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INTERNATIONAL Engineering and Technology ENGINEERING AND International Journal of Civil JOURNAL OF CIVIL(IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME TECHNOLOGY (IJCIET) ISSN 0976 – 6308 (Print) ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), pp. 404-415 IJCIET © IAEME: www.iaeme.com/ijciet.asp Journal Impact Factor (2012): 3.1861 (Calculated by GISI) www.jifactor.com © IAEME FATIGUE BEHAVIOR OF HIGH VOLUME FLY ASH CONCRETE UNDER CONSTANT AMPLITUDE AND COMPOUND LOADING Aravindkumar.B.Harwalkar1 and Dr.S.S.Awanti2 1 Associate Professor, Department of Civil Engineering, P.D.A.College of Engineering, Gulbarga, Karnataka State, India. e-mail: harwalkar_ab@yahoo.co.in 2 Professor and Head, Department of Civil Engineering, P.D.A.College of Engineering, Gulbarga, Karnataka State, India. e-mail: ssawanti@yahoo.co.in ABSTRACT Road projects in future have to be environmental friendly and cost effective apart from being safe so that society at large is benefited by the huge investments made in the infrastructure projects. To achieve this, component materials of the pavement system have to be optimized with reference to cost effectiveness, sustainability and fatigue behavior. This paper presents a study on fatigue behavior of high volume fly ash concrete (HVFAC) and conventional concrete (PCC) under constant amplitude fatigue loading. Also behavior of HVFAC was studied under compound fatigue loading. In the present investigation HVFAC mix with cement replacement level of 60% with low calcium fly ash has been used. A total number of 95 prism specimens of HVFAC were tested under constant amplitude fatigue loading. Also 100 prism specimens of PCC were tested under constant amplitude fatigue loading for comparative studies. All prism specimens were of size 75mm×100mm×500mm and were tested under flexural fatigue loading using haiver sine wave loading. Frequency of fatigue loading was kept at 4Hz. Lognormal model was verified for probability distribution of fatigue life. Studies indicated that lognormal model was acceptable for fatigue life distributions at all stress levels for both HVFAC and PCC. The parameters of distribution exhibited dependency on stress levels and type of concrete. Relations between stress level and fatigue life were developed for both HVFAC and PCC. These relations were found to be dependent on type of concrete. A total number of 24 prism specimens were tested under compound fatigue loading. Based on the results of compound fatigue loading the validity of Miner’s hypothesis for high volume fly ash concrete was verified. It was found that Miner’s hypothesis gives both unsafe and over safe predictions of failure. Miner’s sum was found to be dependent on type of compound loading and sequence of loading. 404 International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME Keywords: Compound fatigue loading; High volume fly ash concrete; Probability distribution; lognormal. 1. INTRODUCTION Fatigue strength is an important property which has to be taken into account in the design of various concrete structures requiring long fatigue life. Especially the understanding of the behavior of a concrete road under fatigue loading is vital for the design and the performance prediction. Also there is a need for optimization of materials in the rigid pavement system with regard to long term fatigue resistance at lowest cost and ecologically sound choices. Many researchers have carried out studies on developing fatigue models for plain concrete. Majority of the researchers [1-3] have developed the fatigue model relating the stress level (S) which is defined as the ratio of maximum stress applied in cyclic loading to static flexural strength, to number load cycles to failure (N), termed as fatigue life. This relation is commonly called as Wholer equation. The second form of fatigue model given by Vesic et al [4] and Treybig et al [5] is a power equation relating S and N. Jakobsen et al [6] included the effect of ratio of minimum stress to maximum stress in cyclic loading, which is known as stress range (R), in the S-N relation for fatigue. Hsu [7] developed a more general expression for fatigue strength involving four variables i.e., S, N, R and period of cyclic loading (T). But the most commonly used fatigue model for design of concrete pavements is the one given by Wholer equation. In literature [8-9] variable amplitude fatigue studies have been carried out on plain concrete to verify the validity of Miner’s hypothesis. Miner’s hypothesis assumes that damage accumulates linearly with the number of cycles applied at a particular stress level. As per Miner’s hypothesis the failure criterion is written as: ----------- ----- (1) Where ni = number of cycles applied at stress level i Ni = number of cycles to failure at stress level i k = number of stress levels used Studies carried out by Siemes (8) on plain concrete proved the validity of Miner’s rule. But the studies carried out by Holmen (9) found variable amplitude loading to be more damaging than that predicted by Miner’s hypothesis. As per the definition given by Mehta [10], a concrete having minimum cement replacement level of 50% by fly ash is termed as high volume fly ash concrete (HVFAC). Limited studies [11-13] are available on fatigue behavior of HVFAC. Ramkrishnan et al [13] have developed an S-N relation for HVFAC with cement replacement level of 58% using third point flexural fatigue loading at a frequency of 20Hz. In the development of S-N model it has been assumed that the non dimensional term ‘S’ eliminates the influence of static ultimate strength of concrete and hence eliminates the effect of water-cement ratio, type and gradation of aggregate, type and amount of cement, age of concrete. But there are concerns over influence of static strength of concrete on S-N relation due to variation in fracture toughness. There is also very limited literature available on fatigue behavior of HVFAC under compound and variable amplitude fatigue loading. 405 International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME 2. RESEARCH SIGNIFICANCE AND SCOPE In the present investigation an attempt has been made to study the fatigue behavior of HVFAC under constant amplitude and compound fatigue loading. Multistage constant amplitude loading has been used as compound fatigue loading in the present investigation. The fatigue test results of HVFAC were compared with that of reference concrete (PCC). To investigate the fatigue behavior a series of prism specimens of size 75mm×100mm×500mm were tested under flexural fatigue loading. In the present investigation HVFAC mix satisfying the criteria of pavement quality concrete was developed using a cement replacement level of 60% with low calcium fly ash. A total number of 100 PCC prism specimens were tested under constant amplitude fatigue loading. For HVFAC, 95 prism specimens were tested under constant amplitude fatigue loading. Probability distributions were developed for experimental results of fatigue lives. S-N relations were established from regression analysis of fatigue data. A total number of 24 specimens of HVFAC have been tested under compound fatigue loading to verify the validity of Miner’s hypothesis. 3. LABORATORY TESTS 3.1 Materials The ordinary Portland cement from single batch has been used in the present investigation. The coarse fraction consisted of equal fractions of crushed stones of maximum size 20mm and 12mm. Low calcium fly ash satisfying the criteria of fineness, lime reactivity and compressive strength requirements [14] has been used in the investigation. Fine aggregate used was natural sand with maximum particle size of 4.75mm. Polycarboxylic based superplasticizer has been used as high range water reducing admixture (HWRA) to get the desired workability. The optimum dosage of superplasticizer for each type of concrete was fixed by carrying out compaction factor test. 3.2 Mixture Proportions A minimum grade of M30 which results in a minimum static flexural strength of 3.8N/mm2 has been specified for pavement quality concrete by Indian Roads Congress [15]. Trial mixes were developed to achieve M35 grade HVFAC at cement replacement of 60%, which was the optimum replacement percentage with water to cementitious ratio of 0.3. Water to cementitious ratio utilized in the investigation i.e., 0.3 was the lowest value that could be used from the limitation of reduction in water content that can be achieved using HWRA and usage of conventional means of mixing and compaction. Corresponding conventional concrete was used as reference concrete (PCC). Mixture proportions of the two types of concrete are shown in table 1. Table 1 Mixture Proportions of Concrete Mixture PCC HVFAC Components Cement (OPC 53 grade) in kg/m3 440 176 Class F fly ash in kg/m3 0 264 Water in kg/m3 132 132 Superplasticizer in liter/m3 15.4 3.5 3 Saturated surface dry sand in kg/m 937.6 858.2 Saturated surface dry coarse aggregate 1059 1059 in kg/m3 406 International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME 3.3 Test Procedure and Test Results 3.3.1 Static Testing Cube specimens of size 150mm×150mm×150mm were used for determining compressive strength. For static flexural strength, specimens of similar size to that of fatigue specimens have been used. An effective span of 400mm has been used for both static flexural strength and fatigue strength determination. All the strength properties were determined after a curing period of 28days. Static compressive strength and flexural strength values are shown in table 2. Table 2 Mechanical Properties of Concrete Property of concrete/ 28 day compressive strength 28 day static flexural strength Type of concrete in MPa in MPa Conventional concrete 62.3* 6.9* HVFAC60 40.8* 5.3* * Mean value of six specimens 3.3.2 Fatigue Testing 3.3.2.1 Constant Amplitude Fatigue Testing: Fatigue test specimens were tested under one-third point loading using frequency of loading as 4Hz. Since the present investigation was aimed at pavement application haiver sine wave form of cyclic loading was used. Typical fatigue test set up and loading pattern used are shown in figures 1 and 2 respectively. All the fatigue specimens were tested after 90 days from casting so as to give allowance for sufficient strength gain. Specimens were cured for 28 days by ponding method and then covered with polythene bags up to 90 days. Minimum stress in fatigue loading was maintained at 1% of maximum stress. Minimum stress was used mainly to prevent any possible movement of specimens at support during testing and to simulate residual stresses in the pavement to a certain degree. Beyond the upper limits of stress levels used for different types of concrete in the present investigation, the fatigue life values were insignificant to be recorded i.e., they were typically less than 10. For HVFAC at all cement replacement levels the lower limit of stress level used was based on the criteria, when none of the test specimens failed even after of application of one lakh cycles of fatigue loading. PCC was tested for eight stress levels and HVFAC was tested at seven stress levels. Constant amplitude Fatigue test results for PCC and HVFAC are tabulated in table 3 and 4 respectively. Fatigue life values have been arranged in the increasing order so as to facilitate probability analysis. Figure 1 Flexural fatigue test setup Figure 2. Typical constsant amplitude fatigue loading 407 International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME Table 3 Fatigue Life of PCC at Different Stress Levels Test S=0.85 S=0.81 S=0.76 S=0.71 S=0.65 S=0.61 S=0.57 S=0.53 specimen no. 1 22 84 158 1327 5289 16488 46582 100000* 2 43 97 284 1489 7213 20312 48270 100000* 3 69 105 312 2596 8863 22268 52164 100000* 4 78 152 382 3642 10322 34511 54416 100000* 5 82 184 411 4149 12723 39920 56005 100000* 6 94 198 474 5218 16523 46718 66012 100000* 7 102 288 578 6629 18708 51512 73676 100000* 8 110 432 694 8383 20391 61512 80520 100000* 9 122 682 916 9558 21262 77812 81891 100000* 10 138 730 1182 12009 23992 81800 100000* 100000* 11 ---- ---- ---- ---- 24771 92477 100000* 100000* 12 ---- ---- ---- ---- 27344 100000* 100000* 100000* 13 ---- ---- ---- ---- 32811 100000* 100000* 100000* 14 ---- ---- ---- ---- 40887 100000* 100000* 100000* 15 ---- ---- ---- ---- 44816 100000* 100000* 100000* *specimen did not fail after the application of given number of cycles of loading -- data not available Table 4 Fatigue Life of HVFAC at Different Stress Levels Test S=0.80 S=0.75 S=0.70 S=0.65 S=0.60 S=0.54 S=0.50 specimen no. 1 44 78 312 4159 5324 18785 100000* 2 48 102 422 5802 6852 19084 100000* 3 52 146 584 6802 7102 21039 100000* 4 65 182 886 7759 8404 22259 100000* 5 72 212 1092 8759 12723 29384 100000* 6 88 292 1109 9259 14785 32911 100000* 7 92 344 1243 10014 15680 45512 100000* 8 99 459 1422 12008 22348 62214 100000* 9 112 582 1586 14620 28109 68743 100000* 10 120 889 1704 14882 36891 76544 100000* 11 ---- ---- 1959 16822 45841 82477 100000* 12 ---- ---- 2390 16822 49869 86792 100000* 13 ---- ---- 3532 18826 52113 100000* 100000* 14 ---- ---- 4426 23426 59641 100000* 100000* 15 ---- ---- 3962 28110 65869 100000* 100000* * specimen did not fail after the application of given number of cycles of loading -- data not available 408 International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME 3.3.2.2. Compound Fatigue Testing: Compound fatigue testing was carried out on HVFAC specimens only. Two stage, three stage and four stage constant amplitude fatigue loadings have been used as compound fatigue loading. In two stage loading test specimen was subjected to a fixed number of load cycles at a particular stress level in the first stage and after the first stage amplitude was changed corresponding to second stress level and maintained constant up to failure. In three stage fatigue loading three stress levels have been applied to the test specimen. Fixed numbers of load cycles were applied for two stress levels and testing was continued up to failure at the third stress level. In four stage loading fixed numbers of load cycles were applied for three stress levels and at fourth stress level specimen was tested up to failure. Minimum stress was maintained at 1% of the corresponding maximum stress for all the specimens. Test results of compound fatigue loading were used to check the validity of Miners hypothesis for HVFAC. 4. PROBABILITY ANALYSIS OF CONSTANT APLITUDE FATIGUE TEST RESULTS Since the fatigue lives for both types of concrete showed larger scatter, an attempt to determine the probabilistic distributions was made. Few researchers [16-17] have developed Weibull distribution models for fatigue lives at different stress levels in case of conventional concrete. In the present study lognormal distribution models were developed and verified for different stress levels. Conservatively for few specimens which did not fail after the application of one lakh cycles of loading at some of the stress levels fatigue life value has been taken as one lakh cycles in the probability analysis. 4.1 Determination of Lognormal Distribution Model The probability density function of lognormal distribution model is given by equation (2). The parameters of lognormal distributions are µ and σ which are mean and standard deviation of observed ln (N) values. In the equation (2), ‘X’ represents ln(N) values. ………… (2) The values lognormal distribution parameters for all the types of concretes and at different stress levels are shown in table 5. It can be seen that the parameters of lognormal distribution are dependent on type of concrete and the stress level. Table 5 Lognormal Distribution Parameters for Fatigue Lives at Different Stress Levels Type of concrete Stress level Parameters of log normal distribution µ σ PCC 0.85 4.3450 0.5501 0.81 5.4036 0.7867 0.76 6.1377 0.5925 0.71 8.3841 0.7565 0.65 9.7882 0.6293 0.61 10.8915 0.6321 0.57 11.2150 0.3007 HVFAC 0.80 4.3158 0.3599 0.75 5.5329 0.7742 0.70 7.2237 0.7795 0.65 9.3603 0.5369 0.60 9.9538 0.8700 0.54 10.7820 0.6580 409 International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME 4.2 Model Verification Probabilistic models developed in the present investigation were tested using Kolmogorov-Smirnov test. For conducting this test, the test statistic D2 was calculated using equation (3) in which FO (Nj) is the observed distribution of N and FN (Nj) is the hypothesized distribution of N and m is the total number of specimens. …………….. (3) The D2 values were compared with critical D2 for the given sample size and significance level of 5%. If calculated value is less than critical D2, model is accepted. The basic calculations for verification of lognormal model for PCC at stress level of 0.85 are shown in table 6. The D2 values and verification of lognormal distributions for both types of concretes at different stress levels are shown in table 7. It can be seen that lognormal model was accepted for both types of concretes at all stress levels. Table 6 Kolmogorov-Smirnov Test for Lognormal Distribution for PCC at Stress Level of 0.85 Nj j FO(Nj) FN(Nj) D2 for Maximum D2 for 5% Inference = j/m from lognormal D2 from significanc lognormal distribution= lognormal e level and distribution | FO (Nj)- distribution m=10 FN(Nj)| 22 1 0.1 0.0113 0.0887 43 2 0.2 0.1443 0.0557 69 3 0.3 0.4201 0.1201 Lognormal 78 4 0.4 0.5085 0.1085 model for 82 5 0.5 0.5447 0.0447 0.1449 0.41 fatigue life 94 6 0.6 0.6408 0.0408 distribution is 102 7 0.7 0.6946 0.0054 accepted 110 8 0.8 0.7409 0.0591 122 9 0.9 0.7980 0.1020 138 10 1 0.8551 0.1449 Table 7 Kolmogorov-Smirnov Test for Lognormal Distribution at Different Stress Levels Type of Stress level Maximum D2 D2 for 5% Inference concrete from lognormal significance distribution level PCC 0.81 0.1583 0.41 0.76 0.0781 0.41 Lognormal 0.71 0.1230 0.41 models for 0.65 0.0739 0.34 fatigue life 0.61 0.1628 0.34 distributions 0.57 0.1609 0.34 are accepted HVFAC 0.80 0.1445 0.41 in all the 0.75 0.0901 0.41 cases 0.70 0.0868 0.34 0.65 0.0757 0.34 0.60 0.1208 0.34 0.54 0.1462 0.34 410 International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME 5. DETERMINATION OF S-N RELATION S-N relations were developed by carrying out regression analysis on fatigue test data of constant amplitude loading. The S-N curves determined for the two types of concretes are shown in figure 3. S-N relations for PCC and HVFAC are shown in equations (4) and (5) respectively along with R2 values where R is the coefficient of correlation. It can be seen that S-N relations are dependent on type of concrete i.e., on the strength of concrete. In the present investigation existence of upper limits of stress levels for fatigue loading, dependent on type of concrete, was observed. The 95% confidence limits using constant variance were determined for both PCC and HVFAC. Upper and lower confidence limits along with S-N curve for PCC and HVFAC are shown in figures 4 and 5 respectively. Lower confidence limits are important in design of structures. S = -0.0358Ln(N) + 0.9948 (R2=0.9332) --------------- (4) S = -0.0338Ln(N) + 0.9389 (R2=0.8759) --------------- (5) S-N Curve for PCC and HVFAC 1 y = -0.0358Ln(x) + 0.9948 0.9 R2 = 0.9332 -- Eqn for PCC y = -0.0338Ln(x) + 0.9389 0.8 R2 = 0.8759 -- Eqn for HVFAC Stress Level (S) 0.7 S-N Curve for PCC 0.6 S-N Curve 0.5 for HVFAC 0.4 Log. (S-N Curve for PCC) 0.3 0 20000 40000 60000 80000 100000 120000 Log. (S-N Curve for Fatigue Life in No. of Cycles of Loading HVFAC) Figure 3. S-N Curves for PCC and HVFAC S-N Curve S-N Curve and 95% Confidence Limits for PCC 1 Upper 95% 0.9 y = -0.0358x + 0.9948 confidence 2 limit R = 0.9332 -- S-N curve 0.8 Lower 95% confidence Stress Level (S) limit 0.7 Linear (S-N y = -0.0358x + 1.0439 Curve) 0.6 -- Upper 95% confidence limit eqn 0.5 Linear y = -0.0358x + 0.9457 (Upper 95% -- Lower 95% confidence limit eqn confidence 0.4 limit) Linear (Lower 95% 0.3 confidence 0 2 4 6 Ln(N) 8 10 12 14 limit) Figure 4. S-N Curve and 95% confidence limits for PCC 411 International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME S-N Curve and 95% Confidence Limits for HVFAC 1 S-N Curve y = -0.0338x + 0.9389 0.9 R2 = 0.8759 -- S-N curve Upper 95% confidence 0.8 limit Stress Level (S) Lower 95% 0.7 confidence limit 0.6 Linear (S-N Curve) y = -0.0338x + 0.9977 0.5 -- Upper 95% confidence limit eqn Linear (Upper y = -0.0338x + 0.8801 95% 0.4 confidence -- Lower 95% confidence limit eqn limit) Linear (Lower 0.3 95% 0.000 2.000 4.000 6.000 8.000 10.000 12.000 14.000 confidence Ln(N) limit) Figure 5. S-N Curve and 95% confidence limits for HVFAC 6. ANALYSIS OF TEST RESULTS OF COMPOUND FATIGUE LOADING Test results of compound fatigue testing along with calculation of cumulative damage factor for HVFAC are shown in tables 8 to 11. Stress levels shown in the tables 8 to 11 are given in the order in which they have been applied to the specimens during testing. Fatigue lives at different stress levels in tables 8 to 11 have been calculated from equation (5). Cumulative damage factor i.e., Miner’s sum varied between 0.824 and 2.103. Miner’s sum showed dependency on type of compound fatigue loading and also on the sequence of loading. Table 8. Cumulative Damage Factors for HVFAC for Two Stage Compound Fatigue Loading No. of load cycles applied at Fatigue Life at Cumulative damage Specimen Stress level Stress Level factor no. S=0.55 S=0.6 S=0.55 S=0.6 (n1) (n2) N1 N2 M=(n1/N1)+(n2/N2) 1 20000 20672 99302 22621 1.115 2 20000 21453 99302 22621 1.150 3 20000 24550 99302 22621 1.287 4 40000 27683 99302 22621 1.627 5 40000 25894 99302 22621 1.548 6 40000 19527 99302 22621 1.266 Table 9. Cumulative Damage Factors for HVFAC for Two Stage Compound Fatigue Loading No. of load cycles Fatigue Life at Cumulative damage Specimen applied at Stress Stress Level factor no. level S=0.65 S=0.6 S=0.65 S=0.6 M=(n1/N1)+(n2/N2) (n1) (n2) (N1) (N2) 1 2000 9861 5153 22621 0.824 2 2000 15683 5153 22621 1.081 3 2000 13187 5153 22621 0.971 4 1000 17122 5153 22621 0.951 5 1000 15566 5153 22621 0.882 6 1000 19891 5153 22621 1.073 412 International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME Table 10. Cumulative Damage Factors for HVFAC for Three Stage Compound Fatigue Loading Fatigue Life at Stress Cumulative Specimen No. of load cycles applied Level damage factor no. at Stress level M=(n1/N1)+(n2/N2) S=0.55 S=0.6 S=0.65 S=0.55 S=0.6 S=0.65 +(n3/N3) (n1) (n2) (n3) (N1) (N2) (N3) 1 40000 6000 2838 99302 22621 5153 1.219 2 40000 6000 3836 99302 22621 5153 1.412 3 40000 6000 4126 99302 22621 5153 1.469 4 20000 10000 3645 99302 22621 5153 1.351 5 20000 10000 3358 99302 22621 5153 1.295 6 20000 10000 5372 99302 22621 5153 1.686 Table 11. Cumulative Damage Factors for HVFAC for Four Stage Compound Fatigue Loading Specimen No. of load cycles applied at Fatigue Life at Stress Level Cumulative no. Stress level damage factor S=0.55 S=0.6 S=0.65 S=0.7 S=0.55 S=0.6 S=0.65 S=0.7 M=(n1/N1)+ (n2/N2)+(n3/N3) (n1) (n2) (n3) (n4) (N1) (N2) (N3) (N4) +(n4/N4) 1 40000 5000 1000 911 99302 22621 5153 1174 1.594 2 40000 5000 1000 811 99302 22621 5153 1174 1.509 3 40000 5000 1000 1025 99302 22621 5153 1174 1.691 4 20000 10000 2000 1258 99302 22621 5153 1174 2.103 5 20000 10000 2000 852 99302 22621 5153 1174 1.757 6 20000 10000 2000 1042 99302 22621 5153 1174 1.919 7. CONCLUSIONS Based on experimental investigations following conclusions were made. • For probability distribution of fatigue life lognormal distribution model was found to be satisfactory for both PCC and HVFAC at all stress levels. • Parameters of lognormal model were found to be dependent on type of concrete and the stress level. • There is an upper limit for stress level in fatigue loading which is dependent on type of concrete, beyond which fatigue life value was insignificant. • S-N relations obtained from regression analysis were found to be dependent on type of concrete i.e., mainly on the static strength of concrete. Following are the S-N relations for PCC and HVFAC S = -0.0358Ln(N) + 0.9948 -------- for PCC S = -0.0338Ln(N) + 0.9389 -------- for HVFAC • Miner’s sum varied between 0.824 and 2.103. Hence Miner’s hypothesis gives both unsafe and over safe predictions for failure of HVFAC under compound fatigue loading. • Miner’s sum shows dependency on type of compound fatigue loading and also on sequence of loading. 413 International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print), ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME 8. ACKNOWLEDEGEMENT The financial support under Research Promotion Scheme from All India Council for Technical Education, New Delhi, India, is gratefully acknowledged. REFERENCES 1. Hilsdorf, H.K., and C.E.Kesler. Fatigue Strength of Concrete under Varying Flexural Stresses. ACI Journal Proceedings, Vol. 63, No. 10, 1966, pp. 1059-1076. 2. Ballinger, C.A. Cumulative Fatigue Damage Characteristics of Plain Concrete. Highway Research Record, No. 370, 1972, pp. 48-60. 3. Tepfers, R., and T.Kutti. Fatigue Strength of Plain, Ordinary, and Lightweight Concrete. 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