Concrete repair can be defined as the act of renewing, restoring, or replacing of any concrete or concrete surfaces after initial placement (Smoak et al, 2002). In general, the need for concrete repair can arise from different causes which may vary from minor concrete holes or cracks- such as resulting from minor imperfections due to snap-tie holes or she-bolt holes, to major damages resulting from external factors or structural failures. In practice, it is found that the majority of the defects can be repaired or made good, while in some cases it is often found that the rebuilding of the individual member or structure is often economical (Allen et al, 2005).
It is agreed in general that the protection and the repair of the concretes are complex tasks which requires special attention and integrated knowledge of different specialist fields. Due to the explosive growth of economies around the world and due to the increased investment in infrastructures (especially in terms of maintenance, restoration and repair) in the recent years, the repair and protection of concrete structures has become a major topic of both scientific and economic interest. As a result, the construction industry has faced many challenges and demands. Underneath these demands lies the fact that the concrete structure deteriorate over time at intolerable rates. As such, it is highly necessary to develop the ways by which the life of the concrete structures can be increased.
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There is also a need for appropriate tools and techniques to carry out the task of repair and maintenance of the concrete structures within the limited budgets available. It is also important to consider the importance of environmental conditions to which the concrete structures are exposed while considering the tools and techniques employed for the repair of the structures.
The field of concrete repair calls for the integrated knowledge (expertise) of different scientific fields. The basic idea behind this fact is that the buildings or the concretes are facing different environmental or external conditions and around the worlds, the field pose many challenges to both the academia and the practitioners alike. Added to this, the fact that many buildings have their own history and nature, it becomes necessary to develop and implement individual strategies and solutions for the protection, repair and maintenance of concretes. As a result of these complications, different basic strategies and solutions has been developed of the repair of the concrete structures (Raupach, cited in Alexander et al, 2009). This highlights the fact that this area requires knowledge from different fields and that the domain of the subject is more of interdisciplinary in nature.
2.2.1 The practice of concrete Repair:
There are different standards to which the practice of repair of the concrete should be carried out. EN 1504 provides 43 methods to repair and protect the concrete structures which are based on eleven different principles. However, it is generally recommended that the designer can choose the appropriate solution depending on different conditions like the economical and technical considerations (Alexander et al, 2009). It has also been recommended in the previous studies that while selecting the optimal methods for different situations, the designer should investigate in detail the special questions such as the required level of maintenance, durability, and the sustainability (ibid).
Many authors have based their works on the topic of concrete repairs (Wall and Shrive, 1998; Kuhlmann, 1990; Plum, 1990, etc). Though there are many methods or standards regarding the concrete repairs, it can also be noted that there is no standard design specification for the repair of the concretes. It can be argued from an outset that a proper understanding of why the patches fail could possibly reduce the number of failures. However, due to the absence of standard design procedure for the repair of the concretes and the lack of understanding, many researchers have now concentrated their efforts towards understanding the durability of repair patches.
2.3 Adhesion between the concretes:
In the field of concrete research, the adhesion between the concrete is one among the mostly researched topics. This has been argued to be an important aspect that influences the durability and the reliability of the patch up or the repair works (Czarnecki et al, 2007).
In general, adhesion which can be considered as a basic issue in the repair of the concretes can be defined as the adherence of two materials in contact (Czarnecki, cited in Alexander, M. G., et al, 2009).
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The EN 1504-10 standards (European Standards) used the term 'bond' to indicate the adhesion that takes place between the old concrete and the new concrete. As such, it is an important factor that can also determine the effectiveness of the repair. Fiebrich (1994) divided the adhesion mechanisms into three types; mechanical interaction, chemical bonding and thermodynamic mechanisms.
The European Standards, EN 1504 provides two ways of expression of the formulated bond level results of the adhesion (Czarnecki, cited in Alexander, M. G., et al, 2009).
Threshold value in MPa; towards structural repair (EN 1504-3): >2,0MPa (Class R4), >1,5 MPa (Class R3);
Pass/fail criteria towards structural bonding (EN1504-4) e.g. hardened concrete-to-hardened concrete or fresh concrete-to-hardened concrete: the test shall result in fracture in the concrete.
Once the bond is formed between the concretes, the bond can then be said to be effective only if the newly formed bond ensures an even distribution of the stresses and an effective transfer of load without failure.
Specifically, it has been found that the increase in adhesion, results in higher tolerance on non-compatibility property of both the old concrete and the new overlaying material (Czarnecki et al, 2007).
2.4 Factors affecting Adhesion:
There are many basic theories on adhesion that describe the mechanisms of the adherence between the old concrete and the new overlay material. The main theories pointed by different authors on the topic are the absorption theories and the mechanical blocking theories (Emmons et al, 1994; Santos et al, 2007; Maerz et al, 2001). Briefly, in the first theory relating to the thermodynamic adsorption the intermolecular forces or the Van der waal forces, the chemical linkage through the interface and the hydrogen liaison increases the bond between the old and the new concrete. The second theory of mechanical blocking can be broken down at two levels firstly macroscopic in which the roughness of surface creates mechanical blockings between overlay and concrete and secondly at microscopic level, the surface porosity at the interface substrate, comparable to a micro-roughness, facilitates anchoring by tangle of hydrates, Bouksani et al (2010).
Czarnecki (2009) in his research about the adhesive aspects of the concrete bonds gave a crucial conclusion. He concluded that there are many factors affecting the adhesive strengths. These included:
The properties of the repair materials: this included, viscosity, shrinkage during setting, surface tension, creep, etc.
Old concrete (concrete substrate): strength of the material, presence of impurities, cracks present on the surface and the micro cracks, porosity of the material, the roughness of the surface, etc,.
The effect of the environment: the level of temperature- the change and the rate of change, presence of moisture- humidity level and the change of humidity level, the phenomenon of transportation- suction due to capillary effect, the loading-mechanical loading, ageing, effect of corrosion etc,.
All these factors mentioned above could influence the bond between the concretes. Many of the factors could increase the bonds between the concretes but, in most of the cases, these reasons cause destruction in concrete bonds. Due to these reasons, it is obvious there is still a need for the High Adhesive Repair Materials to be used in the repair practices in order to address these issues.
It has been identified that adhesion will depend on many factors. These are mainly due to the phenomena that take place at the interface area of the concretes (Slater, 2001). The main phenomena includes, roughness of the material surface, presence of moisture on the surface, the humidity levels of the external environment, external loading, wetability of the surface, the properties or the influence of the additive materials, etc,.
The importance of surface roughness, moisture condition and curing condition is often highlighted in many researches. It has been said that the first step in repair or the maintenance of the concrete after choosing the method is to prepare the surface of the existing concrete on to which the concrete is to be laid. This may include the removal of the existing deteriorated concrete layer so that this can be replaced by the new material. The strength will depend on the degree of adherence of the new material on to the existing material. However, the influence of the roughness and the moisture of the surface also contribute to the overall bond between the old and the new concrete. Therefore, it is very important to ensure that the moisture and the roughness quality of the surface is achieved to the required level before the new concrete is laid on to the existing concrete (Courard, 2000).
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The difference in the result of the researches carried out by different researchers could be observed on; the condition under which the curing is carried out; the humidity condition; the bonding agents used in the experiment; the temperature at which the test is carried out; the age of the specimen; the type of the test employed; the design/setup of the specimen, etc. To this point, Julio et al (2004) argued the importance of different test parameters to be considered while comparing the results. They noted that these differences produced contradictory results and as such, there is no consensus in the findings. Therefore it is generally agreed that the comparison of findings appear to be difficult. But, to get an overview of the current knowledge and substantive findings some of the major factors that affect the adhesion between concretes are discussed below.
2.4.1 A single study considering various factors:
Wall et al (1988) investigated the factors affecting the bond between the old and the new concrete in concrete repairs. The experiment was carried out to explore the effects on the bonds based on different parameters. These parameters included the thickness of the layer of the bonds, the ratio of the cement and water used in preparing the mortar, the influence of different conditions under which the curing process was carried out, the influence of wetting the surface of the old concrete onto which the new concrete is laid, the influence of the delay between mixing the copolymer PVA bonding agent and its application to the old concrete.
The major difference that can be noted from the study carried out by Wall et al (1988) and other researches is the difference in the bonding strength of the specimens containing the Portland cement to the specimens that contained Copolymer PVA agents. It was observed in their study that the specimens that contained Portland cement mortar were stronger than the PVA agent ones.
Major differences were also found in the tests relating to the thickness of the new concrete layer or the repair material applied to the older concretes. It was found that the strength increased when the thickness reduced (1/8 in and 3/16 in proved stronger than the 1/4 in layer). However, the authors do not give further explanation regarding this phenomenon. Notably, the research did not provide clear explanation about the affect of cement to water ratio. The compressive strengths for different water to cement ratios were found to be different. Ultimate compressive stress for a water cement ratio of 0.32 was found to be lower (1870 psi lower) than the one which had a water cement ratio of 0.40 by this observation, Wall et al (1988) concluded that the bond strength between the two concretes decreased as the water cement ratio also decreased.
One another finding was about the surface condition of the substrate concrete before the application of the new layer. It was found that the pre-wetting of the substrate before the application of the new layer resulted in improving the strength of the bond. In cases when the PVA bonding agents dried before the application of the repair material to the surface, it was found that the compressive strength of the bonds decreased by 10%. However, it was also found that the mortars with PVA modified cements produced higher bond strength.
One other important aspect found was the influence of the curing condition on the strength of the bonds. Different specimens were cured under different curing conditions. It was found to affect the ultimate strength. Though it was found that the specimens that were cured under high humidity conditions (100%) had greater strengths than the ones cured in less humid conditions, the differences were not considerable. Hence it was concluded that the degree of influence was small when considering other parameters.
2.3.2 Influence of Material for the Repair:
Though there are many methods that can be followed while repairing the concretes, it is advised that proper care should also be taken while considering the repair material. Once these have been selected, the next important step is to clearly understand the conditions that the concrete will experience over its life. Some of the key factors are the weather conditions, chemical exposures, the range of temperature, the magnitude of the load and the duration to which the concrete is exposed to the load, the nature of the patch (aesthetic or structural), etc. It is important to consider these facts because different researches in to the field of the concretes have shown that the performance of the patch material varies under different conditions. It can be argued that proper care should be taken while selecting the patch materials by giving proper consideration to the material properties of the substrate concrete and the old concrete and the conditions to which the concrete will be exposed over its life. It could be recommended that the materials with different properties could be selected only if the bond strengths are not affected and further it does not cause any durability problems (Wipf et al, 2004). However, it is vital to ensure that the internal stresses do not exceed the tensile stresses of the substrate (Wipf et al, 2004).
Influence of Maturity:
The bond between the substrate and the new concrete has also been linked with the maturity of the overlay. This was pointed by Delatte et al (2000). To elaborate over this topic, they linked the maturity of the overlay to the bond strength, which in turn was related to the overall strength of the material (interface). It was concluded in their research that the early- age strength of the concrete had significant effect on the bond strength and the tensile strength of the material. These groups of researches have also highlighted that overlay or the interface as one of the major zones of the fracture. Thus, according to Beushausen and Alexander (2008), mechanisms of bond failures in relation to the parameters of material and the location of failure will require further research.
2.3.3 Influence of Test Method:
It has been argued in the previous journals that the quality of the bond strength can be influenced by a range of parameters relating to the material and the conditions of the environment (Alexander and Beushausen, 2008). Another important factor is the test method used. There are various test methods proposed to evaluate bond properties and performance of repair materials in general. They include the tensile bond, slant-shear, twist-off, flexural and patch test etc., However each test is influenced by different combinations of factors and cannot give alone a full picture (Austin et al,1999). For instance, Momayez et al (2005) reported in their study that in few cases bond strength from some tests were upto eight times higher than those obtained from other type of tests.
Additionally, it is important to note that the mechanical adhesion in tension varies considerably from that in shear (Alexander and Beushausen, 2008). Therefore, it can be argued that the views of different authors' will also depend on the choice of the methods used in determining the bond strength.
2.3.4 Condition of the surface of the interface:
The bond strength also depends on the general condition of the surface like cleanliness. There is also special industry standards to which the cleaning and preparing of the concrete surface should be carried out before the repair is carried out (Charles and Scott, 1997). In additions to these standards, there are also many standard recommendations provided by different manufacturers of the repair material products. The more commonly known industry standards are: ASTM D 4258 Surface Cleaning Concrete for Coating; ASTM D 4259 Abrading Concrete; ASTM Standards for Cleaning, Surface Preparation, and Testing; ASTM D 4285 Indicating Oil of Water in Compressed Air, etc.
One of the major points to be considered while cleaning the surface is the presence of the surface contaminants. This can be in the form of liquids or solids. There is a potential threat that these contaminants could problems for the application of the material, curing, adhesion or other things which are vital for forming the bond between the two concrete layers (Wipf et al, 2004). It is noted that the presence of dust and unsound concretes are also a major barrier for forming a strong bond between concretes. It is recommended that the use of the jack hammer be avoided when dealing with the damaged concretes. Instead, the use of the chisel and hammer can ensure that the structural properties of the concretes are not affected (Ibid).
2.3.5 Influence of Roughness:
The influence of the roughness to the bond strength between the concretes has been argued by many authors. It has been argued in the previous studies that the addition of the ordinary concrete on to the interface that is rough produce better results than the one that was laid on to the smooth interface (Garbacz et al, 2006). These findings are similar to the ones investigated by other authors too. Matana et al (2005) identified in their research that the bonds between the concretes provided good bond strength when the material had rough interface. Similar results were also given by Courard (2006). Roughness of the substrate surface is often presented as the most important factor to achieve a good bond. This improvement is mainly assigned to the increase of the contact surface (Talbot et al 94, Santos et al 07).
On the other hand through the study done by Perez et al (2009), while studying about the correlation between the roughness of substrate surface and debonding risk, they interpreted by their results that increasing of roughness does not enhance the bond strength.
The debate over the influence of roughness is still an ongoing issue and as such, the effect of the roughness of the concrete surface over adhesion process is not very clear (Austin et al, 1995; Czarnecki, et al, 2003). Some researchers have argued that there is influence of surface aggregates on the level of adhesion that take place at the interface. A visible aggregate of ratios between thirty to forty percentages provided better results (Fiebrich, 1994). Fukuzawa et al (2001), argued the existence of correlation between roughness parameters of the material surface and the adhesion strength. Though the roughness of the material is an important factor, some authors (Silfwerbrand, 1990) have also pointed the affect of micro cracks that are produced on the surface of the concrete when the surface is treated by different methods to achieve the roughness. These kinds of micro cracks are found to deteriorate the quality of the bonds that is formed between the concretes (Garbacz et al, 2005).
2.3.6 Influence of moisture:
With regards to the presence of moisture, recent studies provide a better understanding to the topic of influence of the moisture on the strength of the bond. Bouksani et al (2010), in their study on the influence of the roughness and moisture of the substrate surface on the bond between the old and the new concrete, suggest that the penetration of the overlay material into the substrate concrete is mainly dependent on quality preparation of the substrate surface.
They also suggested that the surface of the substrate should be saturated with dry surface- which means that the surface of the substrate is not stagnated with water, so that it ensures that a good micro roughness and a better thermodynamic adsorption is achieved. They suggest that the stagnation of the water on the substrate surface can prevent the pores from absorbing the overlay material which can result in a weak adhesion process. The overall result of over stagnation is that, it will stand as a barrier in the interface zone reducing the adherence process.
On the other hand if the substrate material surface is dry with very low moisture or no moisture, this will result in the interface absorbing more water from the newly laid material. Such a process will also reduce the overall adhesion process as this could result in higher chance of incomplete hydration of the cement. Thus, it can be concluded that the moisture needs to be present only to a optimum level. Too much moisture or too little presence of moisture will result in a weak adhesion process.
Similarly, Xu (1999) also studied bond strength using oven dry, air dry and saturated surface dry condition of substrate. He reported that prewetting the surface of concrete substrate reduced the bond strength of the specimens used in his experiment from 0.64 to 0.12 MPa
However, Saucier and Pigeon (1991) didn't find any notable difference between the bond strength of lab dried surface and pre-wetted surfaces in slant shear test. In another study, Austin et al.(1995) did work on finding bond strength using substrates with saturated surface dry, saturated surface wet and air dry moisture conditions. They also reported no significant difference in bond strengths (about 2.77 to 2.98 MPa) due to the different moisture condition of substrate.
2.3.7 Influence of Curing Condition:
There are a few researches carried out concerning the affect of curing condition on the shear bond strength. Yee and Ibrahim (2010) in their study on shear capacity of precast slabs investigated the difference in results for water cured samples and air cured samples.
The results they got were varying with similar roughness characteristic of surface but different curing condition. For instance, a rough surface produced higher shear capacity than a smooth one in air cured condition whereas a smooth surface produced 16 to 18 % higher shear capacity in water cured conditions. Another notable point was the influence of curing condition on the interface strength which was extremely variable, as such it decreased by 30 % in smooth and rough surface in water cured condition where as for the air cured condition it increased by 40% between smooth and rough surface.
2.3.6 Influence of Bond Coat:
It is common that some external agents are used in the mix to achieve a higher level of adhesion. Many authors have different opinion regarding this, mainly the bond coats like polymer composites. Some authors (Silfwerbrand, 1998) argue that the bond coats can result in creation of an extra plane of weakness. As a result, it has been advised to avoid the bond coats. Further, the bond coats could also lead to the reduction of the interlocking effect resulting in an overall negative effect (Garbacz et al, 2005). On the other hand these arguments have been questioned by some other authors. Interestingly, according to some other authors (Austin et al, 1995; Pretorius and Kruger, 2001), the presence of the bond coats could increase the adhesion process between the two concretes.
To this point, Garbacz et al (2005) conducted a study to investigate the influence of surface treatment of the concretes on the adhesion mechanism between the concrete layers. Different surface treatment methods were used in order to achieve different qualities of surface on the concrete material surface. Additionally, a repair mortar with coat was applied to the old concrete and the adhesion was measured. The results showed that, in the case of the overlays applied without the bond coats, the adhesion process was influenced by the roughness of the concrete substrate.
Form this, it was concluded that the good bond strength properties can be achieved from the mortar (repair mortar) and the bond coat properties. It was further concluded that the properties of the bond coats and the mortar helped in bridging the micro cracks and concrete pieces, and also filled the surface irregularities. Therefore, it can be concluded on the recent works on influence of the surface on the adhesion between the surfaces the roughness condition of the surface can be regarded as one of the contributing or influencing factor in the repair of the concretes.
From these discussions on the different factors affecting the bond it can be concluded that the results are not always in consensus and there is still need for studies to be carried out in this area for gaining further insight of the actual behaviour of bond with respect to changes in factors affecting it.
2.4 Surface Roughening Technique:
There are different ways in which the surface of the materials can be prepared to achieve different roughness conditions. However, the main aim of performing the surface treatment on the substrate is to the make sure that the unwanted layers that might reduce the adhesion process are removed before the new layers of concretes are laid on to the surface. In addition, this also ensures that the total area of surface contact is increased to the maximum level. The shapes and impressions achieved on the surface will depend upon the energy applied and chosen techniques.
Roughening Techniques which does not deteriorate the mechanical integrity of the substrate material must be used. It is important to consider the local condition of the concrete under consideration. Accordingly, common techniques like chisel and hammers, sandblasting, hydro-jetting, needle gunning, grinding or milling, hammering etc, may be used (Garbacz et al, 2005).
2.4.2 Measurement of Roughness:
The roughness is generally assessed qualitatively. But, identifying the fact that this kind of roughness evaluation leads to subjectiveness of results, there have been some attempts made to quantify the roughness. For, instance mechanical profilometry was used to differentiate polished and sandblasted concrete surfaces (Courard 1998, Courard and Nelis 2003 & Courard and Garbacz 2004). Due to some shortcomings of this process like viability with only short surfaces etc., another method of optical analysis was developed (Perez et al 2003) in order to analyse large surfaces. Later on, these two above mentioned techniques were compared by Courard et al (2006) and they concluded that with the combination of these two methods it is possible to get a very good description of "roughness" at all scales. They also pointed out some limitations of these methods regarding the shape of stylus which would make it impossible to take measurements on very rough surfaces and other limitations being very time consuming etc.,
In another study by Courard et al (2004) a point which is concluded is that a parameter Xa which is the arithmetic mean of the departure of the roughness profile from the mean line is the major discriminating parameter for the comparison of surface preparation techniques.
In a new method for evaluating the surface roughness Abu Tair et al (2000), studied five different kinds of roughness including needle gunning. They used a different approach for quantifying the roughness by relating the amount of roughness to Roughness Gradient of the roughness profile obtained. They concluded that the roughness gradient parameter can give a proper indication of the roughened surfaces.
In this study an attempt is made to find out a similar kind of parameter to which the amount of roughness can be related and the roughened surfaces can suitably be categorized.