Field test on temperature field and thermal stress for prestressed concrete box-girder bridge

Abstract

A field test was conducted to investigate the distribution of temperature field and the variation of thermal stress for a prestressed concrete (PC) box-girder bridge. The change of hydration heat temperature consists of four periods: temperature rising period, constant temperature period, rapid temperature fall period and slow temperature fall period. The peak value of hydration heat temperature increases with the increasing casting temperature of concrete; the relation between them is approximately linear. According to field tests, the thermal stress incurred by hydration heat may induce temperature cracks on the PC box-girder. Furthermore, the nonlinear distribution of temperature gradient and the fluctuation of thermal stress induced by exposure to sunlight were also obtained based on continuous in-situ monitoring. Such results show that the prevailing Chinese Code (2004) is insufficient since it does not take into account the temperature gradient of the bottom slab. Finally, some preventive measures against temperature cracks were proposed based on related studies. The conclusions can provide valuable reference for the design and construction of PC box-girder bridges.

Keywords

box-girder bridge / field test / hydration heat / temperature field / temperature gradient / thermal stress

Introduction

Thermal stresses caused by hydration heat or sunlight exposure may induce temperature cracking of PC box-girder bridges [1-3]. Steel bars in the structure would corrode if they were exposed to moisture on the condition that temperature cracking has taken place, which may lead to potential hazard on structures. Wang et al. and Lv et al. [4,5] analyzed the thermal stress of concrete structures at the early age of curing. Schutter [6] studied the temperature cracking of massive concrete structures by finite element method (FEM). Kim et al. [7] discussed the compressive strength development of concrete with different curing times and temperatures. Kjellsen et al. [8] predicted the later-age strength of concrete. Peng et al. [9] proposed a computational method for determining the self-equilibrium thermal stress caused by sunlight exposure of concrete box girder bridges. Enrique et al. [10] investigated the distributions of temperature and thermal stress in concrete box-girder bridges.

From the studies mentioned above, controlling the temperature difference between interior and surface concrete so as to prevent temperature cracking is the main issue in concrete structures. In this paper, the temperature field and thermal stress caused by heat of hydration or exposure to sunlight were investigated by field monitoring, and some preventive measures against temperature cracks are also proposed.

Instrumented box-girder

The instrumented box-girder bridge presented in this paper is located in the Shiman Expressway in China. The frame of the main bridge is a three-span PC continuous rigid frame with a span of 42.2 m+76 m+42.2 m. The height of the box-girder at the cross-section M-I (or S-I) is 4.5 m, and it is 2.2 m in the middle of the midspan. The cross-sections of the instrumented box-girder are shown in Fig.1. The thickness of the top slab is 30 cm, the thicknesses of the webs and the bottom slab are described in Table 1, and the typical dimensions of the cross-section are shown in Fig. 2. Layouts of the thermocouples in cross-section s6#, the cross-sections from 0# to m9# and layouts of the strain gauges from the cross-sections S-I to S-III and M-I to M-III are shown in Fig. 3.

Temperature field and thermal strain caused by concrete hydration

Variation of temperature field

The variation of temperature caused by concrete hydration in cross-section s6# (mean temperature of air was about 30°C at the early age of concrete curing) and the cross-sections from 0# to m9# (which were constructed from Jan. to July in 2006) is shown in Figs. 4 and 5.The thermocouple No.TA in top slab at cross-section m3 # was mal-functional. During the early age of each concrete segment curing (from 0# to 9#), the observed temperatures of air are described in Fig. 6.

Field test results (see Figs. 4 and 5) show that the variation of hydration heat temperature consists of four periods. The first is temperature rising period (0-16 h after concrete casting). At this stage, the temperature of the interior concrete reaches the peak value rapidly. The temperature increment is about 26.9-30°C and the average rate of the rise in temperature is about 1.29°C-2.32°C/h. The second is constant temperature period (16-20 h after casting). The temperature of the interior concrete is approximately stable around the peak value. The third is rapid temperature fall period (20-30 h after casting). At this stage, the temperature decreases rapidly and the average rate of drop in temperature is about 0.82°C-1.12°C/h. The fourth is slow temperature fall period (2-4 d after casting). At this stage the temperature of the interior concrete gradually regresses to the normal temperature of air and the average rate of decrease in temperature is about 0.18°C/h-0.38°C/h.

Figures 4 and 5 show that the temperatures measured by the thermocouples in the same cross-section almost reach the peak value simultaneously, and the time-lag effect is not obvious. The peak value of temperature of the interior concrete in the top slab and bottom slab is within the range of 4.1-6.2°C, which is lower than that in the web since the temperature in the top/bottom slab is influenced by air temperature readily. As shown in Fig. 4, the peak value of temperature at the corner rib of the flange is higher than any other part of the box-girder, and the duration of temperature stabilization is longer than that in the top slab, bottom slab and web. From Fig. 5, the hydration of concrete is influenced by seasonal variation. The cooler the climate and the higher the speeds of wind are, the slower the hydration velocity and the lower the peak value of temperature of hydration heat will be. Reaching the peak value of temperature of the interior concrete takes about 22 h in winter, but about 14 h in summer. The maximum temperature difference between the interior concrete and surface is about 14°C in winter and about 8°C in the summer based on the field test.

Relation between peak temperature and casting temperature

Peak temperature of hydration heat in box-girder is dependent on casting temperature of concrete and temperature increment of hydration. Controlling the casting temperature of concrete is the main measure for the peak temperature of hydration heat. The peak temperatures of the hydration heat increasing with the casting temperature of concrete are shown in Fig.7. It is obvious that the relation between them is approximately in linear.

Time-dependent behavior of thermal strain

Hydration of concrete will produce plenty of heat during the early age of curing and great temperature gradient will be induced by low thermal conductivity of the concrete. Then the temperature gradient and structural constraint produce the thermal strain in box-girder [11,12]. The variation of the thermal strain induced by the hydration heat in cross-sections M-I and M-II is obtained by field monitoring as shown in Figs. 8 and 9.

As presented in Figs. 8 and 9, at the early age of curing, the temperature of the interior concrete rises rapidly and the volume is expanding, due to the temperature difference between the interior and surface concrete, together with the shrinkage of the surface concrete, the generation of the thermal tensile strain at the surface. After 12-16 h from concrete casting, the thermal tensile strain reaches a maximum value. The maximum values of the thermal tensile strain are 32.8 μϵ and 37.3 μϵ respectively at the top and bottom surface in cross-section M-I, and 28.8 μϵ and 31.3 μϵ at the top and bottom surface in cross-section M-II. However, the temperature of the interior concrete decreases with curing time, and the volume is shrinking. After 24 h from concrete casting, the thermal tensile strain at the surface of the concrete turns into compressive strain, which increases with curing time, while the temperature of the interior concrete continuously decreases. The maximum values of the compressive strain are 123.2 μϵ and 75.7 μϵ respectively at the top and bottom surface in cross-section M-I, and 102.2 μϵ and 65.2 μϵ in cross-section M-II.

Field test results show that the maximum thermal tensile strain and the compressive strain at the corner rib of the flange are larger than that in any other part due to the considerable thickness of the corner rib. The thicker the slab of the concrete box girder is, the higher the tensile or compressive strains on both surface will be. Moreover, the compressive strain at the top surface of the box girder appears earlier than that at the bottom surface, since the top surface is affected by the temperature of the atmosphere readily, the time discrepancy is about 2-4 h.

Temperature field and thermal stress caused by air temperature

Distribution of temperature gradient

The variation of air temperature may cause great temperature gradient, which can induce high thermal stress in the concrete box-girder. The variations in temperature with air temperature at the top and bottom slabs (the thermocouples are about 5 cm beneath or on the top or bottom surface of the box-girder) as well as in the web are shown in Fig.10.

The temperature of the interior concrete increases with air temperature, the rate of the rise in temperature of the top slab is the highest because of the perpendicular incidence of sunlight. The maximum temperatures of the top and bottom slabs are 39.0°C and 27.6°C respectively (at 3:00 p.m.). The temperature increment of the web is only about 1.2°C (at 3:00 p.m.) because of the shading of the top slab. The peak temperature induced by air temperature in the slabs of the concrete box-girder lags by about 1-2 h behind the maximum air temperature, mainly because of the low thermal conductivity of concrete. The maximum difference in temperature between the top and bottom slabs is 11.4°C, which lags by about 1.5 h behind the maximum air temperature. The difference in temperature between the left and right webs is only 0.5°C (after 4:00 p.m.), due to the oblique incidence of sunlight.

The distributions of the temperature field and nonlinear temperature gradient are presented in Figs.11 and 12. The maximum increment of the temperature at the top surface of the concrete box-girder caused by air temperature is about 25.1°C and the average rate of rise in temperature is about 2.39°C/h (from 7:00 a.m. to 16:00 p.m. in summer) based on continuous field monitoring. Accordingly, the maximum increments of temperature at the bottom surface and webs are about 10.6°C and 0.9-1.2°C respectively, and the average velocities are about 1.01°C/h and 0.09°C/h, respectively, as shown in Fig.11. The comparison of the temperature gradient between the field test results and Chinese Code [13], British Standard<BS5400> [14], as well as the New Zealand Code [15] is summarized in Table 2. Based on statistical analysis of the data obtained from field measurement, the thickness of the box-girder influenced by air temperature are respectively about 800 mm beneath the top surface and 300 mm on the bottom surface. The maximum temperature increments at the top and bottom surfaces of the concrete box-girder are 22.7°C and 8.5°C respectively, while the temperature increment beneath the top surface 100 mm is about 12.9°C, as shown in Fig.12. The temperature increment recommended in the Chinese Code (2004) which ignores temperature gradient at the bottom surface is significantly different from the actual data from field monitoring. By contrast, the temperature gradients of the top and bottom surfaces recommended in the British Standard(BS5400) (1982) and the temperature gradient of bottom surface recommended in the New Zealand Code (1976) are lower than the field data.

Thermal stress due to air temperature

The variation of the thermal stress induced by air temperature in cross-sections S-I to S-III and M-I to M- III are obtained by field monitoring, as shown in Figs. 13-15. Based on the field test results, the thermal stresses of air temperature are related to temperature gradient, constraint conditions and geometrical dimension of the box-girder.

In cross-sections S-I and M-I, the thermal stresses are higher than that in the other cross-sections, since the concrete is in rigid constraint conditions. The maximum values of the thermal stresses at the top and bottom surface are 3.05 MPa and 1.55 MPa respectively. In cross-sections S-II and M-II, the maximum values of the thermal stresses are lower than those in cross-sections S-I and M-I, which are respectively 0.87 MPa and 0.58 MPa at the top and bottom surface. In cross-section M-Ⅲ, the constraint conditions are loose. The temperature gradient is lower than that in the other sections and the thermal stress is also lower. In cross-section S- III, the constraint conditions are more rigid than those of uncontrolled cantilever of cross-section M- III, and the maximum thermal stress at the top surface is 1.03 MPa, but at the bottom surface, it is comparatively lower. The thermal stresses caused by the variation of air temperature and sunlight exposure will exceed the stresses induced by live loads based on the results. Moreover, the thermal stress has lag effect, based on field monitoring; the maximum thermal stresses lags by about 1-2 h behind the peak value of air temperature.

Conclusions and suggestions

1) The variation of hydration heat temperature consists of four periods: temperature rising period, constant temperature period, rapid temperature fall period and slow temperature fall period. The change in temperature is related to the dosage of cement, geometrical dimension of the structure, casting temperature of concrete, heat dissipation conditions and curing conditions of concrete. The maximum temperature caused by hydration heat occurs at the corner rib of flange.

2) The peak value of temperature caused by hydration heat increases with the casting temperature of concrete, the relation between them is approximately in linear.

3) At the early age of concrete curing, the thermal tensile strain is generated at the surface of the box-girder. Along with the temperature drop at the interior concrete, the thermal tensile strain at the surface of concrete turns into compressive strain, which increases with curing time while the temperature of the interior concrete decreases continuously. The maximum tensile and compressive strains at the surface are respectively 37.3 μϵ and 123.2 μϵ based on field test.

4) The thermal stress caused by air temperature and sunlight exposure changes with the variation of temperature gradient, constraint conditions and cantilever length of the box-girder. And it may exceed the stress induced by live loads. Moreover, the thermal stress has lag effect, i.e. maximum thermal stresses lags by about 1-2 h behind the peak value of air temperature.

Following preventive measures against temperature cracks are proposed based on the above results:

1) Control casting temperature of concrete in between 25°C-30°C.

2) Control peak temperature of interior concrete to between 55°C-80°C; reduce dosage of cement by adding cementitious material.

3) Improve curing conditions and shorten the temperature difference between interior and exterior of concrete.

4) Choose an appropriate time to remove the moulding board (the maximum temperature difference between interior and surface concrete is less than 15°C), and not remove it when air temperature is changing rapidly.

5) Optimize the sequence of construction and ameliorate boundary constraint conditions.

6) Choose suitable pavement materials to reduce temperature gradient in concrete box-girder.

7) Appropriately arrange constructional reinforced bars to enhance crack resistance capacity.

References

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