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durability notes-2012.pdf

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McMaster University
Civil Engineering
Ghani Razaqpur

DURABILITY OF CONCRETE Concrete structures must be designed to be durable and not deteriorate under normal conditions of operations. Certain chemicals and environments can damage concrete and therefore precautions must be taken at the design level to prevent damage due to attack by these elements. Dense, impermeable concrete would generally resist the intrusion of harmful agents and would be more durable. Fluid transport, either in liquid or gaseous form, depends on the structure of the hydrated cement in the hardened concrete. Both the permeability and diffusivity of concrete may be used to assess its durability. Damage caused by freeze-thaw can be reduced by air-entraining while attack by sulfates can be mitigated by using sulfate resistant cements. Initiation of steel reinforcement corrosion in concrete can be significantly delayed by durable concrete and a reasonable cover thickness ( >50 mm) on the reinforcement. The greater thickness and its dense quality of the cover can protect concrete from damage caused by corrosion of reinforcement initiated by either chlorides penetration or carbonation. To prevent, alkali- silica reaction (ASR) the aggregates for making concrete should be carefully chosen in accordance with current standards requirements. Permeability: The rate at which water can pass through a material is its permeability. A permeable concrete allows water and other low viscosity fluids to pass through it easily, resulting in leakage. High permeability values reduce durability. Low w/c ratio and proper curing (high degree of hydration) lower capillary porosity and lower permeability and diffusivity. Permeability of concrete may be as low as 1 x 101m/sec for w/c = 0.3 to as high as 60 x 10-14m/sec for w/c = 0.7. A p The rate of flow through concrete can be determined experimentally and then using Darcy’s law, one can determine its hydraulic conductivity, or permeability as follows: dq h Water k A Δh dt L Concrete Where dq/dt = rate of fluid flow (m /s) k = permeability coefficient (m/s) Δh = drop in hydraulic head through the sample L = thickness of the sample (m) A = area perpendicular to the flow direction (m ) 1 As permeability of a given material may differ depending on the fluid viscosity, a more basic definition of permeability coefficient can be given by ' k k g ’ 2 where k = intrinsic permeability coefficient (m ) η = dynamic viscosity of the fluid (N.s/m )2 ρ = density of the fluid (kg/m ) 2 g = gravitational acceleration (m/s ) Diffusivity: Movement of a species in a material due to concentration gradient or difference in concentration of the diffusing species between two points within a material. Diffusivity is important because chlorides, carbon dioxide and oxygen enter concrete through diffusion. Both carbon dioxide and chlorides from deicing salts and ocean sprays are harmful to reinforced concrete because they lead to corrosion of steel reinforcement. The rate at which chlorides can diffuse into concrete will determine the time length for the initiation of corrosion of reinforcement. Diffusion is governed by Fick’s law: c 2C D dt 2x The solution of this equation can be found using calculus and is x C x,t C o 1 erf 2 Dt erf = error function (see table on the next page) dC/dx = concentration gradient D = diffusion coefficient dC/dt = rate of diffusion t = time (sec) C (x,t) = diffusing species concentration at depth x from the surface at time t 2 w/c D (for chloride ion diff-12on in cement paste) m /s 0.4 2.60x10 0.5 4.47x10 -12 0.6 12.35x10 -12 The addition of a pozzolan or blast furnace slag produces more C-S-H (calcium silicatehydrate) and more discontinuous pore structure and tends to reduce diffusion rate, which impedes the initiation of corrosion. Pozzolans and slag will be discussed later. - Diffusion occurs through atomic (ionic) diffusion. Diffusing species may be solid (Cl ) or gaseous (CO ,2O )2 2 Values of erf z for given z values to be used in the diffusion equation solution 3 Example: Calculate the chloride concentration after 20 years of constant exposure at 25 mm below the surface of the slab assuming w/c 0.5 for the concrete and a constant 3 surface concentration of 1.5 kg/m of concrete. Solution: x C x,t C o 1 erf 2 Dt 0.025 C 0.025,630.72x10 6 1.5 1 erf 2 4.47x10 12 x630.72x10 6 1.5 1 erf (0.235) 1.5 [1 0.2603] 1.1096 1.11 kg / m 3 Note that in the above equation time is measured in second and depth in meter. So x = 25 mm =0.025 m and t = 20 years = 20 x365 days/year x 24 hrs/day x 3600 sec/hr = 630.72 x 10 sec. The erf of 0.235 is found from the table of error function on the previous page and was obtained by linearly interpolating between z =0.20 and z=0.24. It is reported in the literature [Mehta and Monteiro (1993), Concrete: Structures, nd Properties and Materials, 2 Edition, Prentice Hall, p. 548) that chloride concentration in the range of of 0.6 to 0.9 kg per cubic meter of concrete can initiate corrosion. Other criteria specify the concentration in terms of the ratio of the chloride to hydroxyl ions (Cl - /OH ) in the concrete pore solution. If this ratio exceeds 0.6, the steel may become vulnerable to corrosion. Carbonation Carbon dioxide in the air is absorbed into the concrete, which reacts with the concrete pore solution and lowers the pH of concrete. Concrete ordinarily has pH of 12.5 and is highly alkaline in nature, but carbonation can reduce its pH to as low as 8.5 and the reduction in concrete alkalinity provides a more favourable environment for the onset and propagation of steel reinforcement corrosion. Steel tends to corrode more in less alkaline environments. The chemical reaction responsible for carbonation is as follows: Ca(OH ) 2 CO 2 CaCO 3 H 2 2NaOH CO 2 NaCO 3 H 2 The calcium carbonate CaCO and3sodium carbonate NaCO precipit3te inside the pores of the cement, thereby diminishing its permeability but also reducing the pH level of the pore solution. Alkalinity is reduced because the above reactions remove the hydroxide - ions (OH ) from the concrete pore solution. 4 From basic chemistry the product of the concentration of the hydrogen ion [H ] and + hydroxide ion [OH ] in a solution at 25 C is constant given by 14 kw H OH 1.0x10 - + Hence reduction in [OH ] leads to increase in [H ] and since pH is defined as pH log H + For pH =12.5, the [H ] concentration is 12.5 = -log [H ] Find the antilog of the above, to obtain + -13 [H ] = 3.16 x 10 M (M = moles) Note, one mole of hydrogen = 1.008 grams If pH = 8.5, then the [H ] concentration is given by 8.5 = -log [H ] which gives + -9 [H ] = 3.16 x 10 M Hence there is a 10,000 times increase in hydrogen ion concentration. Note that this is still an alkaline solution, but is less alkaline than it was originally. The depth of penetration of CO in2o concrete as function of time can be approximated using the following equation: d D c t where d= depth of penetration measured from the co0.5ete exposed surface (mm) D c carbonation coefficient in mm/year t = time of exposure in years D cepends on the quality and composition of concrete. Concretes with a dense microstructure with low w/c ratio and incorporating silica fume and certain other admixture will be more resistant to carbonation. This equation does not give the concentration of carbon dioxide in the concrete rather it simply gives the depth of carbonation. For example, if we let D = 3 mm/ year , then after 15 years, the c carbonation will have occurred up to a depth of 5 d 3x 15 11.6mm Alternatively, if the cover to the reinforcement is 25 mm, let us determine the number of years tit will take for the carbonation front to reach the reinforcement t ( d )2 (25 )2 69.44 years D 3 c So by providing adequate and good quality concrete cover, incidence of steel corrosion initiated by carbonation can be minimized. Carbonation occurs the most, or with higher rate, in environments with relative humidity in the range of 50-70%. Thus concrete in relatively dry or wet environments is less prone to carbonation. Alkali-Aggregate Reaction (ASR) Certain siliceous aggregates react with the alkali (sodium and potassium oxides) in the concrete in the presence of water. The reaction causes the formation of a gel which causes swelling of the aggregates and which in turn produce internal stresses and cracking. This is very harmful to concrete and therefore the selection of non-reactive aggregates is an important aspect of concrete production. Today strict standards exist for the selection of the appropriate aggregates in concrete
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