Effect of Re-vibration Process on Axial Capacity of Short Reinforced Concrete Circular Columns

- A considerable amount of research work has been performed on the effects of vibrating of fresh concrete on the reduction of shrinkage and creep, the improvement bond stress between reinforcing bar and concrete, reduction the concrete permeability and improvement of the mechanical properties of concrete (tension and compression). Series of tests on reinforced concrete circular column, cubes and cylinders were carried out to study the effect of re-vibration duration on axial strength of column, compression strength of concrete cubes, and tensile strength of cylinders. Different compressive strengths of concrete and different size of aggregate were considered in this investigation. The test results show that, the re-vibration operation improves the tensile and compressive strength of concrete. The stiffness of columns increased with increasing the re-vibration duration up to1.5 times the initial vibration duration. Size of aggregate has significant effect on the improvement properties of concrete due to re-vibration. Increase the time duration of re-vibration delay the appearance of first crack.


Introduction
Repeating the operation of vibration of the fresh concrete after a period, which is called revibration, which may be beneficial to improve the properties of concrete especially when successive layer of fresh concrete was placed and the upper layer of fresh concrete was partially hardened. Re-vibration after time re-arranged the aggregate particles and eliminates entrapped water, which may improve the tensile strength, compressive strength and bond strength between the reinforcing bar and the concrete. Plastic shrinkage cracks for the exposed concrete can also be eliminated by the operation of re-vibration. Revibration affect significantly by the time duration of re-vibration. Auta et al. 1 studied the flexural strength of revibrated reinforced concrete beam with saw dust ash as partial replacement for cement. They tested seven beams of size 150 x 150 x 600 mm reinforced with 12 mm diameter steel bar. The beams were re-vibrated for 20 seconds at an interval of 10 minutes successions up to one hour after initial vibration. They concluded that the revibrated had improved the flexural strength of reinforced beams. Rao et al. 2 investigated the effect of re-vibration on compressive strength of concrete with a wide range of water to cement ratio varying from 0.35 to 0.7 and with more numbers of re-vibration time ranging from 0.5 to 4 hours. The results show that the compressive strength of concrete increased with re-vibration up to certain time then decreased thereafter. Abdel Rahman 3 tested beams of size 100 x 100 x 500 mm to study the effect of vibration and revibration on the flexural strength of beams. They concluded that the re-vibration improved the flexural strength of concrete within the first one hour. Kassim 4 studied the influence of revibration on the compressive strength of retarded cylinders concrete examined at the time interval ranged from 2 to 8 hrs. The results showed that the maximum compressive strength of concrete is achieved after 2 hrs and 35 minutes. Hashim 5 studied the effect of internal and external vibration on compressive strength of concrete. It was noticed that the internal vibration is more effective on the compressive strength of concrete than the external vibration. Gamal 6 studied the effect of re-vibration on the quality of concrete. The compressive strength of concrete was measured by 150 x 150 x 150 mm cubes and bond strength was measured by pull-out test using cylinder 150 mm in diameter and 300 mm in length. It was showed that the increase in compressive strength and bond strength in revibrated concrete and the maximum gain in compressive strength and bond strength occurred when the re-vibration was at the initial setting time. Larnach 7 studied the effects of external initial vibration and then re-vibration on the bond and compressive strength. He found that the external re-vibration produced reductions in bond strength ranging from 6% for re-vibration after 1/2 an hour to 33% after 3 hrs. In addition, the reduction in compressive strength was 14% and 16% for the same time above.
According to the review of previously experimental results, there is no clear understanding to the effect of time duration of revibration on the compressive and tensile strength of concrete, so, the main objectives of this study are:  Study the effect of re-vibration duration on the compressive and tensile strength of concrete through testing circular columns, concrete cubes, and concrete cylinders.  Study the modes of failure of circular concrete column subjected to axial compressive force after different re-vibration duration.  Determine the optimum period of re-vibration that improved the mechanical properties of concrete.

Experimental Program
At the laboratories of Building and Construction Engineering Department (UOT), experimental program was conducted as follows: Twelve concrete circular columns, thirty-six cubes and thirty six cylinders in three groups were tested. In each group, the first specimen was initially vibrated with 20 second, second specimen was re-vibrated with 20 second, third specimen was re-vibrated 30 second and finally, the fourth specimen was re-vibrated with 40 seconds. In Group A, the designed compressive strength of concrete was 35.26 MPa and maximum size of aggregate was 10 mm. In Group B, the designed compressive strength of concrete was 35.95 MPa and maximum size of aggregate was 19 mm. For Group C the designed compressive strength of concrete was 63 MPa and maximum size of aggregate was 10 mm. The characteristics tested specimens in each group were presented in Table  1.
It may be noted that, the waiting time before the process of re-vibration in all specimens was selected to be one hour. This is due to the fact that, the waiting time should be not more than the initial setting time of concrete (1.22 hrs). The initial setting of concrete was investigated in the present study according to Iraqi specification No.5/1984 8. MPa for group C. Three samples of 150 x 150 x 150 mm cube were used to determine the strength of concrete in compression at 28 days according to ASTMC39 [10]. Three cylinders of 150 mm in diameter and 300 mm in length were used to determine the strength of concrete in tensile according to ASTM C496 11. The mix design compositions for each group are listed in Table 2.

Column Specimens Description
The tested specimens (circular short column) had the circular cross section of 150 mm in diameter and 600 mm in length. These columns were reinforced with the 6-8mm longitudinal bars and 7-4mm stirrups uniformly distributed. The yield stress of main reinforcement (8) was 435 MPa and the ultimate tensile strength was 550 MPa. These values were measured according to ASTM A996M-05 [12]. The concrete cover in all sides was taken to be 30 mm. Figure 1 and 2 shows the details of tested specimens.

Curing Process
After cast the concrete, the specimens were laid in water tank for 28 days with a temperature of 22 C o . Then, the specimens were laid in laboratory till the date of testing.

Test Setup and Test Procedure
The tests for column specimens were conducted using a hydraulic machine of 2500 kN as shown in Figure 3. Top and bottom compressive forces were applied to the tested specimens (columns) through the (7 cm) thick steel plate. The calibrated load cell in hydraulic machine was used to measure the applied load. Dial gages were used to measure the vertical displacement at the top of specimen. The applied vertical load was increased incrementally with (5 kN). The cubes were tested according to BS1881-116 13 using hydraulic jacks of 3000 kN with loading rate increments of 6 kN/sec, Figure 4 shows the concrete cubic under test. The cylinders were tested according to ASTM C496 11 using hydraulic jacks of 3000 kN with loading rate increments of 0.94 kN/sec, Figure 5 shows the concrete cylinder under test.

I. Effect of re-vibration duration on behavior of specimens of Group A
In this section, the circular columns, cubes and cylinders of group A were tested with initial vibration of 20 second and with 20, 30 and 40 second re-vibration. The results are listed in the following sections.

1) Axial Strength
According to test results in The main scope of this study was study the effect of re-vibration duration on the strength of concrete column. This was depicted in Figure 7 through the relation between the axial strength of column and the re-vibration duration, in which, the maximum axial strength of the column was at the 30 second re-vibration (1.5 times the initial vibration duration). During the testing of the circular column, the load at which the first crack appears was recorded and the test results were listed in Table 3. The first crack appeared at approximately of 53% to 55% of the ultimate load. The re-vibration, delay the appearance of the first crack especially at the 30 second re-vibration duration (optimum value of time duration of improving the axial strength of column). Figure 8 shows the location of the first crack for each specimen.

2) Compressive and Tensile Strength of Concrete
Cubes of 100 mm edge length were used to measure the compressive strength of re-vibrated concrete and cylinders of 100 mm diameter and 200 mm length were used to measure the tensile strength of re-vibrated concrete. These specimens have the same re-vibration duration of the corresponding main specimens (circular columns). The compressive strength of cubes were tested according to BS1881-116 and the tensile strength of cylinders were tested according to ASTM C496, the results are listed in Table 4, in which, the compressive and tensile strength of concrete increased with the duration of re-vibration increasing till 1.5 times the initial vibration duration, then compressive and tensile strength decreased. Figure 9 and 10 shows the cubes and cylinders at failure.

II. Effect of Re-vibration duration on behavior of columns of Group B
The specimens of group B have a maximum size of aggregate of 19 mm. The columns, cubes and cylinders were tested with initial vibration 20 second and with 20, 30 and 40 second re-vibration. The results are discussed in the following sections.

1) Axial Strength
According to test results in Table 5, the axial strength of the reference column (Ref.B-V20), which was initially vibrated with 20 seconds, was 660 kN. The re-vibration of the column with duration of 20 seconds increased the axial strength of column by 3%. Increase the re-vibration from 20 to 30 and then to 40 second, decrease the axial strength of column by 2.9% and 5.8% respectively. Figures 11 represent the axial load-longitudinal displacement relationship. These relations show at the beginning linear behavior, then, nonlinear relation till the maximum strength. From Figure 12, the maximum axial strength of the column occurs when the re-vibration was only one times the initial vibration (i.e 20 second); this is due to fact that, the increase in the size of aggregate affects in passive way on the re-arranged the aggregate particles. The first crack appears at a load approximately 50% to 56% of the maximum load. Table 5 shows the values of loads at which the first crack appear, in which, the maximum load at which the first crack appears occurs at 20-second re-vibration. Figure 13 shows the location of the first crack for each main specimen.

2) Compressive and Tensile Strength of Concrete
Concrete cubes and cylinders have the same mix design of the main specimens (columns) were used to study the effect of re-vibration duration on the compressive and tensile strength of plain concrete. Table 6 listed the compressive strength of cubes and tensile strength of cylinders under 0, 20, 30, and 40 second re-vibration duration. Test results show that compressive and tensile strength of plain concrete increased when the re-vibration duration equal to one times the initial vibration and decrease with 1.5 and 2 times the initial vibration. Figure 14 and 15 shows the cubes and cylinders at failure.

III. Effect of Re-vibration Duration on Behavior of Columns of Group C
In this section, the circular columns, cubes and cylinders of group C (specimens with compressive strength of concrete f cu =63 MPa) were tested first with initial vibration of 20 second and then with 20, 30 and 40 second revibration, the test results are listed in the subsequent sections.

1) Axial strength
As mentioned before, the main tested specimens in group C have re-vibrated with 0, 20, 30 and 40 second for Ref.C-V20, C-Dur.1, C-Dur.2, and C-Dur.3 specimen respectively, the test results were listed in Table 7.  The axial strength of the reference column was 870 kN. The re-vibration of circular column increased the axial strength of the column by 3.4% and 16.6% when the re-vibration was 20 and 30 second respectively. While, increase the duration of re-vibration to 40 second decrease the axial strength of the column by 13.4% in comparison with 30 second re-vibration duration. The relation of load-longitudinal displacement was depicted in Figure 16. From these Figures, the stiffness of load-deformation behavior increases with the re-vibration duration of 20 and 30 second, then decrease for 40 second revibration. Figure 17 shows the relation between the maximum strength and revibration duration, in which, the maximum axial strength of the column occurs at re-vibration duration equal to 1.5 times the initial vibration. In addition, from Table 7, the cracks appear at a load of 50% to 55% of the ultimate strength of main specimens. Increase the re-vibration duration from 20 to 30 seconds delay the appearance of the first crack. While, when the revibration duration was 40 second, the crack appears faster than the 30 seconds re-vibration. Figure 18 shows the first crack location in the main specimen.

2) Compressive and Tensile Strength of Concrete
According to test results in Table 8, the tensile and compressive strength of plain concrete increased when the re-vibration duration was 20 and 30 second in comparisons with the 0% revibration of the reference specimen (Ref.C-V-20). Maximum value of tensile and compressive strength was at 30 second re-vibration. While, these strength decreases when the re-vibration duration was at 40 second. Figure 19 and 20 shows the cubes and cylinders at failure.

IV. Modes of Failure
For columns in all groups, before failure, the ties prevent the concrete core and longitudinal reinforcement from moving outwards due to confinement effect. While, at failure, the concrete crushed outwards and the longitudinal reinforcement is buckled between the ties, no being so confined. The outer shell of concrete column spall off, which represent the typical failure of circular column. Figures 21 through 23 shows the failure of columns of group A, B, and C respectively. Figure 24 shows all columns together at failure.    2) The stiffness of columns increased with increasing the re-vibration duration until 1.5 times the initial vibration duration.
3) Optimum time duration to improve the properties of concrete due to re-vibration depend on the size of aggregate. Using 10 mm maximum size of aggregate, improve the tensile and compressive strength of concrete when the revibration was 1.5 times the initial vibration duration. While, Using 19 mm maximum size of aggregate, improve the tensile and compressive strength of concrete when the re-vibration was 1 times the initial vibration duration. 4) The improvement of mechanical properties of concrete remained significant when using concrete with high compressive strength. 5) Increasing the re-vibration duration delayed the appearance of first crack.