J. Cent. South Univ. Technol. (2008) 15: 423-428

DOI: 10.1007/s11771-008-0079-7

Long term behavior of self-compacting reinforced concrete beams

LIU Xiao-jie(刘小洁), YU Zhi-wu(余志武), JIANG Li-zhong(蒋丽忠)

(School of Civil and Architectural Engineering, Central South University, Changsha 410075, China)

Abstract: Tests were carried out on 8 self-compacting reinforced concrete(SCC) beams and 4 normal reinforced concrete beams. The effects of mode of consolidation, load level, reinforcing ratio and structural type on long term behavior of SCC were investigated. Under the same environmental conditions, the shrinkage—time curve of self-compacting concrete beam is very similar to that of normal concrete beam. For both self-compacting reinforced concrete beams and normal reinforced concrete beams, the rate of shrinkage at early stages is higher, the shrinkage strain at 2 months is about 60% of the maximum value at one year. The shrinkage strain of self-compacting reinforced concrete beam after one year is about 450×10^-6. Creep deflection of self-compacting reinforced concrete beam decreases as the tensile reinforcing ratio increases. The deflection creep coefficient of self-compacting reinforced concrete beam after one and a half year is about 1.6, which is very close to that of normal reinforced concrete beams cast with vibration. Extra cautions considering shrinkage and creep behavior are not needed for the use of SCC in engineering practices.

Key words: self-compacting reinforced concrete beams; long term behavior; shrinkage; creep deflection

1 Introduction

In recent 15 years, self-compacting concrete has been widely used around the world for its superior constructive ability and higher durability. Practical application has been accompanied by researches on the physical and mechanical characteristics of self-compacting reinforced concrete(SCC) and the results have been combined in guideline document^[1]. Since 1994, self-compacting concrete has been applied to new buildings^[2], bridges^[3] and strengthening engineering^[4] in China. Up to now, researches have been carried out on SCC, but most of them put more focus on mix design^[5-6], testing method of its workability^[7-8], mechanical properties and short term structural properties^[9-12]. Little information is available about long term performance with time. Deep research on shrinkage and creep properties of self-compacting concrete is of significant importance in the construction of bridge and ensuring good structural working performance^[13]. To further promote the use of self-compacting concrete in civil engineering, systematically deep research on long term performance of SCC has been made^[14]. Part of the experimental results of prestressed concrete beams has been brought into “specification and guidelines for self-compacting concrete” (CCES02-2004) released in 2005. In this work, the test of the second batch of self-compacting reinforced concrete beams was carried out and the effects of mode of consolidation, load level, tensile reinforcing ratio, and structural type on shrinkage and creep property of self-compacting concrete were investigated.

2 Experimental

Among the 12 tested beams, 3 beams were used for contrastive shrinkage beams, and the remaining 9 beams were used for creep testing under long term loading. Parameters of beam specimens for long term test are listed in Table 1. Concrete blocks with dimensions of d200 mm×200 mm×550 mm were cast at the same time, which were used to simulate the sustained concentrated loads. The beam specimens were cast in the Building Material Laboratory of Central South University on 26th, Dec 2005. All the specimens were air cured. Standard cubes (150 mm) and prisms (100 mm×100 mm×300 mm) were cast and cured under the same environmental conditions for tested beams. The mix-proportion of self-compacting concrete by mass is 333?722?900?204.7?180?4.62 (cement to sand to granite to fly ash to water to superplasticizer), and the mix- proportion of normal concrete is 396?590?1 094?180?190 (cement to sand to granite to fly ash to water). Standard 42.5 N grade silicate cement and ultra-pulverized fly ash produced by the Electric Plant of Xiangtan City in Hunan

Table 1 Parameters of beam specimens for long term test

Province and tap water were used. The fine aggregates were from Xiangjiang River and the coarse aggregates were continuous graded. The workability index was measured when concrete is mixed. The measured slump flow of self-compacting concrete was 600 mm, and the height difference for U-type box was 24 mm. The mechanical properties of concrete are shown in Table 2. The elastic modulus tests were carried out at 28 d.

Table 2 Mechanical properties of concrete

Details of specimens and instrument of configuration are shown in Figs.1-4. The deflection at the midpoint and the support of the tested beams were measured by dial gauge with a precision of 0.01 mm and the concrete strain at the same level of reinforcement was measured by dial gauge with a higher precision of 0.001 mm. The experiment was carried out in the Building Material Laboratory of Central South University and the humidity was kept constant by a dehumidifying machine. The test was started on 26th, Jan. 2005 and lasted for 540 d. The test setup is shown in Fig.5 and the variation of temperature with time is shown in Fig.6.

3 Results and discussion

3.1 Shrinkage strain

The drying shrinkage is of great concern in engineering. The cracking resistance of high strength

Fig.1 Reinforcement details of simple beam (unit: mm)

Fig.2 Layout of measuring points of simple beam (unit: mm)

concrete and the decrease of prestress force demand a lower drying shrinkage. The relationship between shrinkage strain for test beams and time is shown in Fig.7. It can be seen that smaller shrinkage deformation is attributed to higher reinforcing ratio, which can provide much higher restraint over concrete. The relationship between shrinkage strain and time for self-compacting concrete and normal concrete without restrain is shown in Fig.8.

As observed from Fig.8, under the same natural environmental conditions, the shrinkage—time curve of self-compacting concrete beam is very similar to that of a normal concrete beam. For both self-compacting concrete beam and normal concrete beam, the rate of shrinkage at early ages is larger, the shrinkage strain at 2 months is about 60% of the maximum value at one year,

Fig.3 Reinforcement details of two-span continuous beam (unit: mm)

Fig.4 Layout of measuring points of two-span continuous beam (unit: mm)

Fig.5 Test setup of long term experiment: (a) Shrinkage beam; (b) Simple beam; (c) Continuous beam

Fig.6 Temperature vs time of long term experiment

Fig.7 Shrinkage vs time for test beams

Fig.8 Shrinkage vs time for test beams without restrain

and the rate of increase becomes smaller with the increase in time. For normal concrete, the more the water used per unit volume of concrete, the larger the shrinkage. However, for self-compacting concrete, the higher the mass ratio of cement to cementitious material, the larger the shrinkage^[15]. Since the measured strength of vibrating concrete is higher than that of self-compacting concrete with lower cement-cementitious ratio, other things being equal, the shrinkage of normal concrete is higher. The shrinkage strain of self-compacting concrete after 1 year is about 450×10^-6.

3.2 Instantaneous deflection

Table 3 lists the tested instantaneous deflection of beam specimens. It can be seen from Table 3 that the instantaneous deflection of normal concrete is smaller than that of self-compacting concrete due to lower modulus of elasticity under loading. The increase in tensile reinforcing ratio can reduce the instantaneous deflection. The instantaneous deflection of continuous beams is about 50% that of simple beams, indicating that the increase in the degree of indeterminacy of a structure can dramatically reduce the deflection.

Table 3 Instantaneous deflection of beam specimens

3.3 Creep deflection and deflection creep coefficient

The concrete strain at the same level of reinforcement was measured with a dial gauge. And the curvature on mid-span section was calculated using the concrete strain and the distance between the top and bottom steel. Deflection creep coefficient is determined by the following formula:

                             (1)

where f(t, τ) is creep deflection; f_g is instantaneous deflection under self-weight and calculated according to structural mechanics using flexural rigidity(EI₀) obtained from the tested instantaneous deflection increment under each load increment; f_p is measured by dial gauge. The relationship between creep curvature and creep deflection and deflection creep coefficient with time are shown in Figs.9-11.

It can be seen from Figs.9-11 that the creep deflection—time curves are very similar to the creep curvature—time curves. Creep deflection was directly measured by a dial gauge and creep curvature was calculated by using the strain of concrete, which shows that the test method is correct and the test data are reliable. The effects of different factors on long term performance of self-compacting concrete beams are discussed as follows.

Fig.9 Creep curvature vs time for test beams

Fig.10 Creep deflection vs time for test beams

The dehumidifying machine was used to keep the humidity of the laboratory a constant at 60%. The experiment reported in Ref.[16] was carried out under natural environmental conditions and the average humidity in Changsha is about 80%. Other conditions being equal, the creep becomes smaller when the relative humidity increases. In addition, it can be seen from the comparison of the test results with that in Ref.[14] that the effect of temperature on the creep of self-compacting reinforced concrete beams is not as remarkable as that due to humidity.

Under the same natural environmental conditions, the creep deflection—time curves of self-compacting concrete beams are very similar to those of vibrating concrete beams. The rate of creep development is very high at early ages and slows down as the age of concrete increases. The creep deflections developed in one month and six months are respectively about 43% and 96% of that developed in one year for self-compacting concrete beams, whilst for vibrating concrete beams they are about 39% and 93%, respectively. The modulus of elasticity of vibrating concrete is higher compared with that of self-compacting concrete under loading, creep deflection of vibrating concrete beams is lower, which is only 76% of that of self-compacting concrete, whilst the deflection creep coefficient for both kinds of beams is almost the same

Fig.11 Effect of different factors on deflection creep coefficient: (a) Mode of consolidation; (b) Tensile reinforcing ratio; (c) Load level; (d) Structural type

Creep deflection decreases as the tensile reinforcing ratio increases. Creep deflection of beam sc-2 is about 90% of that of beam sc-1, whilst the deflection creep coefficient is higher, which is in accordance with the provision of ‘Code for Design on Concrete Structure’ in China regarding effect of long term action on deflection.

As known, the creep deflection increases as the load increases. The difference of the applied load of the tested beams is very small and also the self-weight accounts for half of the total load. Therefore, the creep deflection of beam specimen sc-3 is about 94% of that of beam specimen sc-1, while the deflection creep coefficient is almost the same, which indicates that the concrete creep developed during this test is within the linear creep range.

The creep deflection of continuous beam after 1 year is only 40% of that of the simple beams, indicating that the increase in degree of indeterminacy can dramatically reduce the creep deflection. On the other hand, the deflection creep coefficient is 1.6, it is almost the same for both types of beams due to smaller instantaneous deflection of continuous beam.

4 Conclusions

1) An increase in reinforcing ratio can reduce shrinkage deformation. The shrinkage strain of self- compacting concrete increases as the age increases. The shrinkage strain curves converge after 6 months.

2) Under the same environmental conditions, the shrinkage—time curve of self-compacting concrete is very similar to that of normal concrete. The shrinkage strain of self-compacting concrete after one year is about 450×10^-6.

3) The deflection creep coefficient of self-     compacting reinforced concrete beams after 18 months is about 1.6, which is very close to normal reinforced concrete beams cast with vibration. Extra cautions considering shrinkage and creep behavior are not needed for the use of SCC in engineering practices.

References

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(Edited by YANG Hua)

Foundation item: Project(50278097) supported by the National Natural Science Foundation of China

Received date: 2007-10-19; Accepted date: 2007-12-28

Corresponding author: LIU Xiao-jie, Associate professor; Tel: +86-731-2656842; E-mail: Liuxj@mail.csu.edu.cn