BOND STRENGTH OF BAR USING GROUTING FOR PRECAST CONCRETE CONNECTION

In precast concrete, a connection is needed to unite the components so that they become a whole unified structure. This study aims to determine the reinforcement strength and length of reinforcement in precast concrete connections. To paste reinforcement into precast concrete, giving additional material in the form of grouting which is called sika grout 215 and functions as an adhesive is necessary. Pullout testing is carried out in the laboratory, and its simulation by modeling uses the finite element method based software. This research is divided into 2 phases. The first phase is making specimen to examine the bond strength between the concrete and reinforcement that has been given sika grout 215. So monolithic specimen is made as a comparison. The result of the bond strength of the monolithic test specimen is 6.24 MPa, and the sika grout 215 category is 6.52 MPa. From the experimental results in the laboratory with modeling, it is obtained the bond strength ratio of 0.94. The length of development (ld) based on the results of the testing phase I of 200 mm. The second phase is examining the damage pattern due to the stress that occurred. Specimens are made into 4 categories, namely modeling developments with the length of 120 mm (<40% ld), with the length of 160 mm (<20% ld), with length of 200 mm (= ld), and with the length of 260 mm (> 30% ld) both for monoliths and sika grout 215. The damage pattern, which is in the form of yielding and breaking reinforcement as the result of the pullout experiment in the laboratory shows not much different from the result of simulation using the software.


INTRODUCTION
The precast concrete structure as fabricated components are connected one and another in the work location in order to make the precast component form a complete structure. The precast concrete components assembling requires specific connection techniques. The most common connection method in precast construction is by implanting more extended reinforced steel that forms a pass through the hardened precast concrete. Then an adhesive substance is added to unite the reinforcement and the concrete (Hosseini, 2015, andAbd. Rahman, 2015). The connection is considered stable if there is no slip between the reinforcing steel and the concrete so that the exact length of reinforcement is needed, as well as the durable adhesive substance, to hold the tensile force on the connection (Rosyidah, 2011, andLu, 2012) In order to get the best bonding strength between the hardened concrete and reinforcement requires a robust adhesive substance. Expectedly, the adhesive material in the form of grouting can increase the bond strength between concrete and reinforcement with its advantages in its fast initial strength, resistant to shrinkage compensated, high resistant strength, non-corrosion and nontoxic (Lu, 2017, andRaynor, 2002).
Accepted : 13 August 2019 The bond strength is the maximum bonding value between the reinforcement and its surrounding concrete (Paulay and Priestley, 1992). This adhesive plays an essential role in designing the concrete structure. The factors that influence this stickiness are quite numerous and complex. The factors that determine this bond strength, namely adhesion, friction, and interlocking force (Xing, et al., 2015). In thread reinforcement, besides bond and friction, there is an interlocking force that occurs in reinforcement and concrete. This force causes more exceptional sticking ability of threaded reinforcement compared to plain reinforcement (Xing, et al, 2015, Feldman and Bartlett, 2007, and Hong and Park, 2012. The adhesive stress value of pullout testing can be calculated as adhesive stress along with the embed of reinforcement in concrete (Harajli, et al, 2004, and Wu and Zhao, 2012. The equation of the bonding stress surface average along the reinforcement inserted in the concrete is presented in eq. (1). P DL     (1) P = pullout, D = reinforcement diameter, L = length of reinforcement adhered to the concrete.
In order to get the best result, both the experimental testing and modeling using finite element software. Then, the result of the experimental test including bond strength, length of development and pattern of specimen collapse is compared to the one through modelling.

METHODS
This research is divided into 2 phases. Phase 1 is to examine the bond strength between the concrete and reinforcement given the added grouting material. Phase 2 is to examine the length of development (ld) of reinforcement embedded in the concrete along with the damage pattern that occurs. The specimens used to obtain the bond strength of reinforcement in concrete by grouting use a cubed form standard with a size of 200 × 200 × 200 mm. The concrete used in ready-mix concrete with a quality of 25 MPa. Concrete with its reinforcement, sika grout 215 grouting material is used. The hole in the concrete is formed from a 1-inch diameter pipe at the casting process. When the concrete hardens, the hole is then filled with reinforcement and grouting with a thickness of 11,025 mm ( Figure 1). Reinforcement used is deformed steel with a diameter of 10 mm with BJTS 50 steel quality. The specimens are made as many three pieces. As a comparison, concrete specimens are also made with reinforcement, which is cast monolithically. The same specimens are then modeled in finite element software so that the results of experiments and modeling can be compared.

The Bond Strength
The calculation results of the bond strength of the monolith (M) model, and the one using grouting (SG) are obtained, as shown in Figure 2. grouting has a higher bond than the one at the monolithic specimen.

Result of Development Length of Model Specimen
The development length value of the monolithic specimen model is 194.25 mm, and sika grout of 215 is 193.43 mm. The results of the development length of monolith and sika grout specimens are relatively equal. The result of the specimen using sika grout of 215 only needs a slightly shorter development length than the monolithic specimen. For more details, the development length difference between monolithic specimens and Sikagrout grouting of 215 can be seen in the diagram of the development length requirements in Table 1.

The pattern of Specimen Damage with Development Length (<40% ld) 120 mm.
The stress value at the model is shown to identify the damage occurs. The SGa specimen is yielding at the load of 44970 because the occurred stress value has been over the limit of yielding stress 505,164 MPa.
From the value of the bond strength obtained, the concrete does not undergo slip because both the bond strength of the monolith or sika grout specimen are higher than the value of the concrete stress or the occurred grouting. The occurred damage pattern is different from the pattern of damage to the research. It slips on the Ma monolithic specimen and the SGa broken specimen, as seen in Figure 4. The results of the stress identification at Mb specimen has the maximum stress of 524,623 MPa at step 31 with the load of 39800 N ( Figure 5). The specimen of SGb 556,886 MPa at step 21 with the load of 40220 N. Mb undergoes yielding at 37030 N, and SGb undergoes yielding at the load of 37870 N, because of the value of the stress that occurs is over than the yielding stress limit. The specimen of Mb is increasing when the concrete fault due to the load of 8.10 mm at a maximum load of 39800 N. The modeling of SGb specimens undergoes the concrete fault of 7.5 mm during the same conditions with the load of around 39800 N. From the length comparison, the reinforcement of the SGB test using sika grout has smaller change than the Mb of the specimen made by the monolith.
The pattern of damage occurs at 160 mm variation of test specimens for Mb, and SGb monolithic specimens are yielding on reinforcement. It is different from the pattern of damage in the research which undergoes slip on the monolithic Mb and SGb specimens, as shown in Figure 6. It is caused the broken reinforced steel, and its stress has been over the steel fracture stress limit, which is 632,042 N.
The concrete does not undergo slip since the bond strength of both the Ma and SGa specimens is bigger than the value of the concrete stress or the occurred grouting. The pattern of damage for 200 mm variation specimens for Mc test specimens is yielding on reinforcement while in SGc specimen undergoes breaking on reinforcement. This pattern of damage is almost identical to the pattern of damage in the research, as shown in Figure 8. However, there is still yielding on Mc specimen. The maximum stress of Md reaches on 461.07 MPa. It is on step 14 with the load of 29140 N. After passing through the load, the stress on Md cannot be read because of yielding and breaking, and the SGd specimen reaches the maximum stress of 606.738 MPa in step 21 with the load of 43790 N. The SGd undergoes yielding on the load of 34450 N which is caused by the over-limit value of stress.
Form the value of the bond strength obtained; the concrete does not undergo slip because the bond strength of both Ma and SGa specimens is more significant than the value of the concrete stress or the occurred grouting. The damage pattern occurred to 260 mm specimen type of Md undergoes breaking on the reinforcement while the specimen of Sgd undergoes yielding on the reinforcement, The damage pattern occurs slightly different from the damage pattern in the research which undergoes slip on the monolith Md and Sgd specimen undergoes fracture on the reinforcement, as shown in Figure 10.

CONCLUSION
The conclusion can be drawn from the specimen experiment and modeling as follows: The bond strength of the monolith specimen is 6.24 MPa, and the bond strength of specimen using sika grout 215 is 6.52 MPa. The ratio of the results in the model experiment is 0.9.
The bond stress of the specimen with additional Sikagrout of 215 is as good as the bond stress produced by reinforced steel, which is directly cast together with the concrete or monolith. It indicated by the equal value of the bond stress and the development length needed for each development length variation. The damage pattern occurred is the yielding of reinforcement due to the value of stress occurred in the modeling over the limit of the breaking value of reinforced steel stress.
The occurred bond stress is varied when the reinforcing steel reaches yielding at an equal diameter which has been caused by the area of contact reinforced steel concrete that is also varied because of the different length of the development. The smallest bond stress is 5.773 MPa, and the biggest one is 9,803 MPa, while the force needed to achieve reinforcement steel up to yielding is relatively equal, around 34450 N -37870 N for the equal quality and diameter of reinforced steel.