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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

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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.

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Chafika Djelal, Yannick Vanhove and Laurent Libessart
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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

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Analysis of Friction and Lubrication Conditions of Concrete/Formwork Interfaces
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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

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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

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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

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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

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Analysis of Friction and Lubrication Conditions of Concrete/Formwork Interfaces
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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

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Chafika Djelal, Yannick Vanhove and Laurent Libessart
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(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

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Analysis of Friction and Lubrication Conditions of Concrete/Formwork Interfaces
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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

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Chafika Djelal, Yannick Vanhove and Laurent Libessart
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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

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Analysis of Friction and Lubrication Conditions of Concrete/Formwork Interfaces
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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.
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