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ANALYSIS OF FRICTION AND LUBRICATION CONDITIONS OF CONCRETE/FORMWORK INTERFACES

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Concrete friction plays a fundamental role during various stages of construction and public works operations, including pumping, formwork filling and the production of facings. A tribometer for fluid materials has thus been developed to better study this friction. Tests performed with certain modifications of interface conditions show that friction is governed by interfacial characteristics (e.g. type of demoulding agent, roughness, velocity, pressure). The investigation showed that the tribometer is sensitive to obtain a real understanding of the mechanical behaviour of the Self-Consolidating Concrete (SCC). The tests and observations made reveal that friction mechanisms depend on the properties of the interface. The interface appears to undergo two types of phenomena which depend of the pressure. The demoulding oil generates a reduction of the friction between the SCC and the formwork. Parameters specific to facing appearance are also addressed in this paper.

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ANALYSIS OF FRICTION AND LUBRICATION CONDITIONS OF CONCRETE/FORMWORK INTERFACES

  1. 1. http://www.iaeme.com/IJCIET/index.asp 18 editor@iaeme.com International Journal of Civil Engineering and Technology (IJCIET) Volume 7, Issue 3, May–June 2016, pp. 18–30, Article ID: IJCIET_07_03_003 Available online at http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=7&IType=3 Journal Impact Factor (2016): 9.7820 (Calculated by GISI) www.jifactor.com ISSN Print: 0976-6308 and ISSN Online: 0976-6316 © IAEME Publication ANALYSIS OF FRICTION AND LUBRICATION CONDITIONS OF CONCRETE/FORMWORK INTERFACES Chafika Djelal Professor, Dept. of Civil Engineering Univ. Artois, EA 4515, Laboratoire Génie Civil et géo-Environnement (LGCgE), Béthune, F-62400, France Yannick Vanhove Professor, Dept. of Civil Engineering Univ. Artois, EA 4515, Laboratoire Génie Civil et géo-Environnement (LGCgE), Béthune, F-62400, France Laurent Libessart Assistant Professor, Dept. of Civil Engineering Univ. Artois, EA 4515, Laboratoire Génie Civil et géo-Environnement (LGCgE), Béthune, F-62400, France ABSTRACT Concrete friction plays a fundamental role during various stages of construction and public works operations, including pumping, formwork filling and the production of facings. A tribometer for fluid materials has thus been developed to better study this friction. Tests performed with certain modifications of interface conditions show that friction is governed by interfacial characteristics (e.g. type of demoulding agent, roughness, velocity, pressure). The investigation showed that the tribometer is sensitive to obtain a real understanding of the mechanical behaviour of the Self-Consolidating Concrete (SCC). The tests and observations made reveal that friction mechanisms depend on the properties of the interface. The interface appears to undergo two types of phenomena which depend of the pressure. The demoulding oil generates a reduction of the friction between the SCC and the formwork. Parameters specific to facing appearance are also addressed in this paper. Key words: SCC, Friction, Formwork, Tribometer, Aesthetics
  2. 2. Analysis of Friction and Lubrication Conditions of Concrete/Formwork Interfaces http://www.iaeme.com/IJCIET/index.asp 19 editor@iaeme.com Cite this Article: Chafika Djelal, Yannick Vanhove and Laurent Libessart. Analysis of Friction and Lubrication Conditions of Concrete/Formwork Interfaces, International Journal of Civil Engineering and Technology, 7(3), 2016, pp. 18–30. http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=7&IType=3 1. INTRODUCTION Since the 1980's, the use of Self-Consolidating Concrete (SCC) has grown considerably in popularity. Many significant structures of varying types have now been built with this material. At present, all key building industry actors take into account the progress provided by this material. SCC is also highly attractive to project owners and architects thanks to its finish in terms of facing quality. The appearance of facings constitutes one of the main SCC advantages. The use of effective demoulding agents helps ensure a perfect final product in term of aesthetic quality. While the oils have already been successfully characterized, our understanding of the thickness of oils applied onto formwork walls before casting is still lacking. Facing quality depends primarily on the concrete skin properties, i.e. the layer of material in contact with the formwork skin. This would extend to the first tenths of millimetres of concrete, in influencing both colour and texture. Demoulding oils are also used by formwork manufacturers to limit corrosion phenomena. When subjected to repeat concrete pouring, the oil film actually disappears and wear begins to occur as aggregates need to be included in the design of formwork installations capable of withstanding the concrete pressure. Over the past few years, several researchers have begun focusing on friction at the concrete/formwork interface, as a means of either determining the lateral pressure of concrete against the formwork [1-2] or conducting phenomenological studies [3-6]. Two plane/plane tribometers have been specially designed for such studies. The underlying principle is identical for both devices, i.e. a metal plate in contact with a movable concrete surface. These devices are capable of reproducing the conditions encountered as concrete is being poured into the formwork. Several researchers [7] have proposed predictive models for determining the concrete pressure against formwork. Vanhove et al. [1] and Proske et al. [2] have developed a predictive model based on Janssen's theory in order to evaluate concrete pressure against a formwork. Both these models introduce a coefficient of friction that depends on several parameters. This paper is aimed at studying the influence of these parameters on the coefficient of friction at the concrete/wall interface and, consequently, encompasses their influence on concrete pressure against the formwork as well. The results output concern the behavioural study of a SCC used during the national project ("B@P") held at the Guerville experimental site (France). In order to better understand the mechanisms taking place at the concrete/wall interface, testing was conducted in the laboratory both with and without demoulding oil [8]. Based on a series of tribometric tests [4], complemented by electrochemical impedance spectroscopy, various phenomenological models could be established to explain the mechanisms in effect at the concrete/oil/formwork interface. A study focusing on facing aesthetics has also been conducted for the purpose of identifying a correlation between the protocol for applying demoulding oil and facing aesthetics.
  3. 3. Chafika Djelal, Yannick Vanhove and Laurent Libessart http://www.iaeme.com/IJCIET/index.asp 20 editor@iaeme.com 2. THE TRIBOMETER The principle adopted herein is to press a sample of concrete against a moving metal surface (Fig. 1). The plate has been cut out from formwork walls by a formwork manufacturer. The sample holders were cylinders 120 mm in diameter fitted with a hatch to feed the concrete, which had been pressurized by the use of pneumatic jacks. A sealant system was installed on the sample holders so as to ensure full containment of the concrete without damaging the oil film applied on the plate. A mobile bottom was also placed at the back of the sample in order to transmit the pressure delivered (N) by the pneumatic jack to the concrete. This device has already been described in many publications [1, 3]. Fig. 1 Detailed view of the tribometer For each test, the tangential force (or frictional force) has been recorded according to time. This force corresponds to two separate frictional forces, namely: - on the one hand, the resultant force of the interference friction force (Fpar) on the gasket system acting against the metal plate, as well as the resultant force of the tie against the slide; - on the other hand, the resultant force (Fmes) of the tangential friction force of both material samples against the plate, i.e. 2F, if friction is considered to be similar for the two samples tested. µ = (Fmes - Fpar) / N (1) 3. PROPERTIES OF MATERIALS AND OIL 3.1. The metal plate Previous studies [3, 9] carried out on concretes have revealed that surface roughness exerts a significant influence on the friction coefficient. Roughness measurements of formwork walls were recorded at the Guerville site using a portable roughness meter (Ra = 1 µm, Rt = 9 µm). Ra is the arithmetic mean deviation relative to the average line, while Rt is the distance between the highest maximum and lowest minimum on the roughness profile (Fig. 2). For this study, a plate has been cut out from a formwork wall. Machining ridges run in the direction of plate displacement. Metallic plate Sample-holder
  4. 4. Analysis of Friction and Lubrication Conditions of Concrete/Formwork Interfaces http://www.iaeme.com/IJCIET/index.asp 21 editor@iaeme.com Figure 2 Roughness profile 3.2. Concrete mix design This study has focused on the behaviour of a SCC that had been used during the national B@P project carried out at the Guerville experimental site. The selected concrete classification is commonly employed for civil engineering structures; this concrete features good rheological characteristics as regards both fluidity and stability. Limestone additions (filler) were introduced into the composition of test specimens as a means of improving facing quality in terms of colour uniformity. The concrete composition and characteristics are listed in Table 1. The particle size distribution analysis [10] of the cement, filler, sand and coarse aggregate has served to determine the maximum diameter Dmax of the grains, as well as the percentage of grain diameters (D) capable of becoming lodged within the plate asperities (Table 2). Table 1 Mixture proportions of investigated concrete Mixture (kg/m3 ) Cement CEM I 52,5 CP2 365 Limestone filler 255 Sand 0/5 670 Coarse aggregate 3/8 790 Superplasticizer 6.0 Cohesion agent 0.66 Water 206 Water / (Cement+Limestone filler) 0.35 Density 2.3 Slump (cm) 70 Table 2 Granulometric analysis of the fine elements of the SCC Dmax D < 80 µm 0.1 µm < D < 10µm Cement 60 µm 100% 55% Limestone filler 100 µm 70% 15% Sand 0/5 5 mm 0% 0% Gravel 3/8 8 mm 0% 0% The concrete particle size distribution is very widely spread, extending from roughly a micron for cement grains up to 8 mm for gravel diameter. The cement and filler grains with diameters smaller than 10 µm will potentially become lodged in the tribometer plate asperities. Medium line Rt Ra
  5. 5. Chafika Djelal, Yannick Vanhove and Laurent Libessart http://www.iaeme.com/IJCIET/index.asp 22 editor@iaeme.com The term fine particles or simply fines refers to all cement and filler components whose diameter is less than 80 µm. Mixing was performed in accordance with the NF P 18-404 Standard entitled "Concretes - Analytical, feasibility and control testing - Specimen manufacturing and preservation". The operating protocol implemented was as follows: Figure 3 Mixing sequence 3.3. The oil used The oil chosen for this study has a plant-based composition (denoted V for vegetable). It is 95% biodegradable without requiring the use of solvents. It has been used at the Guerville site; all pertinent properties are provided in Table 3. Table 3 Vegetable oil properties Properties Vegetable based oil (V) Nature of oil Liquide Color Yellow Flash point (°C) > 200°C Density 0.9 Viscosity at 20°C (mm2 s-1 ) 28 3.3. Oil application protocols Demoulding oils must be applied homogeneously over the entire wall of a formwork. Their application requires the use of a sprayer fitted with an adapted nozzle. Any excess product is removed, as needed, with a scraper (Fig 4). Literature gives a different thickness according to the film. Indeed a 2 µm film can gives a good quality facing, but 10 µm can also gives good aesthetic results [5]. (a) Spraying (b) Spraying followed by scraping Figure 4 Demoulding oil application protocols Excess oil however may lead to facing defects (bubbling). The conditions for applying oils on formworks (Guerville, France) were replicated in the laboratory. Two Aggregates + Sand + Binder Water Superplasticizer End of mixing 1 mn 1mn30 1 min
  6. 6. Analysis of Friction and Lubrication Conditions of Concrete/Formwork Interfaces http://www.iaeme.com/IJCIET/index.asp 23 editor@iaeme.com cases were examined in detail: application of the oil by spraying using a conical nozzle followed or not followed by spreading with a rubber scraper. The oil film thickness was measured by means of two distinct methods: weighing and a technique based on alpha radiation [5]. A sample formwork with a dimension of 5x3 cm2 was tested for the first method. In knowing the mass density of both the oil and the plate surface, it is simply necessary to weigh the sample in order to determine the oil thickness [5]. These results are given in Table IV. Measurement uncertainty equals +/- 0.15 µm. The oil film thickness measurement principle relies on the possibilities offered by the PIXE device, as well as on the properties of  rays, which are material particles (i.e. nuclei of helium containing 2 protons and 2 neutrons) launched at high speed (with an energy equal to 5.3 MeV). Oil thickness is measured from the maximum fluorescence X of the steel composing the metal plate. The level of steel fluorescence is directly influenced by attenuation of  X-rays in the oil film. From the detection of emitted X protons (given that the film only absorbs a small amount), the number of  particles reaching the wall (through the oil film) can be measured, according to a simple measurement protocol by metric absorption . Moreover, very strong method sensitivity has been observed. Figure 5 Schematic diagram of the PIXE principle and the oil film measurement on the tribometer plate The results are shown in Table 4 for both methods. The measurements output by these two methods have yielded practically the same results. Table 4 Thickness of the oil films Methods Weighing PIXE Spraying 17 µm 17.5 µm Spraying followed by scraping 0.8 µm 0.7 µm Oil Metallic plate Source Excitation by  x-ray Emission of  X-ray Studied material Detector Oil Metallic plate Source Excitation by  x-ray Emission of  X-ray Studied material Detector
  7. 7. Chafika Djelal, Yannick Vanhove and Laurent Libessart http://www.iaeme.com/IJCIET/index.asp 24 editor@iaeme.com Friction tests, with and without demoulding oil, were then conducted under the casting conditions implemented at the experimental site of the national B@P project. The pressures analysed, which simulate concrete pressure against the formwork, were defined relative to maximum pressures recorded at the formwork base (P = gh, where  is the mass density of the material, g the gravitational acceleration, and h the formwork height). At the Guerville site, 6 concrete walls of 5 and 10 m high were cast. The pressures calculated at the formwork base equalled to 118 kPa (for the 5 meter high wall) and 235 kPa (for the 10 meter high wall). The relative sliding velocity of the concrete against the tribometer plate were calculated based on the concreting speeds and ground surface area of each formwork. These speeds varied from 1.57 to 12.08 mm/s. 4. INFLUENCE OF THE CONTACT PRESSURE The variation of friction coefficient µ with the concrete pressure against the plate without demoulding oil is shown in Fig. 6 for a speed of 5 mm/s. The variation in this coefficient is not linear Figure 6 Variation of the friction coefficient with the concrete pressure Two zones can be distinguished, thus reflecting two distinct types of friction. This curve displays a minimum at a pressure of 150 kPa. This same trend can be observed for other concrete mix designs. This critical value is equal to 110 kPa for a conventional concrete [9] [11]. To explain phenomena taking place at the concrete/wall interface, please refer to the evolution in shear stress (friction) according to contact pressure (Fig. 7). 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0 50 100 150 200 250 300 350 Frictioncoefficientµ Contact presure, kPa
  8. 8. Analysis of Friction and Lubrication Conditions of Concrete/Formwork Interfaces http://www.iaeme.com/IJCIET/index.asp 25 editor@iaeme.com Figure 7 Evolution of the shear friction stress according to the contact pressure The shear stress is lower for pressures applied to concrete of less than 150 kPa. Two distinct types of friction will occur at the concrete/wall interface. Despite its appearance, fluid concrete is not a continuous medium. The various concrete elements will play very specific roles when friction occurs. The pressure stress applied to the material is transmitted to the granular phase as well as to the paste formed by the binder (cement + filler). This pressure will then cause a portion of the liquid phase and fines to migrate towards the interface. A lubricating surface (or boundary) layer (water + fines) of thickness "e" is thus formed at the interface. Experimentally speaking, the difficulty of highlighting sheared interface phenomena stems from the difficulty of instrumenting the materials in contact and, more specifically, the boundary layer. Owing to the cement particle and filler scales, Schwendenmam [10] and Vanhove et al. [11] used two techniques to develop an understanding of this complex interface. Whether by means of ultrasound [11] or ionizing radiation [10], both methods indicated a decrease in aggregate (sand and gravel) concentration near the wall. At low pressure, the phenomenon at the concrete/wall interface is triggered by the onset of microstructural rearrangement tied to initiating concrete pressurization at the interface. The grains contained in the boundary layer have a number of degrees of freedom, which serves to facilitate shear. As of 150 kPa (critical pressure), a portion of the boundary layer will migrate towards less stressed zones. Based on the conclusions drawn from these two studies, a proposed description of the mechanisms at work can be generated. The plate roughness Rt equals to 9 µm, which allows the possibility that a portion of the cement and filler grains (D < 10 µm) becomes lodged in surface asperities. Shear mainly takes place in this layer (Fig. 8a). For pressures exceeding 150 kPa, a part of the boundary layer will also migrate towards less stressed zones (Fig. 8b). 0 2 4 6 8 10 12 14 16 18 20 0 50 100 150 200 250 300 350 Frictionstress,kPa Contact presure, kPa
  9. 9. Chafika Djelal, Yannick Vanhove and Laurent Libessart http://www.iaeme.com/IJCIET/index.asp 26 editor@iaeme.com (a) (b) Figure 8 Schematic representation of a concrete/metal plate interface The sand or gravel grains will be placed in direct contact with the asperity tips (i.e. granular friction). The force exerted by these tips during plate displacement will lead to their rotation, thus giving rise to considerable energy dissipation and resulting in a faster increase in both the friction coefficient and metal surface wear. After a series of tests corresponding to roughly 70 passes of concrete on the plate, the grains added both width and depth to the asperities. Ra value of 2 µm and an Rt of 26.8 µm were found. 5. INFLUENCE OF THE SLIDING VELOCITY The variation of the friction coefficient with the sliding velocity for 3 contact pressures (50, 150 and 300 kPa) is given in Fig. 9. Below a 5 mm/s sliding velocity, the friction coefficient present a slight sensitivity. Beyond this value, a stable evolution of the friction coefficient is observed. N Migration water+ fines V N Migration (a)P  150 kPa V Boundary layer Plate N Migration Flowing grains V N (b) P > 150 kPa V water+ fines VV
  10. 10. Analysis of Friction and Lubrication Conditions of Concrete/Formwork Interfaces http://www.iaeme.com/IJCIET/index.asp 27 editor@iaeme.com Figure 9 Evolution of the friction coefficient with the concrete pressure against the metallic plate Between 0.5 to 5 mm/s, under the pressure effect and with a sufficiently long period, a limit layer and a part of fines elements from the sand becomes lodged in the plate asperities. The shearing is located in this boundary layer (Fig. 8a). When the sliding velocity is greater than 5 mm/s, a granular friction takes place (Fig. 8b). 6. EFFECT MECHANISMS OF THE DEMOULDING OIL WITH THE APPLICATION PROTOCOLE Fig. 10 shows the evolution of the friction coefficient with the contact pressure for both oil application protocols. A reduction in the coefficient of friction can be observed. This decrease is more pronounced for the sprayed oil. Like for friction without oil, the critical pressure lies at 150 kPa regardless of the oil application protocol. Figure 10 Evolution in the coefficient of friction vs. pressure for both oil application protocols 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0 2 4 6 8 10 12 14 Frictioncoefficientµ Sliding velocity, mm/s 50 kPa 150 kPa 300 kPa 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0 50 100 150 200 250 300 350 Frictioncoefficientµ Contact presure, kPa Sprayed oil Sprayed oil followed by scraping SCC
  11. 11. Chafika Djelal, Yannick Vanhove and Laurent Libessart http://www.iaeme.com/IJCIET/index.asp 28 editor@iaeme.com Beyond 150 kPa, the effect of oil minimizes granular friction. Libessart et al. [8], in his study intended to better understand oil/concrete/wall interface mechanisms, performed a series of tests on various components of a particular oil mix design. This author studied the percentage of acidifier and solvent in a plant-based oil and moreover demonstrated that the effect of a base alone depends in large part on the thickness being applied. The physical effect takes precedence over the chemical effect (Fig. 11a). Conversely, the presence of an acidifier strengthens the chemical effect by creating a greater quantity of soap at the interface (Fig.11b). (a) (b) Figure 11 Diagram depicting the sliding of SCC on the oil film [8] The oil introduced in our study is composed of a vegetable base and devoid of any solvents. In this specific case, the oil film thickness determines friction, which explains the results. 7. INFLUENCE OF THE DEMOULDING OIL ON THE AESTHETIC OF THE FACINGS Few results are given regarding the aesthetic flaws [12] encountered on concrete facing after formwork removal. The two application protocols described above have been analyzed. Moulds sized 30 x 30 x 30 cm were designed by the same formwork manufacturer as the one that built the tribometer plate (Fig. 12). Figure 12 Metallic mould 30 x 30 x 30 cm Concrete Metal plate Film of mineral-based oil Concrete (hyrophilic environment) Vegetable oil film Calcium oleate Ester moleculeFormwork Soap film Aggregate
  12. 12. Analysis of Friction and Lubrication Conditions of Concrete/Formwork Interfaces http://www.iaeme.com/IJCIET/index.asp 29 editor@iaeme.com Regardless of the application protocol employed, the facings are of high quality and show very little bubbling. No concrete attachment points exist on the wall (Fig. 13). Figure 13 Facing surface and dirtying of a mould On the other hand, extensive fouling and dust accumulation have been observed on the mould surface for oil scraped after spraying. 8. CONCLUSION This study has shown the importance of interface conditions when pouring self- compacting concretes into the formworks. To understand the role of the demoulding agent, it is essential to achieve understanding of phenomena at the concrete/oil/formwork interface. The static study of the concrete into the mould, show that the oil film of about 0.8 µm of thickness is sufficient to obtain a facing quality. It has been observed on site that an excess of oil entailed a bad quality of the concrete facing. In dynamic, which correspond to the concrete movement against the formwork surface, the friction coefficient decreases by about 30%. The originality of this research lies in the fact that very few studies have previously been conducted in this field. REFRENCES [1] Vanhove Y, Djelal C (2004) Prediction of the lateral pressure exerted by self- compacting concrete on formwork, Magazine Concrete Research 56(1):55-62. [2] Proske T, Graubner CA (2007) Formwork pressures of concretes with high workability, Advances in construction materials 463-470. [3] Djelal C, Vanhove Y, Magnin A (2004) Tribological behaviour of self compacting concrete Cement and Concrete Research 34:821-828. [4] Djelal C, de Caro p, Libessart L, Dubois I (2008) Comprehension of demoulding mechanisms at the formwork/oil/concrete interface, Materials and Structures 41:571-581. [5] Djelal C, Vanhove Y, Chambellan D, Brisset P (2010) Influence of the thickness of demoulding oils on the aesthetic quality of facings, Materials and Structures 43(5):687-698. [6] Bouharoun S, de Caro P, Dubois I, Djelal C, Vanhove Y (2013) Influence of a superplasticizer on the properties of the concrete/oil/formwork interface, Construction and Building Materials 47:1137-1144.
  13. 13. Chafika Djelal, Yannick Vanhove and Laurent Libessart http://www.iaeme.com/IJCIET/index.asp 30 editor@iaeme.com [7] Bilberg PH & al. (2014) Field validation of models for predicting lateral form pressure exerted by SCC, Cement and Concrete composites 54:70-79. [8] de Caro P, Djelal C, Libessart L, Dubois I, Pebert N (2007) Influence of the nature of demoulding agent on the properties of the formwork/concrete/ interface, Magazine of Concrete Research 59(2):141-149. [9] Vanhove Y, Djelal C, Magnin A (2000) Friction behavior of a fluid concrete against a metallic surface, EUROMAT 2000, Conference on Advances on Mechanical Behaviour, Pasticity and Damage, Tours, France. [10] Schwendenmann G (2006) Etude de l'écoulement des bétons autoplaçants dans les coffrages à l'aide de la métrologie des rayonnements ionisants, Thesis report. Civil Engineering, University of Artois, France. [11] Vanhove Y, Djelal C, Chartier T (2008) Ultrasonic wave reflection approach to evaluate fresh concrete friction, Journal of Advanced Concrete Technology 6(2):253-260. [12] Libessart L, Djelal C, de Caro P (2014) Influence of the type of release oil on steel formwork corrosion and facing aesthetics, Construction and Building Materials 68:391-104. [13] N. Krishna Murthy, A.V. Narasimha Rao, I .V. Ramana Reddy, M. Vijaya Sekhar Reddy, P. Ramesh. Properties of Materials Used In Self Compacting Concrete (SCC), International Journal of Civil Engineering and Technology, 3(2), 2012, pp. 353–368. [14] Prabhakara R, Chethankumar N E, Atul Gopinath and Sanjith J. Experimental Investigations on Compression Behavior Parameters of NSC and SCC Intermediate RC Columns, International Journal of Civil Engineering and Technology, 6(8), 2015, pp. 100–117.

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