Rebar coupler or mechanical splicing is the latest technology which is far better than the traditional lapping system. However, if quality rebar couplers are not procured and quality workmanship is not ensured it may be more disastrous than traditional lapping system.

Tensile strength test

Rebar must be tested to confirm the strength of rebar itself. If the rebar strength and elongation is less than desired value, rebar brand must be changed before testing rebar coupler. If the rebar strength is excessively higher than required strength, the rebar coupler strength test criteria carefully judged.
Failure of rebar at a distance from coupler face: this is the usual case for a good quality rebar coupler fitted to a good quality rebar. This indicates that rebar coupler strength is more than rebar strength. So, the rebar coupler is declared satisfactory.
Failure of Coupler: rebar coupler may fail before the rebar failure for many reasons
- Rebar coupler materials strength is low: if the rebar strength is as per code, rebar coupler fails before rebar failure, the rebar coupler shall be rejected.
- Threading of rebar is not proper: neither rebar nor rebar coupler fails; rather rebar is slipped out from the coupler due to improper threading of rebar. Threading of rebar must be changed.
- Threading of coupler is not proper: neither rebar nor rebar coupler fails; rather rebar is slipped out from the coupler due to improper threading of coupler. Rebar coupler must be changed.
- Excessively strong rebar: if the rebar strength is excessively higher than its grade, the rebar coupler may fail by slippage at thread or tearing of coupler materials. If the failure load of the coupler > 1.25*Fy*As (As = cross-sectional area of rebar, Fy = yield strength of rebar taken from its grade), the rebar coupler quality is satisfactory.

Slip test

The slip test is important to ensure seismic performance of rebar coupler. Excessive slip will cause crack in concrete leading poor seismic performance of the structure.

Two rebar parts are joined together using rebar coupler
Gauge length is marked, and extensometer is set up
The sample is loaded up to 0.6*Fy*As, and unloaded to 0.02*Fy*As
Gauge length is measured before loading and after unloading. It may be directly read from the extensometer. If the permanent deformation is less than 0.1 mm, the rebar coupler is satisfactory

Low Cycle Loading Test

ASTM A1034, ICC AC133, and ISO 15835 all provide similar guidelines for low cycle tension-compression testing of couplers. Tensile strength and slip tests are sufficient to ensure the quality of rebar couplers for residential buildings and less critical structures. In critical and important structures (schools, colleges, universities, shelters, hospitals, power plants), low cycle loading tests should be performed before using the coupler. For bridges, high cycle fatigue tests are mandatory.

Low cycle loading tests are of two types:

High stress test: Tensile load = 0.9*Fy*As and compression load = 0.5*Fy*As. This cyclic load is applied 20 times of loading and unloading. The purpose of this test is to evaluate the seismic performance of the coupler by loading and unloading it in a manner similar to the way tension (pull) and compression (push) are applied to the rod during an earthquake. This test is performed with a load equivalent to a medium-sized earthquake. If the permanent length increase is less than 0.3 mm after 20 times of loading and unloading, the quality of the coupler is satisfactory. That means, residual elongation, u20 ≤ 0.3 mm criteria must meet by the good rebar coupler.

High Strain Test: Tensile load is applied up to 2 times and 5 times of the strain or length increase at yield stress and a compression load of 0.5*Fy*As is applied. During this loading and unloading, the permanent length increases and slip of the rod including the coupler are measured. If the permanent length increase is less than 0.3 mm after 4 times of loading and unloading with 2 times of yield strain, the quality of the coupler is satisfactory. If the permanent length increase is less than 0.6 mm after 8 times of loading and unloading with 5 times of yield strain, the quality of the coupler is satisfactory. residual elongation, u4 ≤ 0.3 mm, u8 ≤ 0.6 mm criteria must be fulfilled by the good quality rebar coupler. This test is equivalent to a large earthquake where the structure will be deformed excessively without collapse.

High Cycle Fatigue Test

This test requires only for bridge structures where the number of loading and unloading cycles is high. For other structures, low cycle loading test is enough for quality check.

Checking thread quality at site

To confirm the quality of rod threads at the construction site, the following points must be followed properly.

Inspection after cutting

Quality control of bar cutting is checked with protractor to measure the angle after bar
cutting. Quality control of cutting was done after each change of tool, change of bar diameter or sharpening of tools. When quality control engineer measured the cut face the 90° ± 1° will be acceptable. cutting surface must be plain and smooth.

Thread pitch and diameter inspection

“no-go gauge” is used to measure the diameter of the thread of the rebar at site. It’s a direct way to check whether the thread is good or not. The rebar coupler should slide all along the threaded rebar without forcing. In addition, the “NO-GO Gauge” must not go over 2 pitches. As per ISO 2859-1, it is permitted only one rebar thread to fail this check. If more than one rebar thread fails this quality check, the whole lot is rejected. Alternatively, all rebar thread must be checked, and new forging and threading must be done for the rebars which did not pass this test.

Thread length inspection

Insert the “No-Go gauge” flat surface side into the threaded rod. Turn it slowly and gently into the threaded bar until the gauge is naturally stopped by the rebar ribs. After the gauge is fully screwed, the end of the rebar should be within the limits defined on the picture below. If the thread length is too long or too short, the threading machine needs to be reset.

Proof Loading Test

A proof load represents a load level above the likely service stresses, but below the design yield of the rebar. When loaded to 0.75fy, it will ensure that the rebar coupler installed can achieve an acceptable safety factor against failure. Proof loading will minimize the slip during earthquake increasing the seismic performance of the coupler. In critical and important structures, structural engineers suggest to perform proof loading test for each rebar coupler after fitting it at site. Proof loading test is not necessary for residential projects, because proof loading test for each rebar coupler will increase the project cost.

Concluding remarks

Rebar coupler or mechanical splice is the modern technology, which is economic, robust and seismic resistant. However, without ensuring proper quality coupler and threading works at site, it may cause disaster for the building during earthquake.

Quality Assurance and Quality Control of Rebar Coupler

Prof. Dr. Engr. Jahangir Alam

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