As it was mentioned earlier, the causes of concrete deterioration may be physical or chemical. The typical causes of concrete deterioration may be grouped into two categories; surface wear and cracking as follows:
It should be emphasized that the distinction between the physical and chemical causes of deterioration is purely arbitrary; in practice the two are frequently superimposed on each other. For example, higher permeability of concrete increases the risk of rebar corrosion and corrosion of reinforcement causes cracking and further increase in permeability chemical causes of deterioration will be discussed later. Cracking of concrete due to normal temperature and humidity gradients are out of scope of this course. Here, the physical causes of concrete deterioration will be explained.
3.1.Deterioration by Surface Wear (Abrasion)
Under many conditions, such as abrasion, erosion and cavitation, progressives loss of mass from concrete surface may occur.
Abrasion: refers to dry attrition (e.g. wear on pavements and industrial floors by vehicular traffic)
Erosion: occurs mainly in hydraulic structures, refers to wear by the abrasive action of fluids containing solid particles in suspension. Erosion takes place in spillways, pipes, sewage system.
Cavitation : refers to loss of mass by formation of vapor or bubbles and their subsequent collapse due to sudden charge of direction in rapidly flowing water.
Hardened cement paste has a low resistance to attrition. Service life of concrete can be greatly reduced under repeated cycles of attrition, especially when paste matrix of concrete is of high porosity or low strength, and is inadequately protected by an aggregate which lacks wear resistance. There is a good correlation between the W/C ratio and abrasion resistance of concrete. Thus, ACI Committee 201 recommends a minimum 28 day compressive strength of ~30 MPa. Suitable strengths may be attained by a low W/C ratio, proper grading of fine and coarse aggregate (limiting Dmax to ~25mm), lowest consistency practicable for proper placing and compacting (max. slump 75 mm; for toppings 25mm), and minimum air content consistent with exposure conditions.
When a fluid containing suspended and rolling particles is in contact with concrete, the impinging, sliding and rolling action of particles may cause surface wear. The rate of erosion depends on the quantity, shape, size and hardness of the particles being transported, on the velocity of the moving particles as well as on the porosity or strength of concrete.
For silt-size particles (2-150 μm), erosion will be negligible at bottom velocities up to 1.8 m/s (min. velocity to transport particles).
When severe erosion or abrasion conditions exist, it is recommended that in addition to the use of hard aggregates, the min. 28 day compressive strength of concrete should be ~40 MPa, and before exposure to aggressive conditions concrete should be adequately cured. European Standard ENV206 (1992) recommends a period of curing twice as long as normal in order to achieve good resistance to surface wear.
For a proper attrition resistance, at least the surface of concrete should be of high quality. The properties of concrete surface zone are strongly affected by the finishing operations; vacuum dewatering is beneficial. The presence of laitance should be avoided by delaying floating and troweling until the concrete has lost its surface bleed water. Moreover, the bleeding capacity should be reduced by taking suitable measures such as using mineral admixtures. Industrial floors or pavements may be designed to have a 25 to 75 mm thick topping, consisting of a low W/C ratio concrete containing hard aggregate of ~12 mm Dmax. Due to very low W/C ratio, concrete toppings containing Latex admixtures are becoming increasingly popular for abrasion resistance.
Resistance to abrasion can also be achieved by application of surface hardening solutions to well-cured new floors or abraded old floors. Solutions most commonly used are magnesium or zinc fluosilicates or sodium silicate which react with CH present in hcp to form insoluble reaction products, thus, sealing the capillary pores at or near the surface and hence to improve the resistance to ingress of fluids or dusting due to abrasions.
As far as aggregate is concerned, strong and hard aggregate is preferred with inclusion of some crushed sand. However, the abrasion resistance of aggregate, as determined by the Los Angeles
From cement content point of view, rich mixes are undesirable, a max. cement content of 350 kg/m3 is necessary to allow coarse aggregate particles present just below the surface of concrete.
Shrinkage compensating concrete has a significantly increased abrasion resistance probably due to the absence of fine cracks which would encourage the progress of abrasion.
The non linear flow at velocities exceeding 12 m/s (7 m/s in closed conduits) may cause severe erosion of concrete through cavitation. In flowing water, vapor bubble form when the local absolute pressure at a given point in the water is reduced to ambient vapor pressure of water corresponding to the ambient temperature. As the vapor bubbles flowing downstream with water enter a region of higher pressure, they implode with great impact because of the entry of high-velocity water in to the previously vapor occupied space, thus causing severe local pitting. Therefore, the concrete surface affected by cavitation is irregular or pitted, in contrast to the smoothly worn surface by erosion from suspended solids. The cavitation damage does not progress steadily: usually, after a period of initial small damage, rapid deterioration occurs, followed by damage at a slower rate.
Best resistance to cavitation damage is obtained by the use of high strength concrete. The Dmax near the surface should not exceed 20 mm, because cavitation tends to remove large particles. Hardness of aggregate is not important (unlike the case of erosion resistance) but a strong bond between aggregate and mortar is vital.
Use of steel fibers may improve the cavitation resistance. However, in contrast to erosion or abrasion, a suitable and strong concrete may not necessarily be effective in preventing damage due to cavitation for an indefinite time. The best solution lies in removal of the causes of cavitation, such as surface misalignments, or abrupt changes of slope or curvature that tend to pull the flow away from the surface. If possible, local increase in velocity of water should be avoided as damage is proportional to the sixth or seventh power of velocity.
Test methods for the evaluation of wear resistance of concrete are not always satisfactory, because the damaging action varies depending on the exact cause of wear, and none of the test procedures may satisfactorily simulate the field conditions of wear. Therefore, laboratory methods are not intended to provide a quantitative measurement of the length of service that may be expected from a given concrete surface; they can be used to evaluate the effects of concrete ingredients and curing or finishing procedures on the abrasion resistance of concrete.
ASTM C 418-90 prescribes the procedure for wear determination by sandblasting; the loss of volume of concrete serves as a basis for judgment. ASTM C 779-89 describes three optional methods for testing the relative abrasion resistance of horizontal concrete surfaces. In the steel-ball abrasion test, load is applied to a rotating head containing steel balls while the abraded material is removed by water circulation; in the dressing wheel test, load is applied through rotating dressing wheels of steel; and in the revolving-disk test, revolving disks of steel are used in conjunction with a silicon carbide abrasive. In each of the tests, the degree of wear can be measured in terms of weight loss after a definite time.
Proneness to erosion by solids in flowing water can be measured by means of a shot-blast test. Here, 2000 pieces of broken steel shot (850 µm [#20 ASTM sieve size]) are ejected under air pressure of 0.62 MPa against a concrete specimen 102 mm away. Besides, due to direct relationship between the abrasion and erosion resistance, the abrasion resistance data can be used as a guide for erosion resistance.
3.2.Cracking by Crystallization of Salts in Pores
A purely physical action of crystallization of the (sulfate) salts in the pores may account for considerable damage in concrete. For instance, when one side of a slab or retaining wall of a permeable concrete is in contact with a salt solution and the other sides are subjected to evaporation, the material can deteriorate by stresses resulting from the pressure of salt crystallization in pores.
In many porous materials, the crystallization of salts from super saturated salt solutions can occur only when the concentration of the solute (C) exceeds the saturation concentration (Cs) at a given temperature. As a rule the higher the (C/Cs) ratio (or degree of super saturation) the greater the crystallization pressure. The crystallization pressures for salts that are commonly found in pores of concrete, calculated from the density, molecular weight, and molecular volume for a C/Cs ratio of 2 are shown in Table 3.1. At a degree of super position of 10, the calculated crystallization pressure of NaCl is 1835 atm at 0°C and 2190 atm at 50°C.
Table 3.1. Crystallization Pressures for Some Salts
Salt | Chemical Formula | Density (g/cm3) | Molecular Weight (g/mol) | Molar Volume (cm3/g) | Pressure (atm) C/Cs = 2 | |
0°C | 50°C | |||||
Anhydrite | CaSO4 | 2,96 | 136 | 46 | 335 | 398 |
Epsomite | MgSO4.7H2O | 1,68 | 246 | 147 | 105 | 125 |
Gypsum | CaSO4.2H2O | 2,32 | 127 | 55 | 282 | 334 |
Halite | NaCl | 2,17 | 59 | 28 | 554 | 654 |
Hepta hydrite | Na2CO3.7H2O | 1,51 | 232 | 154 | 100 | 119 |
Hexa hydrite | MgSO4.6H2O | 1,75 | 228 | 130 | 118 | 141 |
Kieserite | MgSO4.H2O | 2,45 | 138 | 57 | 272 | 324 |
Mirabilite | Na2SO4.10H2O | 1,46 | 322 | 220 | 72 | 83 |
Natron | Na2CO3.10H20 | 1,44 | 286 | 199 | 78 | 92 |
Tach hydrite | 2MgCl2.CaCl2. 12H2O | 1,66 | 514 | 310 | 50 | 59 |
Thenardite | Na2SO4 | 2,68 | 142 | 53 | 292 | 345 |
Thermonatrite | Na2CO3.H2O | 2,25 | 124 | 55 | 280 | 333 |
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