The Erosion of Reinforced Concrete Walls by the Flow of Rainwater
International Journal of Concrete Structures and Materials
The Erosion of Reinforced Concrete Walls by the Flow of Rainwater
Kawthar Hadja 0
Fattoum Kharchi 0
0 Built Environment Research Laboratory (LBE), University of Sciences and of Technology Houari Boumediene (USTHB) , Algiers 16111 , Algeria
The action of rainwater on reinforced concrete walls has led to an erosion phenomenon. The erosion is very apparent when the walls are inclined. This phenomenon is studied on a real site characterized by different architectural forms. The site dates back to the seventies; it was designed by the architect, modeler of concrete, Oscar Nie Meyer. On this site, the erosion has damaged the cover of the reinforcements and reduced its depth. In this research work, a method of quantification of the erosion is developed. Using this method, the amount of mass loss by erosion was measured on imprints taken from the site. The results are expressed by the rate of mass loss by erosion; they are associated to the height and the inclination of the walls. Moreover, laboratory analysis was carried out on samples taken from the site. From this study, it is recommended to consider the erosion, in any building code, to determine the cover thickness.
concrete; erosion; rainwater; flow; walls; prints; mass loss
In the literature the wall erosion of buildings throw the
flow of rainwater has not been much studied. The rare works
found in the literature focus rather on the effects of water jets
and those of the water flow of rivers and basins. It is often
associated with the impact of the hard particles and the water
pressure on the surfaces. It arouses the interest of the ACI in
the document (ACI 210 1993).
The phenomenon studied in our case is particular; there
are no great flows of water but of rain falling and flowing on
buildings walls. Degradation is very apparent; the concrete
cover of the reinforcement is greatly reduced. In some areas
the steel bars are apparent.
There is, thus, a mass loss which indicates a phenomenon
of erosion. Our objective is to measure the rate of the mass
loss and also to identify the nature of the phenomenon,
whether it is a pure mechanical effect or combined with a
chemical effect (solvent). In literature, Momber (1998),
worked on the mechanical effects of water jet on several
types of materials for industrial applications such as
machining, drilling, cutting and hydro demolition. For this
author, cement composites are quasi brittle materials. In
Momber et al. (1995) and Momber (2001) he analyzed the
erosion mechanism by the parameters of fracture mechanics
in the sense of pre-existing cracks propagated under the
effect of the water jet. In this case the compression strength
is not sufficient to describe erosion but the fracture depends
greatly on the size of the aggregates used. At high pressure
water jet exceeding 30 times the tensile strength of the
material, Momber et al. (1995) deduced that the interface
paste aggregates has a significant impact on erosion, it is the
origin of the micro-cracks, leading to erosion. Other
researchers like (Liu et al. 2006) and (Liu et al. 2012) had
studied the effect of the flow of river water on concrete
surfaces, in Liu et al. (2006) erosion of concrete is presented
as an abrasion phenomenon that can be corrected by the
addition of silica fume and fibers. The same author in (2012)
has simulated the flow of river water by a jet of water
containing sand combining the water-jet impact load and
sand particle shear/friction. He found that the rate of erosion
is related to the amount of grain moved by the flow of water
and to the angle of the water jet to the impact surface. The
authors agree that the interface between hardened cement
paste and aggregate play the main role in the fracture process
by erosion. Aala Rashad et al. (2014) have studied the
abrasion of industrial floors and have found a direct relation
between the abrasion resistance and the compression
strength; they have concluded that the silicate fume
improves such abrasion resistance.
2. Site Description
The research work concerns the site of our university. All
the structural elements are built with reinforced concrete
with brutal surfaces without any protection this was the time
of the concrete brutalism. The exposure duration of the
studied buildings is 40 years which corresponds to their age.
In the site there are 26 identical buildings (the
amphitheaters) with more than 100 identical walls (Figs. 1 and 2).
Some of them are protected and others are exposed to the
rain. The phenomenon of erosion is repeated with the same
appearance on all the structural elements.
The erosion appears also on other types of structural
elements, such as the elements walls/columns (Fig. 3). There
Fig. 1 Inclined wall intensely eroded.
Fig. 3 Repeated wall elements partially eroded.
are seven buildings of this kind having 60 columns each.
These elements are straight on first 5 m and inclined at the
bottom. They have been subjected to the flow of rainwater
on one side and protected on the other side (Fig. 4). The
measurements were conducted on walls with a protected side
(protected concrete) and another exposed to rain (eroded
concrete) (Fig. 2). By this choice, the study concerns the
same concrete that is poured at the same moment. Thus, it is
the same element of the structure with and without erosion.
3. Measurement of the Material Lost by the Erosion
3.1 Experimental Procedure
The adopted experimental procedure is used to measure
the material lost by erosion. The measurements are
performed on imprints taken from the real wall panel (Kharchi
Fig. 2 The right side protected part, the left side exposed part.
Fig. 4 The right side protected part, the left side exposed part
of the straight wall.
Fig. 5 Plate of the imprint against the wall.
and Hadja 2014). The protected part (from the rainwater)
and the eroded part of the wall are analyzed at the same time.
For the elements such as the type of photos 1 and 4, the
imprints are taken on both sides. The right side is the
situation before erosion and the left is the situation after erosion.
On the site, heavily eroded elements have visible traces of
carbonation, resulting in as reinforcement corrosion and
concrete spalling. In the present approach, particular
attention was given to select only the elements damaged by water
flow to avoid overlapping with several causes of damage.
3.2 The Imprints
A silicone paste mixed with plaster was spread over a
stable plate (20 9 10 cm2). It was then plated against a part
of the reinforced concrete wall during 2 min. Once removed,
the imprint appears on the plate. The relief of the past
expresses clearly the mass that is lost by erosion (Figs. 5 and
6). This experimental process was improved after several
attempts of measurements. Using plates of 60 9 40 cm2 and
adhesive silicone alone, the imprint did not appear on all the
plate and the silicone became deformed. After several
attempts, the plates were reduced to 20 9 10 cm2 and the
paste to a of silicone-plaster mixture. The plaster was added
to the silicone paste to reduce its deformability.
3.3 Depths of Erosion
The depth of the mass taken away by the erosion is
measured directly on the imprint with a displacement gauge.
Measures are taken on several points crossing the studied
surface according to two perpendicular axes with a step
between 1.5 and 2 cm. (Figs. 7 and 8). The term ‘‘erosion
depth’’ is used herein to indicate the thickness of the material
taken away by erosion.
3.4 Experimental Results
By the visual observation of imprints and surfaces of the
walls, it appears that the effect of the erosion is not uniform.
Fig. 6 The imprint taken from the wall.
Fig. 7 Measuring device.
In the less eroded parts, it is rather the cement matrix and the
small aggregate particles and sand which are lost. On the
contrary, in the most eroded areas, the coarse aggregates are
taken away. It can be deduced that the water flow extracts at
first the hardened cement paste. Over time larger grains are
taken away because there is no more past to seal them.
3.4.1 Profiles of Erosion
The erosion profile represents the variation of the erosion
depth (thickness of the material taken away) along the axis
OX. The operation is repeated in several positions of the axis
OY. The axes OX and OY are those considered in (Fig. 8).
The following profiles (Figs. 9 and 10) are drawn from the
same plate (imprint).
In order to ensure the repeatability of the procedure, two
imprints taken in the same place are compared. The profiles
drawn from the two imprints are rather close as shown on
Fig. 8 Imprint with virtual lines.
X Width of the plate[cm]
Fig. 9 Profile of erosion obtained from imprint on typical wall.
where Mtot is the total mass of the area considered (5 cm
long 1 cm depth), which is 50 9 10 9 10 = 5000 (mm3).
The choice of the depth value which is 1 cm corresponds to
the minimum cover to the reinforcement as considered by
most design codes. For the structural elements considered in
the present study (walls/columns), the initial concrete cover
to the reinforcements is 5 cm as measured at the protected
part of the column using two devices of electromagnetic
measurements (rebar detector and a GPR). The rate of
erosion Reros, thus defined, will vary between the values 1
and 0. The value 1 corresponds to the total loss of the
material where the reinforcements are completely uncovered.
The value 0 corresponds to an undamaged concrete.
Meros and Reros values presented in Table 1 were
obtained in the x-direction of the imprint. They correspond
to the position 1 of Fig. 4 (at h = 6 m).
3.4.3 Variation of the Rate of Erosion
on the Height of the Wall
The previous testing procedure is repeated at various
heights of the wall (1–6 m) with a step of 1 m. The results
are reported on the following curves (Figs. 12 and 13). The
first one presents the results obtained on the typical wall and
the second one presents the average calculated on five walls.
The points 6–3 m are on the straight part of the wall,
where the rate of erosion varies very little with the height.
The major change occurs from the point 2 m downward; the
maximum (70 and 80%) is reached at 1 m. These points are
close to the base and correspond to the inclined portion of
the wall. It is clear from this result, that the inclination of
structural element promotes erosion. The flow of rainwater
causes shock waves at the curvature. The pressure is high
compared to that in the straight sides of the wall, so the
impact force generated in the curved part is very important.
It is a singular point which holds potentially the rainwater.
4. Analysis of the Erosion Phenomenon
In order to identify the phenomenon of degradation and
separate the chemical and dissolving effect from the
mechanical effect, the rainwater is analyzed after passing
over the wall. The results (Table 2) indicate that this water is
hard and strongly ionized. A part of the damage was due to
the action of the dissolving effect of rainwater. Moreover, the
observation of the degraded surfaces by the naked eye
(Fig. 14) and by using the microscope, shows the
detachment of grains of various sizes. The observation of the
analyzed rainwater shows the presence of particles.
The results involve two types of processes: a chemical one
and a mechanical one, their effects are cumulative. In each
X Width of the plate[cm]
Fig. 10 Profile of erosion-average of five walls.
X Width of the plate[mm] 50 60
Fig. 11 Two imprints taken on the same surface.
3.4.2 The Material Lost by Erosion and the Rate
The surface under the curve (width (x)/depth (z))
corresponds to the mass loss by erosion by unit of height. On the
basis of an area of 5 cm in length and 1 cm in width, the
previous profiles give successively the following quantities
of lost material 127 and 110 mm3. By defining the quantity
of the mass loss is noted: Meros, the rate of erosion noted
Reros, can be calculated by reporting Meros to a unit mass
corresponding to a depth of 1 cm that is Mtot.
Table 1 Mass loss and erosion rate.
Fig. 12 Typical curve for one wall.
Fig. 13 Average curve for five walls.
case, rainwater plays a major role and is the basis of the
erosion of walls. The mechanical aspect is also supported by
the XRD analysis (Figs. 15 and 16).
The mechanical effect is the consequence of the grain
extraction by dissolution and also by the beating of rain
drops on concrete surface. The concrete is weakened by the
solvent effect as appears in the mineralogical analyses.
The X-ray diffraction applied on concrete debris collected
in the two studied areas: protected (Fig. 15) and exposed
(Fig. 16), indicates that there is a reduction of quartz peak
about 26 % in the exposed area compared to the protected
Observations were carried out using the technique of
scanning electron microscope (SEM) on concrete sections.
The first one corresponds to the eroded part (Fig. 17) and the
second one corresponds to the protected part (Fig. 18).
Fig. 14 The detachment of grain of various sizes on the
exposed concrete surface.
The image corresponding to the eroded area presents a
rough aspect with distributed form in relief as well as the
presence of the whitish spots of the lime due to the
precipitation of carbonates on the surface of concrete. The
image of the protected area presents an aspect slightly
worn under the effect of its exposure to weak attacks
4.1 Impact of the Carbonation Phenomenon
Concrete cores were taken to determine the carbonation
depth of the two studied areas of concrete. The
phenolphthalein indicates 2.5 and 2.1 cm carbonation depths
respectively for the protected and exposed parts (Figs. 19 and 20).
Carbonation is more advanced in the case of protected
areas than areas exposed to rainwater. This can be explained
by the fact that the carbonation is slowed down by the
saturation with water during the pluvial periods. When there is
production of calcite, it is on the surface and hence
Table 2 Comparison between the rainwater collected at the bottom of the wall (drained rainwater) and rainwater collected in a
reservoir (ordinary rainwater).
Drained rainwater (mg/l)
Ordinary rainwater (mg/l)
Fig. 15 XRD spectrum-protected area.
Fig. 16 XRD spectrum-eroded (exposed) area.
Fig. 17 Image (SEM) of the eroded concrete area.
vulnerable to the effect of erosion. The roughness due to
erosion also favoured the growth of micro-organisms
(algae). Darlington (1981) and (Dubosc 2000) consider the
roughness as an important factor in the colonization of
concrete walls by the micro-organisms. The filamentous
algae consume some mineral precipitants on the concrete
surface like calcite.
4.2 Impact on Strengths
In order to study the effect of the erosion of walls by the
flow of the rainwater on the mechanical strengths, the
Fig. 18 Image (SEM) of the protected concrete area.
compressive strength was measured on the concrete
corresponding to the two sides of the wall, the protected part and
the exposed part using two methods. The first method is the
destructive test; the compressive strength is measured by
crushing cylindrical-concrete cores (Fig. 21) in a
compression testing machine (Fig. 22). The second method is
nondestructive testing by the determination of the rebound
hammer (Fig. 23) and the measurement of the ultrasound
velocity (Fig. 24 and Table 3).’’
Fig. 19 The carbonation depth corresponding to the
Fig. 22 Crushing core concrete.
Fig. 20 The carbonation depth corresponding to the exposed
and eroded face.
Fig. 23 Determination of the rebound.
Fig. 21 Core drilling concrete.
Fig. 24 Estimation of the ultrasound velocity.
Table 3 Mechanical characteristics obtained in the exposed and protected parts of the wall.
Compressive strength r (MPa)
Crushing of concrete core The average value of the rebound
Ultrasound velocity (m/s)
The compressive strength obtained in the exposed face by
crushing of the concrete core is lower than that of the
protected part of concrete. This is due to two reasons:
Firstly, the depth of carbonation in the protected area is
higher than the depth obtained in the exposed part of
concrete. The protected part being more carbonated (see
Sect. 4.1), more calcite is formed which increases the
density of the concrete material and hence gives higher
strengths. According to Breccolotti et al. (2013) and Jong
Yun et al. (2016) and others, the variation of the
microstructure of the carbonated concrete decreases the
porosity which leads to the augmentation of concrete
strength. In this sense Pham and William (2014) indicates
that the carbonation decreases, in particular, the volume of
the micropores (radius \2 nm).
Secondly, in the case of the exposed concrete, the layer
of the calcite has been drained away by the erosion and
hence does not lead to an increase in the density of the
material. This explains the lower strengths of the eroded
concrete. The results of ultrasound indicate a concrete
with a low compressive strength (B10 MPa according to
Rilem) because le measurement was applied only on the
Erosion by rainwater is observed on a real site built
entirely of reinforced concrete. This natural phenomenon is
repeated on several structural elements of the same type.
The erosion has damaged the cover of the reinforcements
and reduced its depth. A method of quantification of mass
lost by erosion was developed. The experimental procedure
is based on imprints taken from the structural element in the
Erosion is expressed by mass loss as a function of the
height and the inclinaison of structural elements. At the scale
of the studied walls, mass loss by erosion is more affected by
the inclination than by the height. In 40 years of exposure,
erosion can remove up to 70 % of the concrete cover. This
phenomenon is slow but very detrimental to the
sustainability of buildings.
Other laboratory measurements such as SEM
observations, XRD and chemical analysis of rainwater, lead to
conclude that the erosion by rainwater is due to two
cumulative processes, a chemical one (solvent) and a mechanical
The carbonation depth and the mechanical strengths were
also measured. They indicate that the erosion removes the
calcite produced by carbonation, leading to a decrease in the
mechanical strengths, and leaves the reinforcing steel
without concrete cover and hence free to corrode.
From the present study, it is suggested to take into
consideration the erosion phenomenon in any building design
code in order to determine the adequate concrete cover to the
Thanks to the LMDC Laboratory of Toulouse (France) for
their collaboration in laboratory analyses (XRD and SEM).
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