The Erosion of Reinforced Concrete Walls by the Flow of Rainwater
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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.
Keywordsconcrete erosion rainwater flow walls prints mass loss
Ground penetrating radar
Height of the wall
The mass loss
The mass of the measuring base
The rate of erosion
Scanning Electron Microscopy
Width of the plate
Réunion Internationale des Laboratoires et Experts des Matériaux
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
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 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
3.3 Depths of 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. 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
3.4.2 The Material Lost by Erosion and the Rate of Erosion
Mass loss and erosion rate.
M eros (mm3)
R eros (%)
3.4.3 Variation of the Rate of Erosion on the Height of the Wall
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
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)
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 area.
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 (wind).
4.1 Impact of the Carbonation Phenomenon
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 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
Mechanical characteristics obtained in the exposed and protected parts of the wall.
Compressive strength σ (MPa)
Ultrasound velocity (m/s)
Crushing of concrete core
The average value of the rebound
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 (≤10 MPa according to Rilem) because le measurement was applied only on the surface.
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 site.
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 one.
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 reinforcement.
Thanks to the LMDC Laboratory of Toulouse (France) for their collaboration in laboratory analyses (XRD and SEM).
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