Abstract
The large size concrete structures are continuously placed with time delay due to practical reasons. The bond strength between old concrete and new concrete is related with a performance of concrete structures. In order to investigate bond strength of slant shear specimens, the specimens (100 × 100 × 300 mm) were fabricated and tested. Two fractions of setting retarder (0% and 1%), surface roughness (none, brushed, and chipping), and curing hours (8, 16, 24, and 72 hours) were considered as variables. Failure modes, compressive strength, and displacement were measured to evaluate the bond strength of specimens. As a result, chipping treatment, which is commonly applied to increase the bond strength between old and new concrete, did not show significant increase of bond strength. The addition of retarder shows that the increased bond strength compared with the chipping treatment. The addition of the retarder in concrete shows sufficient bond strength after the previous placing of 16 hours. Furthermore, the retarder-added specimens with additional brushing treatment of concrete surfaces showed increase of bond strength more than the specimens without brushing. From the test results, it is confirmed that the addition of the retarder can be a new method to increase the bond strength of the concrete surface in continuous placement of concrete.
1. Introduction
In a construction field, the concrete is commonly placed at different time intervals because of the limited form work process. It is also known that the bond strength between the old concrete and new concrete is considerably related to the performance of buildings [1–7]. Therefore, many researchers have conducted various studies such as the slant shear test, pull out test, and bisurface test of concrete surface [8–12].
Hu et al. [8] conducted the slant shear test to investigate the bond strength of old and new concrete. All specimens have 70.4 mm of diameter and 144 mm of height. The slant angles (30°, 40°), the depths of valley (1.2, 2.4 mm), the ages of old concrete (60, 120 days), and the loading rates (0.2 kN/s; quasistatic loading, 10.79, 14.7, 18.19 m/s; dynamic loading) were considered as variables. Total 95 cylindrical specimens were tested in different variable conditions. As a result, the bond failure occurred in specimens with a slant angle of 30°. When the slant angle is changed from 30° to 40°, the bond strength between new and old concrete increased. The tests were carried out in both quasistatic and dynamic loading conditions, and the significant difference of failure modes could not be found in both tests.
Tian et al. [9] fabricated 100 × 100 × 300 mm slant shear specimen and tested to investigate the bond strength between normal concrete and ultrahigh performance concrete. The slant shear specimens have 60° of slant angle. The compressive strength of normal concrete (30, 40, and 50 MPa) and interface groove densities (0.000, 0.133, 0.266, and 0.399) were considered as variables. As a result, the bond strength between NC (normal concrete) and UHPC (ultrahigh performance concrete) was increased, while the compressive strength of normal concrete was increased. Furthermore, with the increasing groove density, the bond strength gradually increased.
Al-Osta et al. [10, 11] investigated the bond strength of UHPC to NC interface. In order to investigate the bond strength, the slant shear test, bisurface test, splitting tensile strength test, and four-point flexural strength test were conducted. The NC surface roughness (as cast, drill hole, and sand blasting) was considered as a variable. The test result confirmed that the surface preparation between old and new concrete can be used to increase the bond strength of concrete.
Pereira Prado et al. [12] carried out the slant shear test, push-off test, and splitting tensile test. The test specimens have interface between HSC (high-strength concrete) and UHPC. The surface roughness such as exposed coarse aggregate, exposed fine aggregate, smooth, shear key, and expanded mesh. As a result, the specimens with shear key show a higher value of bond strength in the slant shear test. While the shear key showed good performance of bond strength, the exposed coarse aggregate shows appropriate performance of bond strength in all tests accompanied with compressive failure of specimens.
The previous studies showed that the surface treatments of concrete can increase the bond strength between the old concrete and new concrete, but these treatments have limitations of efficiency for the practical use. In addition, the surface treatment methods are not proper to the automatic construction process because these methods need an additional process after the previous placement of concrete.
In this study, adding retarder in concrete was investigated to figure out that it can be used to increase the bond strength between old concrete and new concrete for the continuous placement. Because commonly used methods such as chipping, brushing treatment cannot be applied in automatic construction process. The slant shear tests were carried out with the considerations of addition of the retarder, surface treatments, and curing hours in order to compare the differences of the bond strength among the various specimens.
2. Experimental Program
2.1. Test Methods
The slant shear test was conducted to evaluate the bond strength of the specimens. 100 × 100 × 300 mm prismatic specimens were fabricated. As shown in Figure 1, the specimens had slant slope θ of 30° from vertical. In order to make slant slope, the new concrete casted after starting the setting of the old concrete. The mix proportion of the new and old concrete is shown in Table 1. The setup for the slant shear test is shown in Figure 1(a). The formwork setup for slant specimens is shown in Figure 1(b). The compressive load was applied to the specimens under static loading rate of 0.02 mm/s by using the actuator which has a capacity of 10,000 kN. As shown in Figure 2, the LVDT (CDP-10) which has a capacity of 10 mm was attached on the specimen with slant angle of 30° to measure a relative displacement between the old and new concretes.
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The specimens with a diameter of 100 mm and height of 200 mm were fabricated in accordance with ASTM C39. The specimens were made of setting retarder added concrete and normal concrete, respectively. The cylindrical specimens were tested to measure the compressive strength after curing the specimens for 7 and 28 days based on ASTM C39.
2.2. Mix Proportions and Variables
The mix proportions are presented in Table 1. Type 1 Portland cement, fly ash, and blast furnace slag were used as a binder. The blast furnace slag and fly ash were used to increase the durability of concrete. Several studies reported that the addition of blast furnace slag and fly ash can improve durability of concrete such as reduction of drying shrinkage, inhibition of alkali aggregate reaction, and increase of water tightness [13–15]. The maximum aggregate size was 10 mm. INFRACON-R4 (Samyoung Oil and Chemical Inc.) was used for the setting retarder. INFRACON-R4 is admixture of lignin-themed organic acid compositions. INFRACON_R4 can be used to improve the workability of concrete.
Adding proportion of the setting retarder (1%) was considered as variables in this study. The retarder was related with amount of cement, and the proportion was mass percent. It is known that the addition of the retarder can improve the bonding condition between old and new concrete surface when the concrete was continuously placed. However, the placement of concrete should be completed in two hours after adding the retarder into the concrete in accordance with ACI 347 because using over dosage of the retarder in concrete causes the segregation of concrete [16]. In many cases of continuous placement, the chipping method is practically used to increase roughness of the previously placed concrete surface. But the chipping method has difficulties for the automatic construction process. In this context, the study for using the retarder in continuous placement of concrete is demanded and the design code for using a retarder is to be suggested during the automatic construction process.
In this study, the addition of the setting retarder, roughness of new-to-old concrete surface, and curing hour of old concretes were considered as variables. As illustrated in Figure 3, the specimen ID shows concrete types, roughness configuration, and curing hours. The sample notation of the specimen ID is noted at Figure 3.
3. Test Results
3.1. Compressive Strength
The compressive strength of the old and new concrete is shown in Figure 4. The old concrete specimens had the setting retarder of 1%, while the new concrete specimens were fabricated without the retarder. The test was carried out on the four specimens with a loading rate of 0.02 mm/s in accordance with ASTM C39. The design compressive strength of the old and new concrete was 40 MPa. For the new concrete, the 7-day-old compressive strength developed approximately 84% of the design strength that was 33.5 MPa. The 28-day-old compressive strength of the new concrete was 49.5 MPa. The ratio of the 28-day-old strength to the design strength was approximately 124%. In case of the old concrete, strength development of 7-day-old specimen was approximately 86% of the design strength and the strength was 34.5 MPa. 28-day-old compressive strength developed approximately 129% of the design strength that was 51.5 MPa.
3.2. Failure Modes of Slant Shear Specimens on the 7th Day
The typical failure modes are depicted in Figure 5. The failure modes of the slant shear specimens are shown in Table 2. The failure modes can be classified into the typical modes such as compressive failure and bond failure. The nontreated specimens (R_N_8, 16) and the brushed specimens (R_B_8, 16) failed in compressive failure. These failure modes were affected by increased bond strength between old concrete and new concrete. Because the addition of the retarder prolonged the setting time of the old concrete, then the concrete surfaces between old concrete and new concrete were blended together. The failure modes of the other specimens (R_N_24, 72, R_B_24, 72, N_N_8, 16, N_C_16) were bond failure.
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3.3. Failure Modes of Slant Shear Specimens on the 28th Day
The failure modes of the slant shear specimens on the 28th day are shown in Table 3. The failure mode of nontreated specimens (R_N_8, 16, 24), the brushed specimens (R_B_8, 16, 24), and the control specimen (N_N_8) was the compressive failure. The specimens (R_N_24, R_B_24, and N_N_8) also showed the compressive failure on the 28th day, while the same specimens failed in the bond failure mode on the 7th day. This is because the bond strength was developed faster than the compressive strength during 28 days. The failure modes of the other specimens (R_N_72, R_B_72, N_N_16, and N_C_16) showed the bond failure. For the practical use of automatic concrete placement, the compressive failure is an appropriate failure mode instead of bond failure. This is because increased bond strength between old concrete and new concrete shows efficient performances when unexpected transverse loading condition. As shown in Table 3, a significant difference cannot be shown in the failure mode among the existing surface treatment methods such as wire brushing and chipping. Therefore, it can be confirmed that the failure modes were governed by the setting time of concrete.
3.4. Bond Strength of Slant Shear Specimens on the 7th Day
The slant shear tests have been conducted to measure the bond strength of specimens. The test results of the 7th day and 28th day are shown in Figure 6 and 7. The figures show compressive strength-displacement curves of the specimens. The stroke of the actuator is used as the displacement of the tests in the figures. Every figure has a black horizontal line that has the same value of 200 kN in Figures 6 and 7. The horizontal line, which means the half of design compressive strength, is approximately suggested as a reference value for the convenience of comparing between the curves.
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Figure 6(a) shows a difference between the control specimen (N_N_16) and the chipping-treated specimen (N_C_16) on the 7th day. The bond strength of N_N_16 and the bond strength of N_C_16 were 108.9 kN and 156.3 kN, respectively. Both specimens failed in bond failure. The maximum load of the chipping-treated specimen (N_C_16) shows a 43.5% higher value than the value of the control specimen (N_N_16). It showed a significant increase in the maximum load, but it could not reach the reference value. Therefore, the chipping treatment should be carefully applied when the concrete is continuously placed.
Figures 6(b) and 6(c) depict differences between control specimens (N_N_8, N_N_16) and the retarder-added specimens (R_N_8, R_N_16) on the 7th day. The bond strength of R_N_8 and the bond strength of R_N_16 were 247.9 kN and 199.2 kN, respectively. The addition of the retarder resulted in considerable increases of 64.2% and 83.0% in the maximum load of the specimens, compared to the control specimens (N_N_8 and N_N_16). The retarder-added specimens show enhanced performance; even the new concrete is placed after 16 hours of the first placement of old concrete.
Figures 6(d)–6(g) describe differences between the retarder-added specimens and the brushed specimens on the 7th day. The increases of 2.7%, 19.3%, 59.2%, and 6.3% are shown in the maximum load comparing brushed specimens and the retarder-added specimens. The value of R_B_24 showed a significant increase in bond strength. It means that the brushing treatment of surfaces can be used until 24 hours after the previous placement.
3.5. Bond Strength of Slant Shear Specimens on the 28th Day
The test results of specimens after curing 28 days are illustrated in Figure 7. Figure 7(a) depicts a difference between the control specimen (N_N_16) and the chipping-treated specimen (N_C_16). The specimens failed at 180.4 kN and 146.8 kN in bond failure, respectively. While the additional chipping treatment was applied to surfaces of the specimen (N_C_16), but the significant increase in the bond strength could not be found.
Figures 7(b) and 7(c) describe differences between control specimens (N_N_8 and N_N_16) and retarder-added specimens (R_N_8 and R_N_16). The maximum load of the retarder-added specimens (R_N_8 and R_N_16) shows 10.8%, 21% higher value than the value of the control specimens (N_N_8 and N_N_16), respectively. As a result, it can be noted that the addition of the retarder can increase the bond strength between concrete surfaces.
Figures 7(d)–7(g) show differences between the retarder-added specimens and the brushed specimens on the 28th day. The brushing treatment in surfaces resulted in increases of 2.7%, 16.2%, 37%, and 6% in maximum load of the brushed specimens (R_B_8, R_B_16, R_B_24, and R_B_72), compared to the retarder-added specimens (R_N_8, R_N_16, R_N_24, and R_N_72).
Figure 8 illustrates compressive strength-relative displacement curves on the 28th day. The compressive strength is the same value of Figure 7. The relative displacement was measured to estimate the slip between the surfaces by LVDTs as shown in Figure 2(b). However, relatively small slips between the surfaces were measured in the tests. Therefore, the relative displacement can be ignored when investigating the bond strength between the old concrete and new concrete.
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4. Conclusion
This study investigated the bond strength of old and new concretes considering the addition of the setting retarder, surface roughness between old and new concretes, and curing hours of old concrete. From the previous results, the following conclusions are drawn: the chipping treatment of concrete surface is a commonly used method to increase the bond strength between the old and new concrete surfaces. The chipping treatment could not show significant increase in bond strength in this study. However, it can be concluded that the chipping treatment can be used as a surface treatment to increase the bond strength between the old and new concrete.
This study suggested the new method, that is, the adding retarder to improve the bond strength between the old and new concretes. The setting retarder added method showed significant increase of bond strength rather than chipping treatment. Furthermore, the 8-hour-aged and 16-hour-aged specimens that contain the retarder failed in compression. Based on the experimental results, the addition of the retarder can be used to increase the bond strength until 16 hours after the placement of old concrete.
Brushing treatment on the setting retarder added concrete shows significant increase of bond strength in comparison with only the setting retarder added method. From the test results of the brushing method, it is confirmed that the additional treatment on concrete surface can be valid in 24 hours after the previous placement.
The setting retarder added method is highly efficient in the construction process compared with the general method such as chipping a surface of concrete. However, it is not allowed to use the setting retarder for delaying the setting time more than 2 hours in ACI code, although the setting retarder added method shows the increase of the bond strength in this study. Therefore, further research is needed for practical use of the setting retarder as a new method to increase the bond strength between old concrete and new concrete. Moreover, the code is to be revised to allow the use of the retarder for the continuous placement of concrete, based on the accumulated experimental results.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This research was conducted with the support of the “National R&D Project for Smart Construction Technology (21SMIP-A158708-02)” funded by the Korea Agency for Infrastructure Technology Advancement under the Ministry of Land, Infrastructure, and Transport and managed by the Korea Expressway Corporation and the support of the “National Research Foundation of Korea (NRF-2020R1F1A 1050623).”