Issue 41

Y. Yang et alii, Frattura ed Integrità Strutturale, 41 (2017) 339-349; DOI: 10.3221/IGF-ESIS.41.45 347 From the above fitting results, it can be seen that the tensile modulus, compression modulus and flexural modulus increase with the increase of the loading rate, showing a good power function relationship. It has been proved in the existing research that when the loading rate increases, the stress state inside the specimen is not exactly one-dimensional stress state, but rather a mechanical response characteristic towards the one-dimensional strain state. In particular, in the middle part of the specimen, under a higher loading rate, due to the inertia of the material, the lateral strain of the specimen is restricted, and the higher the strain rate is, the more obvious the restriction will be, showing an obvious strain ratio effect, which causes the material modulus and strength to increase with the growth of loading rate [16]. The modulus and strength of asphalt mixture has a similar change pattern [17-20]. In the same way, in this paper, the change rule that the modulus of the cement stabilized macadam material changes with the loading rate also proves this conclusion. Comparison and analysis of the three moduli According to the test results in Tab. 2, we get to know: (1) Under different loading rates, the ratio between the compression and tensile moduli is 1.71, 1.78, 1.81 and 1.80, with an average value of 1.78. The cement stabilized macadam material shows significant differences between the tensile and compression moduli, and the compression modulus is greater than the tensile modulus, it proved the differences between the tensile and compressive modulus of cement stabilized macadam material. (2) The tensile modulus is respectively about 3.97, 3.77, 3.65 and 3.63 times the flexural modulus, with a mean value of 3.76; the compression modulus is about 6.81, 6.71, 6.63 and 6.54 times the flexural modulus, respectively, with a mean of 6.68. In other words, the tensile and compression moduli are much greater than the flexural modulus. It can be seen that, regardless of the stress state inside the cement stabilized macadam semi-rigid base structure, it is obviously inappropriate to simply use the unconfined compressive resilient modulus to calculate the structural load response. As the compressive resilient modulus is the largest among the three moduli, the structural deformation response calculated based on this modulus will be the smallest. As a result, the asphalt pavement structure, which takes the surface deflection as the indicator, will be thin and unsafe, and the road pavement is likely to be damaged at an early stage. (3) Under different loading rates, the ratios between each two of the tensile, compressive and flexural moduli, are stable, showing that even the loading rate has direct impact on the moduli of cement stabilized macadam material, but it does not affect the ratio relationship between the three moduli[19]. This provides basis and convenience for the conversion between the three moduli. Conversion relations between the three moduli With the loading rate as the intermediate variable, according to Formula (19) and the fitting results in Tab. 3, we can establish the conversion relations between the three moduli. (1) Conversion relation between the tensile modulus and the flexural modulus: ′ = 0.27 1164.79 t f E E (20) (2) Conversion relation between the compression modulus and the flexural modulus: ′ = 0.7 72.04 p f E E (21) (3) Conversion relation between the tensile modulus and the compression modulus: = 0.39 224.81 t p E E (22) According to the conversion relations between the three moduli in Formula (20), (21) and (22), as long as we can get one modulus value, we can easily calculate the other two modulus, which facilitates the selection of modulus design parameters based on the stress state inside the pavement structure. C ONCLUSIONS (1) When the shear effect is considered, the flexural modulus of the beam specimen of cement stabilized macadam will be greatly increased. Therefore, when measuring the modulus of the indoor middle beam made of semi-rigid material, we should consider the shear effect.