Early Strain Measuring And Separating Method For Cement Concrete Pavement Slabs Based On Zero-stress Moment

In the field of health monitoring technology of cement concrete pavement,using strain gauge to measure strain and multiplying modulus to calculate stress is an effective way and common method to evaluate the service condition of cement concrete slab.However,the conventional strain measurement method has the following two problems: there is no theoretical basis for the selection of the initial moment of strain measurement,and the measured strain contains strain components unrelated to structural stress.This leads to a significant decrease in the reliability of the measurement results of the cement concrete strain,especially the early strain,which seriously affects the accuracy of the stress calculation and safety evaluation of the concrete slab.Therefore,it is necessary to propose early strain measurement and separation methods for cement concrete slabs.In this paper,a prediction method for the zero-stress moment of cement concrete is established,which provides a theoretical basis for the selection of the initial moment of strain collection,and a separation method for each strain component is proposed,which has important theoretical and practical value for improving the accuracy of stress calculation and safety evaluation technology.Aiming at the problem that the conventional strain measurement method cannot distinguish whether the strain is related to the stress,a zero-stress cylinder made of PVC pipe is designed,and three strain testing environments are constructed.The idea of separating stressdependent strain component by regulating the strain testing environment is proposed.The results show that: according to whether the strain is related to the stress,the measured strain can be divided into two categories: the stress-dependent strain component and the non-stressdependent strain component.The former mainly includes the strain generated when the temperature and drying shrinkage are constrained,the creep strain and the load strain.The zerostress environment of concrete should have two basic conditions: the concrete can expand and contract freely,and the strain generated by the free expansion and contraction is consistent with the unstress-dependent strain component in the conventional environment.The three strain testing environments can simulate the sum of stress-dependent and non-stress-dependent strain components,the sum of stress-independent strain components and additional dry shrinkage strains,and the tests of stress-independent strain components.Through small-scale tests,the time-varying characteristics of early strain in three strain environments were determined for both natural and recycled aggregate concrete.On the basis of summarizing the trend of the curve and analyzing the reasons,the time-varying law of the early strain of concrete under the condition of constant temperature and humidity is summarized.The results show that: the early strain of concrete generally presents a trend of first rising,then falling,and finally stable.Which development can be divided into five stages: the acceleration rising stage,the convex curve descending stage,the concave curved descending stage,the stabilizing and the stable stage.The higher the ambient temperature is,the earlier the normal strain appears and reaches the extreme value,and the extreme value continues to increase.In addition,recycled aggregate has a certain inhibitory effect on the early expansion and contraction of concrete,and the higher the ambient temperature,the more significant the inhibitory effect.Based on the one-dimensional heat conduction differential equation,a program for predicting the early temperature field of the concrete slab is written.It is verified that the program can reflect the change law of the early temperature field of the pavement slab.The results show that: the variation of concrete temperature field mainly depends on solar radiation and atmospheric temperature.During the day,the temperature of the concrete slab increases with the intensity of solar radiation,and the time when the temperature on the top of the slab reaches the maximum value is about 2 hours earlier than the time when the maximum value of the air temperature is reached.At night,the time for the temperature at the top of the slab to drop to a very low value is about 0.5 to 1 hour later than the time for the very low value of the air temperature.The temperature of the top of the concrete slab is most affected by solar radiation and atmospheric temperature.With the increase of depth,the range of concrete temperature changes decreases and the reaction time increases.Aiming at the problem that the selection of initial time is lack of theoretical basis when collecting concrete strain by conventional methods,the prediction method of zero-stress time under constant temperature and humidity is established.The direct and indirect equivalent formulas of concrete maturity under general environment and constant temperature and humidity are derived,and based on the daily air temperature model,the indirect prediction method of zero-stress time under general environmental conditions is proposed.The results show that the indirect prediction method based on daily air temperature model has the characteristics of simple calculation and high stability of results,and can be used as the first prediction method for zero-stress time.The final setting time of concrete mixture can be used as an alternative method for determining zero-stress time.Based on the results of zero-stress moment and stress-independent strain,the calculation method of early age structural stress of concrete pavement slab is proposed.A test road with a length of about 120 m is paved relying on G109 pavement reconstruction project.The impact of the new method on the stress and cracking risk of concrete slab is evaluated through numerical simulation and test road verification.The results show that the total strain is significantly greater than the stress-dependent strain component,and the error can be reduced by more than 42%when the stress-dependent strain component is used to calculate the structural stress only under temperature load;The 13 th to 24 th hours after the concrete is laid is the high incidence period of early cracking.From the perspective of preventing cracking,the concrete paving time should be selected before 16:00 and after 22:00,and should be avoided around 20:00.