International Journal of Advance Engineering and Research Development

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1 Scientific Journal of Impact Factor (SJIF): 4.72 e-issn (O): p-issn (P): International Journal of Advance Engineering and Research Development Volume 4, Issue 11, November -17 SERVICE LIFE DESIGN OF REINFORCED CONCRETE STRUCTURES IN JODHPUR CITY OF RAJASTHAN 1 Ms.Rachana Sharma, 2 Dr. Archana Bohra Gupta 1 M.E. (Structural Engineering), ** Assistant Professor, Department of Civil Engineering,Surendera Group of Institutions,Sri Ganganagr Rajathan. Rajasthan Technical University,Kota,Rajasthan,India(335001). 2 Ph.D. (Structural Engineering), Associate Professor, Department of Structural Engineering, M.B.M. Engineering College, Jai Narain Vyas University, Jodhpur, Rajasthan, India (3411). Abstract Today the reinforced concrete structures need to be designed for a very high service life. Usually this seems impossible owing to their deterioration due to durability problems. Thus a methodology to achieve the desired life is the need of the hour. This study proposes the application of service life model Life-365 TM for designing concrete construction exposed to chloride laden environment in service for Jodhpur District of Rajasthan state in India. The critical parameters for estimation of service life are identified and their role in designing service life is evaluated. The parametric study provides a guideline for achieving the targeted service life for beam / column elements of RC structures. I. INTRODUCTION Jodhpur is located in western Rajasthan between 26 0 and North latitude and and East longitude. The climate of Jodhpur features extremes of temperature, uncertain rainfall and dryness, i.e. hot and arid climate. The average normal annual rainfall is mm. there is appreciable variation between maximum and minimum temperatures (about ) observed during the year. The ground water in Jodhpur district has varied nature of chemical quality which is influenced to great extent by regional geomorphic and hydro geological features. The salinity of ground water varies from less than 500 µs/cm to more than 10,000 µs/cm when measured in terms of electrical conductivity (EC). Nearly 37% of water sources fall in the salinity level of moderately saline (EC between 00 to 8000 µs/cm) to saline class (EC > 8000 µs/cm). Nearly 23.4 % area in district falls in saline class. Remaining area is in upto moderate salinity. Sodium occurs as major cation in 73.9% water samples and its concentration varies from 5 mg/l to 8700 mg/l. it is mainly associated with chloride (63.3%) along with mixed cations (27.5%) and bicarbonate (9.1%) among anionic species ( The high chloride content in the ground water which is used for all civil engineering construction work is a major cause of corrosion leading to early deterioration of RC structures in this area. SERVICE LIFE As per Eurocode 1, the service life for a structure can be defined as: A structure shall be designed and executed in such a way that it will, during its intended life with appropriate degrees of reliability and in an economic way: - remain fit for the use which it is required; and - sustain all actions and influences likely to occur during execution and use. Weyers (1998) gave that the end of service life is the time of the occurrence of first cracking. Amey et al. (1998) said that when the percentage of reinforcement subjected to corrosion exceeds a certain threshold value, it shows the end of the service life. Martín-Péreza B., Lounis Z., (03) said that the service life is defined as the time until damage accumulation reaches an unacceptable level or limit state, i.e., it is the time when the failure probability reaches an unacceptable level (which depends on the type of structure and failure mode). Lounis Z., Amleh L.,04 said that service life can also be defined as the time at which any of the following limit states are reached: onset of corrosion, cracking, delamination, spalling, or accumulated damage reaching a specified limiting value. An appropriate definition of failure, and consequently service life should consider the acceptable risk of failure, which depends on the risk of loss of life, structure type, failure mode etc. Song H. W., et.al. (07) defined the service life for their study as the time for the corrosion initiation, i.e., when a chloride concentration reaches the critical threshold value (1.2kg/m 3 of total chloride content) at surface of rebar. They also suggested that the time interval from the initiation of corrosion to corrosion cracking in concrete cover is the cracking time of structures. Service life (t s ) of reinforced concrete is also defined as the sum of the time to initiation of corrosion (t i ) and the propogation time (t p ) required for corroding steel to cause sufficient damage to require repair, hence also called time All rights Reserved 1342

2 first repair (t r ) (Ehlen M.A., Thomas M.D.A., Bentz E.C (09)). This is based on Tuutti s corrosion model (Tuutti 1982). II. OBJECTIVES To calculate the service life of beam/column elements of structures existing in upto moderately saline environments subject to deterioration due to corrosion because of chloride ingress. To study the variation of service life with various parameters affecting it viz increase in clear cover, increase in section of beam/column, providing membranes, reducing efficiency of membranes provided, providing sealers and reapplying sealers during the life of member. There are a number of factors coupled together to effect the life of any structure and it is difficult to isolate the effect of each on the structure. Thereby a single parameter is chosen to calculate the service life of these structures which can give a calculation as to when the problem first starts in a structure, when is it required to adopt a repair schedule for the structure. Chloride diffusion is taken as the governing parameter which is responsible for the corrosion of rebars, leading to the deterioration of reinforced concrete structures. The process is more adaptive for new structures and can be used appropriately for the existing structures. A software Life-365 Service Life Prediction Model is used to calculate the service life. The process used is not specific and is similar for any reinforced concrete structure. Square beam column element is considered for the analysis. 21 cases are taken to study the variation of service life of RC structures in the area chosen. Cases I to IV use a square beam/column of size 300 mm square and no barriers. Clear cover is varied as 30mm, mm, 50 mm and 54.5 mm respectively from case I to case IV. Cases V to VIII consider membranes with 100% efficiency and age of failure as years for a 300 mm square section. Clear cover is varied as 30mm, mm, 50 mm and 54.5 mm respectively from case V to case VIII. Cases IX to XII consider membranes with varying efficiency as 80%, %, % and % respectively for a 300 mm square section, clear cover mm and age of failure of membranes as years. Cases XIII to XVI include sealers, then sealer reapplied once, then sealer reapplied twice and then sealer reapplied thrice respectively with efficiency 90% and age of failure 5 years for a 300 mm square section and clear cover mm. Cases XVII to XXI are for studying the effect of increasing size of square beam/column as 250mm, 0mm, 0mm, 700mm and 500mm respectively with clear cover mm and no barriers provided. Length of beam/column is taken as 5m and percentage of reinforcement provided in concrete is taken as 1% for all cases. III. APPLICATION OF LIFE 365 Life-365 analysis includes the calculation of the initiation period, (t i ), taking the propagation period, (t p ) constant and calculating the service life t s as the time for first repair t r = t i + t p. Life-365 assumes diffusion to be the dominant mechanism for chloride ingress and uses a simplified approach based on Fick s second law of diffusion: dc = D. d2 C (1) dt dx 2 where, C = the chloride content, D = the apparent diffusion coefficient, x = the depth from the exposed surface, and t = time. The chloride diffusion coefficient is a function of both time and temperature, and Life-365 uses the following relationship to account for time and temperature dependent changes in diffusion: t ref m D(t) = D ref (2) t where D(t) = diffusion coefficient at time t, D ref = diffusion coefficient at time, t ref (= 28 days in Life-365) and m = diffusion decay index, a constant. In order to prevent the diffusion coefficient decreasing with time indefinitely, the relationship shown in Eq. 3.2 is assumed to be valid only up to 25 years hydration period, beyond which D(t) stays constant at the D(25 years) value, based on data from the University of Toronto and other published data. Life-365 uses the following relationship to account for temperature-dependent changes in diffusion: D T = D ref. exp U. 1 1 (3) R T ref T where D(T) = diffusion coefficient at time t and temperature All rights Reserved 1343

3 D ref = diffusion coefficient at time t ref and temperature T ref, U = activation energy of the diffusion process (35000 J/mol), R = gas constant, T = absolute temperature. In literature these equations are available in combined form, as given by (Martín-Péreza B., Lounis Z. (03) D t, T = D ref. t ref t m. exp U R. 1 T ref 1 T (4) In the model, t ref = 28 days and T ref = 293K ( C). The solution for time for initiation of corrosion is carried out using a finite difference implementation of Eq. 3.1 where the value of D is modified at every time step using Eq. 3.2 and Eq IV. ESTIMATION OF SERVICE LIFE PARAMETERS LIFE 365 provides two alternatives for each service life parameter. The user can use the default curves and values for the selected location or else input his/her own profile and values TEMPERATURE PROFILE The software requires the annual temperature cycle to which the structure is exposed. These temperatures are part of the service life calculations that determine the effects of temperature on concrete diffusivity. Monthly mean maximum & minimum temperature based upon data collected between data for Jodhpur is given in table 4.1 ( The mean monthly temperatures are calculated and shown in the last column of the Table 4.1. The temperature profile is generated on the basis of Table 1 and shown in Figure 1a. Table 4.1: Monthly temperature data for Jodhpur for 1982 to 12 Station Name Month Mean Temperature o C Mean monthly Temperature o C Maximum Minimum Jodhpur January Jodhpur February Jodhpur March Jodhpur April Jodhpur May Jodhpur June Jodhpur July Jodhpur August Jodhpur September Jodhpur October Jodhpur November Jodhpur December Figure 1a: Temperature profile for Jodhpur All rights Reserved 1344

4 DIFFUSION DECAY INDEX (m) This dimensionless property describes the time-dependent changes in the diffusion coefficient due to the continued hydration of the concrete. D(t) = D ref t ref t m (5) Takewaka and Mastumoto (1988) and Maage, et al. (1993) calculated the diffusion coefficient as: D a = D aex t ex t α (6) Where, D aex is the apparent diffusion coefficient at the time of first exposure, t ex is the time of first exposure and α is an aging factor. For ordinary concrete with 0.25 w/c < 0., α can be calculated from Poulsen s (Hetek, 1997) expression: α = m = e w /c e w /c (7) The aging factor α used by Poulsen is same as the aging factor m used by Life 365 model. INITIAL DIFFUSION COEFFICIENT D 28 - LIFE-365 TM uses the following equation based on the results from Bulk Diffusion Test (NordTest NTBuild 443) performed at the University of Toronto and some other published data from the same type of test, to determine D 28 for the mixtures that does not contain silica fume: D 28 = w/c (8) MAX SURFACE CHLORIDE CONCENTRATION C s - The rate of buildup and maximum level of external chloride concentrations affect the rate of chloride ingress and ultimately concrete service life. The maximum level of chloride buildup that the concrete structure will experience over its lifetime, C s measured either in % wt. conc. or base unit-specific units, i.e., either kg/m 3. Time to build to max t max. is the number of years for the chloride buildup to reach its maximum level. It is assumed that the buildup is zero at the beginning of the structure s life and it increases linearly upto a maximum level C s beyond which it remains constant. They depend upon the type of structure, its geographic location, and exposure. The chloride profile for Jodhpur has been generated using LIFE365 TM from data taken from a Ph.D. thesis (Gupta A.B. (13)). The chloride profile is shown in figure 1b. Figure 1b: Chloride Profile for R.C. Structures for Jodhpur City Maximum surface chloride concentration 0.33% by weight of concrete Time to build maximum surface chloride concentration 36 years CHLORIDE THRESHOLD C t - The chloride threshold, C t, is the mass of total chloride per unit volume of concrete that results in permanent depassivation of the steel. This is the concentration of chloride required to initiate corrosion of the embedded steel reinforcement, measured in kg/m 3. The value of C t is selected by the software on the basis of the type and quantity of corrosion inhibitor, and the nature of the reinforcement or inputted. For the black steel, this value is 0.05 % by weight of cement. PROPAGATION PERIOD t p - This is the time taken (in years) for the corrosion process to cause sufficient damage to warrant repair. The calculation of service life is done for the various cases. The results of Cases I to IV are shown in Table All rights Reserved 1345

5 Table- 2: Service Life Calculation for Case I- Case IV Case I Without membrane & Cover 30 mm Case II Without membrane & Cover mm Case III Without membrane & Cover 50 mm Case IV Without membrane & Cover 54.5 mm Service Life Parameters w/c D 28 (m 2 /sec) m t s (years) E E E E E E E E E E E E E E E E E E E E E RESULTS AND DISCUSSIONS Variation of reference diffusion coefficient: The value of reference diffusion coefficient D 28 reduces with the decrease in water cement ratio (Table 2). The reduction in water cement ratio makes the concrete less porous hence the diffusion will reduce. Variation of diffusion decay index m: The diffusion decay index or the ageing factor m increases with the decrease in the water cement ratio (Table 2). The index shows that rate of diffusion reduces with time due to hydration of concrete. This is because the pore size will reduce with time hence diffusion and rate of diffusion will reduce thus increasing the diffusion decay index. Effect of change in w/c: Studying the variation of service life with increase in water cement ratio from 0.3 to 0.5 for a 300mm x 300 mm square beam/column having 30 mm clear cover, it is found that the service life increases from 16.4 years to 41.9 years with the reduction in the w/c from 0.5 to 0.3 respectively. There is an increase of 53.7% in service life on reducing the w/c from 0.5 to 0.4. There is a further increase in service life of 66.3% on reducing the w/c from 0.4 to 0.3. The reduction in w/c reduces the porosity of concrete and further reduces pore sizes with time, thus reduces the reference diffusion coefficient while increases the diffusion decay index m which cause the increase in service life. The increase in increase of service life is more for lower w/c. Further studying the variation of service life with increase in water cement ratio from 0.3 to 0.5 for a 300 mm x 300 mm square beam/column having mm clear cover. The service life increases from 18.8 years to 58.2 years with the reduction in the w/c from 0.5 to 0.3 respectively. There is a total increase of 9.6% in service life on reducing the w/c from 0.5 to 0.3. The increase in increase of service life is more for lower w/c. Also the service lives are more for Case II as compared to Case I. Studying the variation of service life with increase in water cement ratio from 0.3 to 0.5 for a 300mm x 300 mm square beam/column having 50 mm clear cover, the service life increases from 22 years to 81.2 years with the reduction in the w/c from 0.5 to 0.3 respectively. There is a total increase of 269.1% in service life on reducing the w/c from 0.5 to 0.3. Also the service lives are more for Case III as compared to Case II and even more than in Case I. The increase in increase of service life is increasing on increase of clear All rights Reserved 1346

6 Service Life (Years) for CasesV-VIII Service Life (years) International Journal of Advance Engineering and Research Development (IJAERD) Observing the variation of service life with increase in water cement ratio from 0.3 to 0.5 for a 300mm x 300 mm square beam/column having 54.5 mm clear cover, it is found that service life increases from 23.6 years to 92.7 years with the reduction in the w/c from 0.5 to 0.3 respectively. There is a total increase of 292.8% in service life on reducing the w/c from 0.5 to 0.3. Also the service lives are more for Case IV as compared to Case I, II and III. The increase in increase of service life is increasing on increase of clear cover. The maximum value of cover is taken as 54.5 mm since it is the maximum allowed value by the software. Thus providing a clear cover greater than this value would not be considered for service life determination since the increase in self weight due to such a thick cover makes this value unsuitable. Effect of change in clear cover: There is a tremendous increase in service life values by just increasing the values of clear cover. There is an increase of 43.9% in service life by just increasing the cover from 30 mm to 54.5mm for w/c of 0.5 while there is an increase of % in service life by increasing the cover from 30 mm to 54.5mm for w/c of 0.3. Figure 2 shows the variation of service life with increase in cover for w/c of 0.5 (Series 1), 0.4 (Series 2), 0.3 (Series 3) respectively. The curves show that the increase of service life increases with increasing cover values. Increases in cover increases the path of chloride diffusion thus increasing the number of years for corrosion to initiate and thus increases service life. Increase in cover does not affect the corrosion propagation time Clear cover (mm) Series1 Series2 Series3 Effect of providing protective membranes: Figure 3 shows the variation of service life with membrane provided and for various cover thicknesses. There is increase in service life values on providing protective membranes though the trend of increase for different cover values is similar as when membranes weren t provided. There is an increase of 63.4% in service life for w/c ratio of 0.5 and cover 30 mm when membrane has been provided in comparison to the value when it wasn t provided. The increase reduces to 31.7% for w/c of 0.3 and cover 30 mm. This shows that the effect of providing protective membranes is more for higher w/c ratios. The increase is 58% for w/c ratio of 0.5 and cover mm which reduces to 24.1% for w/c of 0.3 and cover mm. The increase is 51.8% for w/c of 0.5 and cover 50 mm which reduces to 11.1% for w/c of 0.3 and cover 50 mm. The increase is 48.7% for w/c of 0.5 and cover 54.4 mm which reduces to 11% for w/c of 0.3 and cover 54.5 mm. The increase in service life on providing membranes is reducing with increase in cover thickness Cover30 mm Cover mm Cover 50 mm Cover 54.5mm Water Cement Ratio Figure 3: Graph between Water Cement Ratio and Service life (t s ) for Case All rights Reserved 1347

7 Service Life (Yrs) for cases XIII- XVI Service Life (Years) for Cases VI, IX- XII International Journal of Advance Engineering and Research Development (IJAERD) Effect of variation in membrane efficiency: Figure 4 shows the variation of service life with varying degrees of membrane efficiencies. Membrane efficiency is decreasing from 100% to % with decrement of % each time. With the decreasing membrane efficiency, service life reduces every time since full protection cannot be ensured with less efficiency of membrane but even with % efficiency, service life increases by minimum 1.6 years for the maximum w/c of 0.5. For % efficiency, the maximum increase is 10 years for w/c of 0.3. There is an increase of 2-3 years for every % increase in membrane efficiency for any w/c M Effi 100% M Effi 80% M Effi % M Effi % M Effi % Water Cement Ratio Figure 4: Graph between Water Cement Ratio and Service life (t s ) for Case IX-XII Effect of reapplying sealants: Figure 5 shows the variation of service life with variation in the number of times the sealant is applied. Service lives have increased by 5 years for w/c of 0.5 to upto years for w/c of 0.25 on application of sealant once. There is an increase of 2.5 years on reapplying the sealant once. The increase is 4.5 years if sealant is reapplied twice and the increase is 6 years if the sealant is reapplied thrice. Thus, applying the sealant once is maximum beneficial Water Cement Ratio (w/c) Sealant applied once Sealant reapplied once Sealant reapplied twice Selant reapplied thrice Figure 5: Graph between Water Cement Ratio and Service life (t s ) for Case XIII-XVI Effect of change in sectional dimension: Figure 6 shows that there is hardly any variation in service life on increasing the dimension of the square beam/column. This is because service life mainly depends on the cover. The cover is same for all cases here i.e. mm. So there is negligible change (less than 1 year) in service life All rights Reserved 1348

8 Service Life (Yrs) for Cases XVII- XX, II, XXII International Journal of Advance Engineering and Research Development (IJAERD) mm section 0 mm section 0 mm section 700 mm section 300 mm section 500 mm section Water Cement Ratio Figure 6: Graph between Water Cement Ratio and Service life (t s ) for Case XVII-XX, XXII, II There is increase in service life values on providing membrane or sealant for both 300mm section and 500 mm section but the percentage increase is more for 500 mm section. However there is maximum difference of 5% in the percentage increase. The increase in service life for all cases is non-linear between w/c of 0.5 to 0.36 since service life depends on diffusion coefficient, time, temperature, diffusion decay index mainly. The software LIFE TM limits the value of diffusion decay index m at 0.6. So after that i.e. below w/c ratio of 0.36, the service life depends mainly on reference diffusion coefficient and since service life is directly proportional to reference diffusion coefficient, the curve is linear. The increase in service life has reduced below water cement ratio of This is because for water cement ratios below 0.36, the value of m calculated using Poulsen (Hetek) expression exceeds 0.6 as seen in the Tables 2; while the software takes the upper limit of m as 0.6. So below w/c of 0.36, the effect of increasing ageing factor on increase in service life has ceased. CONCLUSIONS Based on the study carried out, the following conclusions are drawn: Service life of reinforced concrete structures subjected to deterioration due to chloride ingress can be designed effectively using LIFE 365 TM. The value of reference diffusion coefficient D 28 reduces with the decrease in water cement ratio while the diffusion decay index or the ageing factor m increases linearly with the decrease in the water cement ratio. However a maximum limit of 0.6 has been adopted. The maximum value of m = 0.6 is obtained at w/c ratio of 0.36, so for w/c below 0.36, the service life depends mainly on reference diffusion coefficient and since service life is directly proportional to reference diffusion coefficient, its variation is linear. The increase in service life is non-linear between w/c of 0.5 to 0.36 since service life depends on diffusion coefficient, time, temperature, diffusion decay index mainly. The service life increases from 16.4 years to 41.9 years with the reduction in the w/c from 0.5 to 0.3 respectively for a 300mm square beam/column and having clear cover of 30 mm. The service life increases from 18.8 years to 58.2 years with the reduction in the w/c from 0.5 to 0.3 respectively for a 300mm square beam/column and having clear cover of mm. The increase in increase of service life is more for lower w/c. The increase in increase of service life is increasing on increase of clear cover. There is a tremendous increase in service life values by just increasing the values of clear cover. The increase is 43.9% by just increasing the cover from 30 mm to 54.5mm for w/c of 0.5 while the increase is % by increasing the cover from 30 mm to 54.5mm for w/c of 0.3. There is increase in service life values on providing protective membranes though the trend of increase for different cover values is similar as when membranes weren t provided. The maximum increase is 63.4% for w/c ratio of 0.5 and cover 30 All rights Reserved 1349

9 With the decreasing membrane efficiency, service life reduces every time. There is an increase of 2-3 years for every % increase in membrane efficiency for any w/c. Service lives have increased by 5 years for w/c of 0.5 to upto years for w/c of 0.25 on application of sealant once. There is further an increase of 2.5 years on reapplying the sealant once. Applying the sealant once is maximum beneficial. There is negligible variation (< 1 year) in service life on increasing the dimension of the square All rights Reserved 1350