What To Do If Lap Length Is Not Enough
Prof. Dr. Engr. Jahangir Alam
Professor, Department of Civil Engineering, BUET
Table of Contents
Introduction
Appropriate solution is needed if the available lapping length is not enough in any construction site. When Lap Length Cannot Be Found or not adequate as per code, alternative safe, reliable and sustainable solutions must be adopted. This article describes the alternative solutions.
Understanding Lap Length and Its Importance
In reinforced concrete structures, lap length refers to the overlapping portion of two reinforcement bars that are essential in transferring loads between the bars. It is a crucial element in ensuring the structural integrity, seismic performance and strength of the building.
Installation Method of Threaded Rebar Coupler
Threaded rebar coupler is the economical and robust solution to join two rebars. But it needs prior preparation.
- Straightening and cutting: First, the end of the rod must be cut by circular saw so that the rod end surface is even and smooth. In addition, the rebar must be straightened properly.
- Cold forging: rebar diameter of certain length shall be increased by cold forging machine so that after threading the net diameter of the rebar is equal or greater than the original diameter of the rebar. It is called rebar upsetting or cold forging.
- Threading: threading shall be done using a threading machine which matches the coupler threads pitch, angle and diameter. This is the most important part of the work in the site which may lead to a disaster for the building if quality is not ensured. To ensure quality, manual checking for each rebar thread is mandatory by coupler trial fitting to the threaded rebar. If it is too loose or excessively tight, engineers must reject the operator and threading machine. In addition, samples shall be made at site for testing randomly. Tensile strength test and slip test shall be done regularly to ensure quality of threading.
- Installation: rebars are placed in beam, column and shear walls before casting concrete. Pipe wrench should be used to tighten the threaded coupler to join the two ends of rebar.
Solution where lapping length is not adequate
In civil engineering construction, rebar joints are made in 3 ways:
(i) lapping
(ii) welding and
(iii) rebar coupler or mechanical splice.
Lapping length depends on rebar diameter, rebar strength, concrete strength and location of rebar in the RCC member. Lapping length increases with rebar diameter and strength and decreases with concrete strength. As a thumb rule for regular structures, lapping length is 50D. That is, lapping length is 50 times its diameter. For welded lapping, lapping length is approximately 10D. for mechanical splice or rebar coupler, no lapping length is required. Rebars are joined together end to end using one type of round nut called rebar coupler, specially designed to transfer tensile and compression load. There are many types of rebar couplers. Among them, threaded coupler is the most common type which is used in regular construction. It needs preparation of rebar-end before installing in structure. In any situation where lapping length required is not available and rebar end is not prepared before installation in structure, there are some alternative options to solve the issue.
- Epoxy grouted anchoring of rebar
- cold pressed coupler
- welded lapping
- MBT rebar coupler
- Grout filled coupler
Epoxy grouted anchoring of rebar
- The following situations are ideal for epoxy grouted anchoring of rebar
retrofitting and strengthening of RCC members by RCC jacketing - construction of additional floors on existing structure
Disadvantage of epoxy grouted anchoring
- Workmanship may affect quality and strength of anchoring.
- Anchoring depth depends on diameter and strength of rebar and strength of concrete. If concrete strength is less than 3000 psi and rebar diameter is more than 20 mm, anchoring depth may not be sufficient. However, this needs proper calculation for designing anchoring depth.
Cold pressed coupler
A cold pressed rebar coupler is a mechanical sleeve (steel pipe) designed to connect two pieces of reinforcing steel by compressing the sleeve onto the bars using a hydraulic press. The process, known as “cold pressing”, involves inserting both rebar ends into the coupler and applying a specific amount of pressure to deform the coupler around the rebar, forming a tight, non-slipping connection. Unlike threaded or welded couplers, cold pressed couplers do not require bar-end preparation such as threading or heating. This makes them ideal for projects where rebars are already placed without end preparation.
Situations where cold pressed coupler is ideal
- Lapping length is not enough but at least 100 mm rebar length is available for cold pressed coupler installation.
- No exposed length for installation of cold pressed coupler but breaking of concrete is possible to expose 100 mm rebar length
Installation Process of cold pressed couplers
- Insert Rebars into Coupler – Align properly before pressing.
- Suspend the press machine from a tripod beside the rebar
- Apply Hydraulic Pressure – Compress the sleeve (cold pressed coupler) at 6-8 places with uniform spacing
- Inspect the Joint manually – Check for proper tightness.
Welded Lapping
If approximately 10D lapping length is available, welded lapping may be done with the condition of weldable rebar grade. Welding quality must be good. Welding length must be calculated using rebar grade and diameter. But, as per BNBC 2020, welded lapping is prohibited. because, welding may deteriorate quality of rebar.
Grout Filled Rebar Coupler
Grout-filled rebar couplers rely on a specialized chemical grout to create a strong, permanent bond between two reinforcing bars, effectively splicing them together. This is expensive compared to all other alternatives. After inserting rebars into the coupler, chemical grout is pumped into the inner void through one vent and continued until overflowed through the other vent. This liquid chemical grout will be hardened within a few minutes making strong connection between the bars.
One touch rebar coupler
This is the latest invention in the Rebar coupler series. This has a mechanical lock system which allows the rebar to be pushed inside and locked. Anyone can push the rod into the coupler for installation without. No machine, no chemical, no threading is required in one touch rebar coupler. This type of coupler has a great disadvantage that it cannot pass the slip test. So, it is not permitted to be used in seismic applications.
MBT Rebar Coupler
MBT (Mechanically Bolted) couplers join reinforcing bars in concrete construction without the need for welding or threading the bar ends. They utilize a system of serrated saddles and special lock shear bolts to create a secure connection. When the bolts are tightened to a specific torque, they embed into the rebar and the saddles bite into the bar and coupler shell, effectively transferring the load. Bolt heads are sheared and broken automatically if the tightening reaches its desired position. So, quality control is easy for MBT rebar coupler. This coupler is more expensive than cold pressed coupler.
Why can’t the threaded standard rebar couplers be used?
Rebar end must be prepared before installing it in the structure. Rebar is threaded using threading machine before placement. If rebar is already placed without threading, concrete is already cast, there is no scope of using threaded standard rebar coupler. In this situation, cold pressed couplers are the best option to solve the issue.
Quality Assurance and Quality Control of Rebar Coupler
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. So, the tensile strength test and slip test must be performed before selecting the rebar coupler.
Concluding remarks
Welded lapping is risky if the rebar grade is not suitable for welding. As per BNBC 2020, welded lapping is prohibited. Epoxy grouted anchoring is not suitable if the concrete quality is not good. Epoxy grouted anchoring is not feasible at all if there is no additional space to insert additional rebars by anchoring. MBT couplers and grout filled couplers are expensive. So, the best solution is the cold pressed coupler. Cold pressed rebar couplers are a kind of blessing in modern construction, offering superior strength, efficiency, and reliability compared to traditional splicing methods.
