chemicals or saltwater. The passivity of the reinforcement can be protected by using inhibitors. Once the passivity of the embedded reinforcement is destroyed, either by carbonation or penetration of chlorides, corrosion will start and the life of the structure depends on the speed of the corrosion process.

Passive self-healing – The natural process

Centuries-old buildings have been said to have survived these centuries because of the inherent self-healing capacity (due to ongoing hydration) of the binders used for cementing building blocks together. Similar observations have been monitored in cement-based composites incorporating large amounts of cement and pozzolanic materials. Typically, such composites have very low water content which makes impossible the full hydration of the binder. Research revealed that exposure of such materials at ambient conditions leads to self-healing phenomena. However, that was not something that came from proper design but mainly as an inherent property of the composite system. It is obvious that appropriate design for passive self-healing will lead to better exploitation of the capabilities of natural healing in concrete structures simply by taking advantage of the materials already incorporated into the mixtures.

Encapsulation – Artificial self-healing

Encapsulation has been used extensively in the pharmaceutical, paper and other industries. Many encapsulation techniques have been developed to produce microcapsules in bulk quantities and at low enough cost to justify diverse requirements. Examples of high volume/low cost applications come from the agrochemical industry where pesticides and fertilizers have been successfully encapsulated and from the paper industry where carbonless paper is produced by depositing dye containing capsules on paper. Depending on the type of encapsulation system various problems have to be solved:

• Controlled release over specific time period
• Triggered release
• Minimize diffusion losses of volatile components
• Prevent interaction of reactive ingredients
• Improve handling of viscous or tacky materials

The microcapsule design can allow various mechanisms to trigger the release of the active ingredients. Some of the most common are listed below

• Temperature
• Diffusion
• Chemical reaction
• pH changes
• Solubility
• Mechanical rupture

The wall materials will determine the way a particular capsule will release its active ingredients. Fats and waxes, for example, release the active ingredient when the temperature exceeds the melting point of the wall or when the capsule is mechanically ruptured. Various protein and starch wall material are subject to biodegradation, carbohydrate or gelatin walls dissolve in water, fatty acid walls dissolve with pH changes. It is important that the desired release as well as the physical and chemical properties of the entire system under consideration are well defined and understood before the most appropriate encapsulation technique is applied.

Various wall materials can be studied to determine which is the material that releases when the local alkalinity decreases (pH<9). Fatty acids, fats, and polyols are potential wall materials. The size of the capsules is another important factor which is controlled by the original size of the corrosion inhibitor crystals to be used.

On the nature of the interface between concrete and reinforcement

A very common problem which, often, is underestimated is the issue of reinforcement bond with the surrounding concrete. Generally it is believed that proper compaction leads to a homogeneous material which is assumed having adequate bond with the reinforcement. The most common experimental technique to assess the bond is the typical pull-out test, which however has an inherent discrepancy. Although the problem of the bond is practically significant when tensile forces are present, the pull-out test is based on the principle of exerting compressive forces around the reinforcement in order to make the “pulling” possible. Such compressive forces create toughening zone around the reinforcement leading that way in misleading results, since in order for the bar to “slip” the applied force has to overcome not only the mechanical bond between concrete and the bar but also the toughening forces created by the test itself. Over the years many other experimental set-ups have been developed offering more accurate results.

However, what we have not assessed yet is the quality of the bond itself. We have not studied the nature of the interface between concrete and the steel bar. That is an emerging need, since the nature of the interface is the one that affects directly the results of the mechanical tests. In the Building Materials Laboratory of University of Cyprus we have initiated such a study and soon enough we will be able to demonstrate some rather interesting findings.

SCC: Breaking through concrete technology barriers!

Concrete is the most versatile and widely used construction material in the world. Every year, fifteen billion tones of concrete are utilized all around the world. In the design of structures, durability aspects are gaining more and more interest from the engineers. The desired lifetime of a structure as well as the quality of the materials used in its construction play a key role in this. In case of concrete structures, durability is strongly connected with the microstructure of the material in addition to the chemical and mechanical processes taking place in it.

New types of concrete have been developed over the years in order to meet the increasing demand for materials with improved mechanical properties and durability. In early 1970s high strength concrete (HSC) introduced and within a decade fibre reinforced concrete (FRC) as well as high performance concrete (HPC) were also developed. However, the improved microstructure of these new types of concrete might have led to better mechanical properties and durability but did not solve a major issue of concrete technology; the issue of compaction.

It is well established fact that careful and thorough consolidation of concrete is necessary. Inadequate consolidation may lead to the sustainability of the capillary pore network within the concrete and therefore resulting in lack of homogeneity that strongly affects the properties of the material. Nonetheless, it has to be mentioned that the term inadequate consolidation does not necessarily imply a state of less vibration than it is required. Inadequate consolidation also refers to situations of excessive vibration, which can lead to severe segregation and extensive bleeding.

The technique for casting concrete has remained practically unchanged for more than 40 years. Transport equipment and compaction tools have evolved through the years but the basic concepts of casting concrete members and using vibration to consolidate the material remained the same. In contractual documents that define a project, the demands for sufficient concrete vibration are normally formulated in a very general way. A consequence of such broad specification is the existence of a large number of definitions of what is adequate concrete consolidation on construction sites.

In the concrete construction industry over the last two decades, increased productivity and improved working environment have gained high priority. However, the shortage in skilled workers who could sufficiently compact fresh concrete put the integrity and the long term performance of structures into risk. The concept of a material that will be compacted on its own and at the same time will exhibit better mechanical properties compared to conventional concrete was initially introduced in early 1980s in Japan by Okamura (Okamura, 1997). This new material named self-compacting concrete (SCC) and presented to scientific community in 1986 (Okamura and Ouchi, 2003). The prototype of SCC was first completed in 1988 utilizing materials already in the construction market. Self compacting concrete gained rapidly attention and became the most innovative type of concrete. SCC offers a rapid rate of concrete placement, with faster construction times and ease of flow around congested reinforcement. The fluidity and segregation resistance of SCC ensures a high level of homogeneity, minimal concrete voids and uniform concrete strength, providing the potential for better level of finish and durability to the structure.

Background

Self compacting concrete is defined by its workability. The three essential properties of SCC are its ability to flow under its own weight (filling ability), its ability to pass through congested reinforcement (passing ability) and its ability to resist segregation (segregation resistance). The development of mixtures proportions often requires more effort for SCC than for conventionally placed concrete (CPC). The exact choice of mixture proportions depends on material availability and performance requirements. SCC mixtures always incorporate high range water reducing admixtures to ensure concrete is able to flow without the need of any external mechanical vibration. Additionally, the water to powder ratio is reduced to ensure segregation resistance. Comparing the mixture proportions, it can be observed that SCC proportions exhibit some combination of higher volume of paste, higher powder content, smaller maximum aggregate size and lower coarse aggregate content. Moreover, pozzolanic ashes and mineral fillers are commonly used in the production of SCC in order to decrease cost, improve workability and improve hardened properties and durability.

SCC is highly sensitive to changes in material properties and proportions and therefore requires increased quality control. Moreover, the consequences of deviations in workability are more significant for SCC. For example, a slight change in the water content may have a considerable impact on the rheological properties of the resulting SCC.

Principles Governing FRC

According to ACI, fibre reinforced concrete is that kind of concrete made of hydraulic cements containing aggregate of various sizes and amounts and discontinuous discrete fibres (ACI Committee 544, May 1982). The fibres used have to be not more than 50mm and 500mm in length and diameter respectively. Furthermore, the volume fraction of fibres used must not exceed 2% of total volume of mix. A typical water to cement ratio used in FRC mixes is in the range of 0.35.

 

Fibre Reinforced Concrete is generally a very durable material regardless of the type of fibre used. Steel fibres used these days are mainly brass coated in order to avoid the corrosion problems of the past. Generally, steel fibres are well protected in uncracked concrete where the high alkalinity provides a passive layer on the fibre surface. But even in the past where non-stainless steel fibres were used serious corrosion needed many years to occur[i]. In the case of cracked concrete, fibres (if not corrosion protected) may corrode very rapidly in the presence of chlorides.

 

Durability

In the case of polypropylene fibres, the problems due to chemical degradation are not present since these fibres are generally chemically inert[ii]. These fibres are used mainly to improve the impact resistance of the composite. Furthermore, research work has shown that composites using polypropylene fibres retain their durability for long periods under normal working conditions[iii]. It is useful to note that polymeric fibres (polypropylene or nylon) have an essential function during the early stage strength of the mix since they prevent early shrinkage cracking due to the compatibility of their Young’s Modulus with that of concrete at early ages[iv].

Glass fibres, as it was mentioned previously, were initially found to suffer from alkaline attack. Further research on that subject led to the development of alkali resistant glass fibres. The durability of composites containing glass fibres is strongly related to the exposure environment. Under dry conditions the toughness of the composite remains almost unaffected even after a ten-year period whereas under water or natural weathering conditions the flexural strength may exhibit severe reduction[v].

Fracture Toughness and Tensile Strength

A typical load-deflection diagram for FRC is shown below.

Figure 1. Load Deflection diagram for FRC.

No matter what kind of fibre is incorporated in the concrete mix the tensile strength of the composite is only slightly greater than that of the cement matrix. Fibres have no little or no influence on the matrix behavior until first cracking. The effect of fibre reinforcement on the composite starts soon after the first crack, influencing the pre-peak strain hardening (region 1 to 2) and especially the post peak tension softening response of the matrix (region between points 2 to 3) by bridging across the microcracks and reducing their width. This enables the matrix to resist more applied load experiencing ductile response to the continuous loading and not brittle as it was previously without the reinforcement. The post first crack behavior depends on the volume fraction of the fibres used in the mix as well as on the aspect ratio[1] of the fibres. Thus, the area under the load deflection diagram, which represents the fracture toughness, is much larger for the FRC than for the ordinary concrete. With increasing deflection, load carrying capacity drops gradually and almost linearly as a result of frictional pull-out of fibres. The increased fracture toughness and the extended strain hardening response exhibited by fibre reinforced concrete are due to the fact that fibres operate perfectly as crack arrestors, bridging the microcracks and preventing in that way their localization and the formation of a large single crack.

In order to achieve improved tensile strength it is apparent to use large volume fractions of fibre. However, if that is the case, care has to be taken to avoid mix workability problems. The marginal improvement in the tensile strength of concrete by the incorporation of fibres (unless large proportions are used) makes FRC more suitable for durability purposes, while other, more improved cementitious composites (e.g. HPFRC) are suitable for structural applications.

 

Load Deformation Response for Conventional FRC and HPFRC

FRC and HPFRC have similar tensile/flexural load-deflection response, which is shown in the figure below.

 

Figure 2. Tensile/Flexural load deformation response of conventional and high performance FRC.

 

Both composites exhibit linear elastic behavior until they reach point 1 on the curve. At this point the first cracks initiated. Both materials are able to resist more applied load until they reach the load carrying capacity, at point 2. This region between points 1 and 2 is termed strain hardening and it is similar in both materials. When the crack saturation point is reached (point 2) both materials exhibit the so-called strain softening response, which is the region between points 2 and 3 on the curve. Although, the load carrying capacity has been reached at point 2 failure does not takes place immediately. The localized microcracks assemble forming larger cracks, which are held by the fibre reinforcement, and progressive failure takes place in a ductile manner in both the conventional and high performance FRC.

 

There are however important differences between these two materials in terms of the above graph.

Since the slope of the linear part (up to point 1) is an indication for the E-value of the material it can be said that the stiffness of HPFRC is larger than that of conventional FRC. Secondly, the strain hardening region is much larger for the HPFRC in relation to the conventional one. This is because a large volume fraction of fibres has been incorporated in the mix. As a result of this, the load carrying capacity for the HPFRC is much larger, causing a larger amount of deformation, as well. This behavior indicates that HPFRC is much more ductile material. Finally, it can be observed that the energy absorption capacity G_F, which is represented by the area under the curve, is much larger than the corresponding value for the conventional FRC, i.e. HPFRC is more tough due to the inclusion of long fibres (13mm in length). The following table shows the differences in the basic properties discussed above.

 

 

Table 1 Typical values for principal properties of conventional and high-performance FRC.

 

	E	ft	GF
Conventional FRC	30 – 35 GPa	6 – 8 MPa	1000 J/m2
HPFRC	40 – 45 GPa	25 – 40 MPa	40000 J/m2

 

Workability

Generally, workability can be defined as that property of concrete, which indicates its ability to be mixed, handled, transported and placed with minimum loss of homogeneity[vi]. Furthermore, the term workability can be used in order to describe all the essential properties of early stage concrete, such as mobility, compactibility, stability and finishability[vii].

There are various ways of measuring the workability of concrete mixes. Slump is a common, fast and relatively economical test but unfortunately it does not give reliable results in the case of FRC. The compacting factor test has been used in the past but experimental evidence has shown that the success of that test is very limited³. The most reliable workability test for FRC mixes seems to be the VeBe test. This test takes into account the effect of aggregate shape, air content, admixture effects and surface friction of fibres. It is useful to notice that back in late 1970’s an alternative workability test for FRC mixes was introduced, named “inverted slump cone”[viii] but its effectiveness was questioned[ix].

The workability in FRC mixes is influenced by many factors; the most important of these are the aspect ratio and the volume fraction of the fibres used. The aggregate size and volume have also an influential role in the workability. The uniform distribution of fibres becomes more difficult as the size and volume of aggregates increase. Thus, care should be taken during the mix design in order for adequate compaction properties to achieved.

Later developments in the FRC technology, connected with the addition of superplasticisers and microsilica in the mix aim at producing a composite with reduced water to cement ratio and increased strength

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[v] Illston, J.M., “Construction Materials” Second Edition, 1996, E & FN Spon, pp 392-395.

[vi] ACI Committee 544.2R-78 (Revised 1983), “Measurement of Properties of FRC”, pp 433-448.

[vii] Swamy, R.N., Stavrides, H., “Some properties of high workability steel fibre concrete”, RILEM International Symposium in Fibre Reinforced Cement and Concrete, 1975, pp 197-208.

[viii] Schrader, E.K., “Formulating guidance for testing of fibre concrete in ACI Committee 544”, RILEM International Symposium in Testing and Test Methods of Fibre Cement Composites, 1977, pp 9-21.

[ix] Tattersall, G.H., “Discussion Topics”, RILEM International Symposium in Testing and Test Methods of Fibre Cement Composites, 1977, pp 61-63.

Historic development of Fibre Reinforced Concrete

The concept of using fibres in order to reinforce matrices weak in tension is more than 4500 years old. Ancient civilizations used straw fibres in sun-dried mud bricks in order to create a composite with increased toughness, i.e. a matrix with a better resistance to cracking and an improved post cracking response. Since Portland cement concrete started to be used widely as a construction material attempts were made to use fibres for arresting cracks. Engineers had to overcome the major deficiencies of concrete, which were the low tensile strength and the high brittleness. A French engineer, named Joseph Lambot, in 1847 came out with the idea of adding continuous fibres into the concrete, in the form of wires or wire meshes[i]. This led to the development of ferrocement and reinforced concrete as known today. The use of continuous steel reinforcing bars in the tensile zone of concrete undoubtfully helped to overcome the problem of the low tensile strength of concrete. However, the idea of using discontinuous fibres in the concrete was always a challenge.

The development of fibre reinforcement for concrete was very slow before 1960’s. Until then there were some papers describing the basic concept of using fibres for reinforcement in concrete mixes but there was no application. Nevertheless, research on glass fibres had been conducted in USA, UK and Russia in early 1950’s. Actually, in Russia glass fibres were not only under research but were also used in the construction industry. However, this kind of fibres was found to be prone to alkaline attacks. In late 1950’s Portland Cement Association started investigating fibre reinforcement[ii].

Since early 1960’s there has been an increased interest in fibre reinforced concrete (FRC). This period is the turning point for the development of FRC. More rapid modern advances are paralleled by increasing applications. While more new applications were identified a wide range of fibres was introduced. These include:

· Steel Fibres.

· Glass Fibres.

· Carbon Fibres.

· Natural Organic Fibres.

· Polypropylene Fibres.

Generally, the fibres used to reinforce concrete can be characterised as discontinuous, discrete fibres with length less than 50mm and diameter no more than 500mm.

The actual purpose of incorporating fibres in the concrete matrix was the development of a composite with improved strength, both compressive and tensile. By analysing the results of the earliest developments in this field it can be observed that neither the compressive nor the tensile strength were increased by any appreciable amount. The actual benefits of fibre reinforcement were difficult to highlight by the researchers at that time.

Later on, during the modern development of FRC in late 1970’s and early 1980’s, when the testing equipment and analysis procedures became more quantitative and better qualitatively the concept of energy absorption (or fracture toughness) was introduced. This concept enabled the toughness measurement of materials. It was then that the major advantage of FRC was discovered and it was not other than the outstanding property of absorbing large amounts of energy compared to Ordinary Portland Cement Concrete. Even today, after more than three decades of research in this field it can be said that the principal benefit of FRC is the high fracture toughness. However, further research with different types of fibres and admixtures targets the development of a composite with increased tensile and compressive strengths, besides the fracture toughness. These FRC composites are now known as the high performance fibre reinforced concrete (HPFRC).

The production of a cement based material having high tensile and compressive strengths, remarkable energy absorption capacity and which will be homogeneous and isotropic (almost similar to cast iron) is no longer an utopia any more. The incessant research in the field of FRC has led to the production of HPFRC, which shows a combination of amazing properties compared to other cementitious composites.

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1 Naaman, A.E., “Fiber Reinforcement for Concrete” Concrete International Journal, March 1985, pp 21-25.

2 ACI Committee 544, “State-of-the-Art Report on Fiber Reinforced Concrete”, May 1982, pp 411-413.