Fatigue Property of Mill Cut Steel Fiber Reinforced Concrete

Transcription

1 Fatigue Property of Mill Cut Steel Fiber Reinforced Concrete Shanghai Research Institute of Building Sciences Group Shanghai municipal Engineering Institute July 1997

2 Fatigue Property of Mill Cut Steel Fiber Concrete 1 Introduction Application of mill cut steel fiber concrete in the traffic road surface is still in the testing stage in China, because the road surface bears the repeated load and this results in the fatigue breakdown of the concrete which leads to the pavement cracking, therefore, the fatigue property is a significant parameter to design the mill cut steel fiber concrete road surface. This text researches the fatigue property of both mill cut steel fiber concrete and normal concrete through test and analysis, and proposes a fatigue equation of mill cut steel fiber concrete for the reference of the road surface designers. 2 Raw materials mix proportion and test method 2.1 Test material Cement: 525 # Normal Silicate Cement Manufactured by Ningguo Cement Factory Fine aggregate: normal river sand with the fineness modulus being 2. 4 Coarse aggregate: continuous grading macadam with the maximum particle size of 25mm Steel fiber: HAREX Mill cut steel fiber (SF01 32). 2.2 Mix proportion of the concrete The mix proportion of the concrete for test use is as shown in Table 1. Table 1 Steel fiber mix proportion of the concrete Unit: k/m 3 Group Water Cement Sand Stone Steel fiber Preparation and maintenance of the test specimen The mixing materials of steel fiber concrete in this test is prepared at the commercial concrete mixing plant of Shanghai Cement Finished Products Factory and then delivered to the factory laboratory to prepare the test specimen. The test specimen demoulds 24 hours after the shake molding and immediately send to the standard curing room after demoulding (with the temperature of 20 ±3 and relative humidity of over 90%) to maintain 28 days, the fatigue test will be conducted after 90 days of air curing. 2.4 Fatigue test method The fatigue test machine applied in this test is the electro-hydraulic servo type fatigue test machine (model ) manufactured by American MTS company, the dimension of the test specimen is mm, loading according to third point (Figure 1). Figure 1 Loading location figure of the fatigue test

3 Impose 100N preloading to the test specimen before the fatigue test, and repeat for several times to eliminate the error resulted from bad contact and make the instrument run faultlessly, then add the load to the fatigue test upper limit load P max, and unloading, and circulate for several times, conduct the alternating test after stabilization, load rangeability is from P max to P min (Figure 2). Figure 2 Loading schematic diagram of the fatigue test 2.5 Confirmation of the test parameters The average flexural-tensile strength R ω of the concrete Take the flexural-tensile strength average value of 3 test specimens as the R ω of this group of test specimen R ω= 1 / 3(R ω1+r ω2+r ω3) R ω average flexural-tensile strength (MPa) R ω1, R ω2, R ω3 The flexural-tensile strength (MPa) of 3 test specimens in the same group Load circulation characteristic value ρ ρ Load circulation characteristic value P min minimum load (KN) effected on the test specimen P max maximum load (KN) effected on the test specimen Load circulation characteristic value ρ taken in this test is the same with that of the normal concrete, it is stress ratio n n stress ratio σ ω flexural-tensile fatigue strength (MPa) R ω calculate flexural-tensile strength (MPa) Frequency of loading action Frequency of loading action is 6~8HZ, the low value shall be applied when the stress ratio is large, and the high value shall be applied when the stress ratio is small. 3 Test Result 3.1 Flexural-tensile strength Result of the concrete flexural-tensile strength test can be referred to in Table 2.

4 Table 2Mill cut steel fiber concrete flexural-tensile strength Group No. No. of test specimen Admixture amount of steel fiber (kg/m 3 ) Average flexural-tensile strength (MPa) 0 SF SF SF SF Fatigue Property Fatigue test results of the normal concrete and mill cut steel fiber concrete with three different types of admixture amount are shown in Table 3. Table 3 Result of the flexural-tensile fatigue test No. of test specimenfiber Content (kg/m 3 )Stress ratio NMaximum load Pmax(KN) Minimum load Pmin (KN) Time of circulation N 10 4 SF >100 SF * > >100 SF * >100 *SF * >100 * >100

5 Flexural fatigue equation obtained through the regression analysis of the data in Table 3: Normal concrete: n= lgn (1) r (correlation coefficient) = s (regression error) = Mixed with 30kg/m 3 steel fiber: n= lgn (2) r= s= Mixed with 60 kg/m 3 steel fiber: n = lgN.. (3) r= s= Mixed with 90 kg/m 3 steel fiber: n= lgn (4) r= s= After deducting 3 times of the regression error, the fatigue equation with 99.7% security assurances rate is obtained. The above (1), (2), (3) and (4) equations change in turn to: 0 kg/m 3 : n= lgN.(5) 30 kg/m 3 : n= lgN (6) 60 kg/m 3 : n=l gN (7) 90 kg/m 3 : n=l lgN. (8) If the service life of the mill cut steel fiber road surface concrete is to be estimated, the load effect frequency N can be calculated according to the known stress ratio n. Substitute into (5), (6), (7) and (8) equations respectively, the corresponding flexural-tensile fatigue strength equation can be obtained: 0 kg/m 3 : σ ω=( lgN) R ω.(9) 30 kg/m 3 : σ ω=(l lgn) R ω (10) 60 kg/m 3 : σ ω=( lgn) R ω.(11) 90 kg/m 3 : σ ω=(l lgn) R ω (12) When N = 10 6, Substitute flexural-tensile strength calculated value R ω of the concrete in Table 2 into (9), (10), (11) and (12) equations respectively, the respective fatigue strength obtained are: Normal concrete: σ ω= 1.75MPa Mixed with 30kg/m 3 : σ ω=2.90mpa Mixed with 60 kg/m 3 : σ ω=3.17mpa Mixed with 90 kg/m 3 : σ ω=3.95mpa i.e., when the admixture amount of the fiber is 30kg/m 3, fatigue strength is increased by about 65.7%; when the admixture amount is 60kg/m 3, fatigue strength is increased by 81.1%, when the admixture amount is 90kg/m 3, fatigue strength is increased by 125.7%. 4 Discussions The above test result fully illustrated that flexural tensile Fatigue Property of the mill cut steel fiber premixed concrete is greatly improved compared to normal concrete. When the fatigue frequency reaches one million times, fatigue strength of the doped mill cut steel fiber concrete is 1.65 ~ 2.25 times of that of the normal concrete. Similarly, when the stress ratio is fixed, fatigue times of the mill cut steel fiber concrete can be greatly improved, thus greatly extend the service

6 life of concrete pavement. Because concrete is a multiphase heterogeneous material, in its hardening process with the impact from different environmental factors and the external influence, shrinkage with varying degrees will happen to the aggregate and cement mortar interface, and uneven distribution of the concrete internal stress will lead to microcrack, which is the potential cause for fatigue damage resulted from concrete. With the influence of repeated load, the high stress concentration at the concrete internal microcrack tip has prompted the crack to gradually expand until it breaks. And with the admixture of steel fiber, the trait of the concrete is improved, and the steel fiber suppresses crack initiation and expansion in formation process of the concrete structure, thereby the size and number of cracks are reduced and the source of cracks is limited. In the fatigue process, when the crack tip meets the steel fiber, because cracks cannot directly pass and have to deflect the direction, thereby it buffers the stress concentration degree of the crack tip so that the fatigue service life of the concrete is extended correspondingly, and the higher the steel fiber admixture amount, the stronger the ability of inhibiting crack initiation and expansion, and the longer the fatigue life. 5 Conclusions The fatigue property of the mill cut steel fiber concrete has great improvement compared with that of the normal concrete, when the fatigue times is 1 Million, the flexural-tensile strength of the mill cut steel fiber concrete is 1.65~2.25 times of that of the normal concrete. The flexural-tensile fatigue equation of the mill cut steel fiber concrete for the reference of the road surface thickness gauge is: σ ω=( lgn) R ω (30 kg/m 3 ) σ ω= lgn) R ω (60 kg/m 3 ) σ ω= ( lgN) R ω (90kg/m 3 )