Master GRP Pipe Joining Methods: 8 Essential Techniques for Engineers

GRP pipe, also known as glass-reinforced plastic pipe, is made from glass fibers in a resin matrix. These pipes are joined via various methods to achieve a long lifespan and low maintenance requirements. Additionally, central factors such as pressure class, soil and weather conditions, axial or hoop loads, and installation methods can influence the selection of the jointing method.

GRP pipe joining methods, whether restrained or unrestrained joints, create a durable and leak-free connection in water, sewage, or industrial applications. Unrestrained joints, such as bell and spigot or REKA couplings, handle low-pressure flows in stable soils, while restrained joints, including key-lock, adhesive-bonded, and laminated joints, are suitable for high-pressure, unstable conditions. Additionally, flanged joints connect GRP to metal. All these joints cover international standards such as ASTM D4161 to shape an installation for over 50 years.

Whether an engineer or a project manager is seeking a reliable and leak-proof installation, this article is a helpful guide to choosing the most proper jointing method based on your project conditions.

Classification of GRP Pipe Joints: Unrestrained vs. Restrained Solutions

GRP pipe joints are divided into two major pieces: unrestrained and restrained methods. Each represents specific characteristics that are mentioned below:

Unrestrained GRP Pipe Joints

Unrestrained GRP pipe joints, including bell and spigot or REKA couplings, show such a performance in the water supply or sewage systems where flexibility is needed for stable soil conditions.

Also, according to SCRIBD, unrestrained GRP pipe joints like REKA systems can be used rapidly, while their flexibility allows installation in aggressive environments. However, it’s usually used in low- to medium-pressure applications and may increase the cost of additional engineering for thrust blocks.

Restrained GRP Pipe Joints

Laminated, key-lock, and adhesive-bonded couplings are such restrained GRP pipe joints which shine in high-pressure pipelines or in complex networks and unstable soils, due to their permanent and tight jointing systems.

Moreover, this type of joint typically needs more time and cost for installation, while they include a long lifespan and simple installation in a tight situation with no movement.

Aspect	Unrestrained Joints	Restrained Joints
Characteristics	Allow axial/angular movement; need thrust blocks at bends/tees. E.g., bell and spigot, REKA couplings.	Bonded/locked to resist axial/hoop forces; no external supports. E.g., adhesive-bonded, laminated joints.
Use Cases	Uniaxial pipelines, water/sewage in stable soils, low-pressure systems.	High-pressure pipelines, complex networks, seismic/unstable soils.
Advantages	Fast installation (<10 min) Cost-effective Flexible for misalignments	No thrust blocks, simpler design High-pressure reliability Space-efficient
Disadvantages	Thrust blocks add complexity Limited to ≤6 bar Weak in unstable soils	Skilled labor needed Higher costs Less flexible for movement
Installation Complexity	Low; minimal training needed for GRP pipe couplings.	High; requires skilled personnel for GRP pipe bonding methods.
Standards Compliance	Meets ASTM D4161 for basic joint construction.	Complies with ASTM D4161, ASME NM.2 for high-pressure systems.

Types of GRP Pipe Joints and Couplings

The world of GRP pipe joining has expanded significantly. From bells and spigots to steel couplings, these systems are crucially needed in specific conditions. Below, we’re providing eight essential GRP pipe joining systems and the requirements of each.

1.      Bell and Spigot Joints

One of the unrestrained GRP pipe couplings is used in both aboveground and underground pipelines like water and sewage systems and irrigation networks.

Pressure Ratings and Types: Single O-ring and double O-ring are two main types of bell and spigot joints. A single O-ring is designed for low-pressure systems, such as sewage systems (typically up to 6 bar), while a double O-ring can handle pressures exceeding 6 bar in industrial pipelines.

Sealing and Alignment Considerations: a leak-proof sealing in O-ring joints is reached by applying non-petroleum-based lubricants. Also, to avoid predictable damage of O-ring, a proper alignment via witness marks on the spigot shapes a safe insertion, while controlling the pipe’s deflection.

2.      REKA Coupling (Flexible)

REKA couplings, as an unrestrained GRP pipe coupling, use EPDM sealings to prevent leaks, and internal gaskets secure those sealings in low-pressure systems such as water distribution and sewage systems.

Features and Installation procedure: REKA couplings handle ~3° angular deflection while containing a fast installation (usually less than 10 mins) to show a perfect performance in complex piping projects.

Moreover, for proper resistance against water and mild chemicals, dry EPDM gaskets are used in clean pipe ends to make an even alignment and insertion via witness marks.

3.      Key-Lock Coupling

Unlike REKA coupling, key-lock couplings are a restrained version that avoids pipe fraction or separation under axial forces via mechanical key-locks (~3° angular deflection for flexibility). This type of joint is used in water or wastewater where unstable soils and high-pressure conditions require such resistance.

Installation and Key Considerations: The installation method of this joint is partly like REKA coupling, but due to the mechanism of locks, it claims higher costs, while it is perfect for high-pressure projects with axial restraint.

4.      Adhesive-Bonded Coupling (Epoxy Bonded)

Via applying epoxy bondage over the pipe ends, adhesive-bonded couplings shape a rigid and permanent connection in high-pressure applications such as chemical processing, oil and gas pipelines, and desalination systems. (Source: ScienceDirect)

Key Features and Installation: To achieve a flexible design, tapered or straight spigots are available, and an epoxy bond creates a seal that prevents axial or hoop loads while handling pressures of 6 to 32 bars in chemical transportation systems.

To install this joint, the pipe ends should be cleaned, and then the epoxy is applied to the coupling and pipe surfaces. The assembly should then rest for a day before proceeding to GRP pipe testing. So, this process requires trained personnel, longer curing time, and controlled conditions like temperature or humidity.

5.      Glued Coupling (COMBI Type)

The combination of EPDM seals with epoxy makes the COMBI-type glued coupling used in water treatment plants or hybrid systems where flexibility and resistance against pressure is critical. This type is similar to adhesive-bonded joints, as it provides axial restraint.

Installation Process: Like the last types, installation begins with preparing pipe ends to insert dry EPDM and applying epoxy adhesive, then goes through curing (time matters) and pipe testing.

6.      Laminated Joint (Lay-up / Butt & Wrap)

Laminated joints or butt and wrap joints, or lay-up joints are made of layers of fiberglass and resin over pipe ends to shape such a strong and permanent bonding in marine or industrial pipelines where strength and high-pressure resistance are crucially needed.

Installation Tips and General Considerations: To start the installation, pipe ends should be butt and clean to apply resin and wrapping the fiberglass layers, then gets cured for a day or two. Due to material used in this method labor cost are higher than other types.

7.      Flanged Joint

Via bolted flanges, flanged joints as a restraint create such a mechanical and detachable connection in GRP-to-GRP or GRP-to-metal/equipment to ease maintenance. This type of joint is used in water treatment or industrial pipelines due to its easy connection to pumps, valves, or steel pipes. (Source: SCRIBD)

Installation Tips: To avoid leaks, align flanges tightly and insert the gaskets carefully, then tighten the bolts and check for final alignment before GRP pipe testing.

Additional Considerations: For high-pressure applications, use O-ring gaskets and flat gaskets for low pressure, followed by standards like ANSI B16.5, ISO 7005, or EN 1092. This joint eases the maintenance process while ensuring leakproof installation.

8.      Mechanical Steel Coupling

To join GRP pipes, mechanical couplings include steel clamp systems to connect or repair dissimilar materials. Based on the design, it can be used both as restrained or unrestrained joints in water, sewage, or industrial systems.

Installation Tips: To make a tight seal, clean pipe ends, tighten bolts, and compress the gaskets in a way to avoid any leaks in hydrostatic pressure testing. Remember, this installation method is less durable than bonded joints and if it’s used for steel pipes, a corrosion-resistant coating is required in harsh soils.

Method	Joint Type	Pressure Rating	Installation Time	Applications	Advantages	Disadvantages
Bell and Spigot	Unrestrained	≤6 bar (single O-ring), >6 bar (double O-ring)	<10 min	Water, sewage, stable soils	Quick, cost-effective, ~3° deflection	Needs thrust blocks, limited pressure
REKA Coupling	Unrestrained	Low to medium (≤6 bar)	<10 min	Flexible water/sewage systems	Fastest install, flexible, low skill	Thrust blocks, low pressure only
Key-Lock Coupling	Restrained	Medium to high (6–16 bar)	<10 min	Seismic zones, unstable soils	Axial restraint, quick, reliable	Higher cost, limited pressure
Adhesive-Bonded	Restrained	High (6–32 bar)	24+ hours (curing)	Chemical, oil/gas pipelines	High-pressure, permanent bond	Long curing, skilled labor
Glued (COMBI)	Restrained	Medium to high (6–16 bar)	24+ hours (curing)	Hybrid water/industrial systems	Flexible + strong, dual seals	Curing delays, complex
Laminated Joint	Restrained	High (up to 32 bar)	24–48 hours	Fixed points, repairs	High axial strength, customizable	Labor-intensive, costly
Flanged Joint	Restrained	Varies (1–25 bar)	30–60 min	GRP-to-metal/equipment	Detachable, standardized	Costly, bolt corrosion risk
Mechanical Steel Coupling	Unrestrained/Restrained	Varies (1–16 bar)	<30 min	Repairs, GRP-to-HDPE/steel	Versatile, quick repair	Less durable, corrosion risk

Jointing for Hybrid Applications

To shape resistance against corrosion, pressure, and harsh environments in hybrid piping systems, such as water treatment and industrial pipelines, a leak-proof connection between GRP pipes and HDPE or steel pipes is required.

Below are three crucial considerations, including flat-face flanges with full-face gaskets through matching pipe’s diameter size and bolt torque.

1. Flat-Face Flanges with Full-Face Gaskets: To connect GRP pipes to HDPE or steel pipes, flat-face flanges and full-face gaskets are used to avoid leak and cracks in high-pressure flows. Full-face gaskets provide an even pressure on the flanges while having no reaction with fluids like water or mild chemicals to reach a long pipe lifespan.
2. Proper Diameter and Bolt Torque: As noted in ResearchGate, to prevent leaks or stress on GRP flanges, matching the flanges diameter and tightening bolts in a star pattern is essential. Furthermore, Torque (e.g., 70 Nm for DN 200, per ANSI B16.5) prevents over-tightening, which cracks GRP.
3. Optional Use of Machined Sleeves: To fill the minor diameter gaps, steel or GRP adapters are used as a bridge in high-pressure systems or harsh soils. Additionally, sleeves can support the joint structures against dynamic loads, such as those in seismic zones.

GRP Pipe Laying & Joint Assembly Procedures

GRP pipes are assembled in several different forms to create leak-free, durable, and resistant piping systems for use in water supply, wastewater, or industrial applications. These procedures include trenching, bedding, backfilling, compaction, and coupling assembly, which are thoroughly explained below:

Trenching and Bedding

Trenching and bedding methods create a safe base for GRP pipes to keep the alignment and GRP pipe protection during the installation process. The trench size should be about 0.4 times the pipe diameter wide (e.g., 400 mm for a 1000 mm pipe) to allow workers space without extracting too much soil.

Adjustments Based on Soil Type: For stable soils (SC1, like gravel), a thin 50–100 mm sand layer works, also, stable soils (SC2) need 100–150 mm of compressed gravel, while soft or unstable soils (SC3–SC4) require 150–200 mm of crushed stone plus geotextile fabric. A curved bed supports the pipe’s bottom third.

Pipe Backfilling & Compaction

Backfilling and compacting around GRP pipes avoids loads like traffic while trying to keep their shape during GRP pipe installation. Backfill is added in 100–300 mm layers: up to 70% of the pipe’s height, by adding sand or gravel in 100–150 mm layers, tampered to 70% firmness. Additionally, use soil or gravel in 200–300 mm layers, compressed to 90% tenacity.

Pipe Backfilling & Compaction Using the Appropriate Compactors: To start the process, use hand tools near the pipe to avoid damage, then switch to vibratory compactors higher up.

Monitor Vertical Deflection: Be careful about over-tampering above the pipe to keep deflection at 2–3% of the pipe diameter (e.g., 20–30 mm for a 1000 mm pipe); checked with tools like laser levels to make sure of the correct ratio.

Coupling Assembly Procedure

REKA or bell and spigot joints are known as assembling GRP pipe couplings that contain strong, leak-free connections as a central part of GRP pipe joining methods. Now, let’s break them into multiple steps:

1. It starts with cleaning pipe ends and gasket grooves to remove dirt from the surface, ensuring a permanent joint.
2. Then, place dry EPDM gaskets in the grooves, ensuring they sit flat without twists, as wet gaskets may slip.
3. A non-petroleum lubricant is applied to pipe ends for smooth insertion without causing damage to gaskets and pushing them together evenly to prevent any misalignments.
4. After all, witness marks on pipes should be checked to insert them entirely into the right place while following standards like ASTM D4161.
5. Eventually, GRP pipe joints go through hydrostatic testing with water pressure (1.5 times design pressure) to verify no leaks.

Quality Standards and Testing

GRP pipe installation, joining, and testing follow industrial standards, which are critically needed for water, sewage, and chemical systems. Here are the main standards used for GRP pipe construction:

ASTM D4161: Mainly used for joints like bell and spigot or adhesive-bonded couplings.

ASME B31.1 and B31.3: Checked whether for Power Piping or Process Piping in high-pressure or chemical systems in design, fabrication, or inspection protocols.

ASME NM.2: Generally, goes for GRP piping in terms of material properties and joint performance.

Bonding Procedure & Personnel Certification

To reach high-quality GRP pipe joints, bonding procedures for restrained GRP joints like adhesive-bonded or laminated joints and personnel certifications are required.

• Bonding Procedure Specification (BPS): It indicates how each step of joining should be run (e.g., surface preparation, adhesive application, and curing).
• Procedure Qualification Record (PQR): To verify the BPS special components, such as adhesive strength and curing time, these should be tested and documented.
• Bonder Qualification Record (BQR): Via practical and theoretical exams, workers are checked and certified on how skillful they are.

Hydrostatic Testing

Hydrostatic testing demonstrates the efficiency of GRP pipe joints by applying water pressure beyond design conditions while containing no leaks, ensuring a perfect and long-lasting performance of GRP pipe joining methods.

Testing pressure ranges from 1.33 to 3 times the design pressure (e.g., 8–18 bar for a 6-bar system), held for at least 10 minutes to detect potential problems. The pipeline is filled with clean water while the air is released, and pressure is gradually increased to avoid damage.

Moreover, Characteristics such as leaks, cracks, or deformation are revealed by pressure gauges, dye penetrants, or soap solutions, following standards like ASME B31.3 or AWWA C950.

Engineering Considerations and Manufacturer Guidance

There are many factors for managing to achieve the right GRP pipe joining methods. Factors like pressure class, pipe diameter, soil and trench conditions, and accessibility for future maintenance will represent a great performance in water, sewage, or chemical applications.

Pressure class varies from joints type; for example, in unrestrained joints (e.g., bell and spigot, REKA), low to medium pressures are allowed (≤6 bar), while in restrained joints (e.g., adhesive-bonded, laminated), high pressures, up to 32 bar can be handled.

Pipe Diameter changes the joint’s choice; for large diameters (>600 mm), laminated or flanged joints are properly used.

Soil and trench conditions include the utility of restrained joints for seismic zones, and in stable soils unrestrained joints are critically used.

Referenced manufacturers of GRP Pipe Joining Methods

These manufacturers play such a role in designing a guideline to choose the most appropriate GRP pipe joining methods for your project:

1. FLOWTITE®: Including GRP pipe systems in sewage or irrigation systems, in the transmission of chemical, drinking water, or other industrial applications.
2. ROREX: As a leader in the manufacture of GRP pipes, ROREX provides a wide range of services, from manufacturing processes to testing and standards support, such as ISO.
3. GRANDPIPE: Guides piping systems for reliable manufacturing, coating and linings, installation, and further maintenance
4. Zhongyi: Focuses on adhesive-bonded and laminated joints for industrial pipelines while developing methods to avoid corrosion and process problems.
5. Azkompozit: The company recommends pipelines for drinking water supply, sewage systems, and irrigation plants, both in Azerbaijan and globally.
6. ARPOL: As one the leading suppliers in Europe, shapes a complete plan from installing to fixing and repairing pipe joining systems.

Conclusion

To select the best joining methods for GRP pipe, a corrosion-resistant, durable, and lightweight material, several joint types are introduced to meet the demands of each project. Whether in stable or unstable soils, high-pressure or low-pressure flows, restrained and unrestrained joints consistently demonstrate reliable and leak-free performance, meeting standards such as ASME codes. Continuing the ideal practices for joint assembly and engineering consideration to reach the manufacturing aims.

FAQs

1- What are GRP pipe joining methods used for?

These pipes are joined via various methods (such as adhesive-bonded or flanged joints) to reach a long lifespan and low maintenance requirements in water, sewage, and chemical transmission applications.

2- How do unrestrained and restrained GRP pipe joints differ?

Unrestrained joints allow axial and angular movement, requiring thrust blocks, and suit low to medium-pressure systems in stable soils .Restrained joints resist axial and hoop forces without external support, making them ideal for high-pressure pipelines or unstable soils; however, they require skilled labor and longer installation times.

3- What standards cover GRP pipe joint construction?

ASTM D4161 is primarily used for joints such as bells and spigots or adhesive-bonded couplings. ASME B31.1 and B31.3 cover whether Power Piping or Process Piping in high-pressure systems in design or inspection protocols.

4- What is the purpose of hydrostatic testing for GRP pipe joints?

Hydrostatic testing assesses the efficiency of GRP pipe joints by applying water pressure beyond design conditions ensuring that no leaks occur and achieving a perfect and long-lasting performance of GRP pipe joining methods.