Erosion Abrasion and Cavitation of Concrete

Surface wear Progressive loss of mass from a concrete surface can occur due to abrasion, erosion, and cavitation.  The term abrasion generally refers to dry attrition, such as in the case of wear on pavements and industrial floors by vehicular traffic.  The term erosion is normally used to describe wear by the abrasive action of fluids containing solid particles in suspension. Erosion takes place in hydraulic structures, for instance canal linings, spillways, and concrete pipes for water or sewage transport.  Another possibility of damage to hydraulic structures is by cavitation, which relates to loss of mass by formation of vapor bubbles and their subsequent collapse due to sudden change of direction in rapidly flowing water. Mechanism of abrasion and erosion The term ‗abrasion‘ covers a wide range of processes acting on a concrete surface to cause it to progressively lose material. In general terms, we normally think of abrasion as being the action of sandpaper on a surface – one solid surface slides over another, with one of the surfaces (the sandpaper) being harder and consequently removing a proportion of the softer surface. From experience, we know that we must also apply some pressure to the sandpaper – there must be a force acting to push the surfaces together at right angles to the direction of movement. The aforementioned process is indeed one of the mechanisms that cause abrasion of concrete surfaces (Figure 6). It is referred to as ‗abrasion by friction‘ and is one of the main processes occurring during erosion, caused by particles being carried over a concrete surface by water. However, the same effect can be caused by skidding of a wheel or foot on a pavement or floor surface. Moreover, debris, such as dust and grit, between the wheel or foot and the concrete surface will also have a similar and potentially more damaging effect. Moving particles may also abrade a surface by simply impacting with it head-on (Figure 7). Impacts of this type can cause fragments to break free from the surface or initiate surface cracks that may grow to produce fragmentation after a period of time. Of course, interactions between moving particles and concrete surfaces need not be at either of these extremes, and impacts can have angles of incidence between 90° and very low angles, with a combination of friction and impact abrasion occurring with each particle strike.

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Abrasion is the wearing away of the surface by rubbing and friction. Generally, the surface is uniformly worn away, including the cement matrix and aggregates. Factors affecting abrasion resistance include 1. Compressive strength 2. Aggregate properties 3. Finishing methods 4. Use of toppings 5. Curing

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Erosion Figure-9 shows the effect of two different angles of incidence (15° and 90°) on the rate of abrasion of a concrete surface with the higher angle being more damaging. The figure also illustrates the influence of another factor that influences erosion rates – fluid velocity – with higher velocities producing greater rates of erosion. Other factors that influence the rate of erosion are the mass of particles carried in a suspension, particle size, shape, and hardness. Figure-10 illustrates the effect of the quantity of particles, with an increase in the concentration of particles producing faster erosion. Typically, larger particles travelling at a given velocity will produce a larger rate of abrasion. Particle size is related to fluid velocity, since a higher velocity will allow moving water to carry larger articles. The influence of particle shape on erosion rates appears dependent on the angle of incidence, with angular particles causing erosion at slightly lower rates compared with rounded particles at low angles, but causing erosion at much greater rates at 90°. It has been proposed that this is because at 90°, angular particles striking the surface will be more effective at inducing cracking in particles of aggregate in the concrete, since their sharp corners will apply higher stresses during impact. At low angles, the manner in which the angular particles strike the concrete is such that this concentration of stress is less likely to occur. The rate of erosion increases with the hardness of the particles carried by a fluid. Cavitation Cavitation causes erosion of concrete surfaces resulting from the collapse of vapor bubbles formed by pressure changes within a high velocity water flow. When vapor bubbles form, they flow downstream with the water. When they enter a region of higher pressure, they collapse (implode) with great impact. The formation of vapor bubbles and their subsequent collapse is called cavitation. The energy released upon their collapse causes ―cavitation damage‖. Cavitation damage results in the erosion of the cement matrix, leaving harder aggregate in place. At higher velocities, the forces of cavitation may be great enough to wear away large quantities of concrete. The bubbles of water vapour violently collapse in a manner similar to that shown in Figure 11. This collapse produces a fast-moving jet of water that leads to the generation of significant local stresses. It is these stresses that, when they occur against a concrete surface, produce damage. Generally speaking, damage can start where the cavitation index exceeds a value of 0.2.Figure-12 shows Cavitation damage is avoided by producing smooth surfaces and avoiding protruding obstructions to flow. Damage from individual cavitation events takes the form of small pits in the surface, but since the formation of bubbles will typically occur with great frequency, the cumulative effect can be significant. Generally, accumulation of damage is initially slow, but increases with time until the rate of loss of material peaks and declines. The action of traffic over a concrete surface will also generate stresses that can cause microcracking, ultimately leading to abrasion. Figure-13 shows the different stresses that take the form of vertical compressive stresses, horizontal tensile and compressive stresses and shear stresses. The relatively low strength of concrete in tension and shear makes these types of stress the largest threat to the integrity of a pavement or floor, although large vehicles will induce significant compressive strengths. Although the diagram shows a wheel, it is clear that the action of a human foot will produce similar stress distributions, albeit of a smaller magnitude. Complied by S.Praveenkumar/Assistant Professor/Department of Civil Engineering/PSGCT

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Figure-12 Complied by S.Praveenkumar/Assistant Professor/Department of Civil Engineering/PSGCT

Figure-13 Factors Influencing resistance to Abrasion & Erosion When considering the results of research into the variables that influence abrasion and erosion resistance of concrete, it is important to note that this can be measured using a wide variety of tests. Specifically, the manner in which abrasive action is achieved varies considerably, and it is important to stress that, although in most instances a similar behaviour is likely to be observed regardless of the test being used, cases have been reported where this is not the case. ASTM C779 addresses abrasion of horizontal concrete surfaces. The standard describes three different techniques. The first two techniques involve a test machine consisting of a revolving carousel that is able to apply a constant load to the surface that is being tested. In the first test, the load is applied through three revolving steel discs. The rate of the revolution of both the carousel and the discs is defined in the standard. The carousel is run for a period of 30 or 60 min depending on whether long-term performance is a key concern. The machine feeds abrasive silicon carbide grit, which is fed onto the test surface at a constant defined rate. Abrasion resistance is evaluated by measuring the depth of wear at the end of the test. Complied by S.Praveenkumar/Assistant Professor/Department of Civil Engineering/PSGCT

The second of the horizontal surface tests does not use grit, and the revolving discs are replaced by three dressing wheels—spiked steel rollers that travel over the concrete surface. Again, after the machine is run for anperiod of either 30 or 60 min, depth of wear measurements are taken. The third test uses a different type of machine that consists of a revolving hollow drive shaft that applies a constant load to a ring containing a series of captive, but freely moving, ball bearings held against the concrete surface. The drive shaft is revolved at a defined rate, and the depth of wear is measured using a micrometer gauge that is part of the apparatus. The abrasion resistance is expressed in terms of the time required to reach a specified depth of wear. There are additionally two ASTM tests for concrete resistance to erosion type processes. The first of these, ASTM C418, uses sandblasting in a cabinet using a standardized gun nozzle and a standardized abrasive delivered onto the surface of a concrete specimen at a specified rate under a specified pressure for 1 min. The rate of abrasion is estimated by measuring the mean volume of the cavities produced on a number of spots by determining the mass of plastic clay of known density required to fill the cavity. The second method, ASTM 1138M , is the ‗underwater method‘ and attempts to more closely mimic the conditions encountered when concrete is exposed to sediment carried by moving water. In the method, the cylindrical concrete specimen is placed in a cylindrical tank, with the surface to be tested facing upward. Steel balls of various sizes are placed on the surface, and the tank is filled with water. A paddle of specified dimensions and geometry is submerged in the water at a fixed distance above the concrete surface and rotated at a speed of 1200 revolutions per minute. The concrete specimen is periodically removed and weighed to determine the loss of mass resulting from abrasion up to a test age of 72 h. Because abrasion is purely mechanical process acting on the surface of the concrete, it is not surprising that strength plays a very important role in defining the resistance of concrete to this mode of deterioration. 1) The strength of both the aggregate and the cement matrix plays a role, and the importance of these properties is very much dependent on whether the cement matrix or the aggregate is the stronger material. 2) The influence of cement content on abrasion resistance is dependent on the strength of the cement matrix. 3) With very low W/C ratios, where the strength of the paste is higher than that of the aggregate, increases in resistance with an increase in cement content are observed (for a fixed W/C ratio). However, the opposite is true where the strength of the concrete is lower than the aggregate. 4) Stronger aggregate will typically produce greater resistance to abrasion. However, this effect is more pronounced where the strength of the concrete is lower. 5) Maximum aggregate size appears to be important, with a larger value producing greater resistance. 6) It should be stressed that since normal mix design techniques reduce the cement content as maximum aggregate size increases, this factor may have an influence on experimental results. 7) Nonetheless, when the proportion of coarse aggregate is reduced, there is an increase in abrasion resistance, indicating that aggregate size has a genuine influence. Complied by S.Praveenkumar/Assistant Professor/Department of Civil Engineering/PSGCT

8) As with most durability characteristics, adequate curing of concrete is of great importance.

Complied by S.Praveenkumar/Assistant Professor/Department of Civil Engineering/PSGCT