Overview of Long-term Durability of Composite Pultruded Profiles

 

 

What does this Paper do?

 

This paper comprehensively summarizes the aging performance of the mechanical properties of FRP pultruded profiles under various long-term environmental factors, including water immersion/humidity, alkaline solution immersion, acid solution immersion, high/low temperature, ultraviolet radiation, freeze-thaw cycle, dry-wet cycle and comprehensive natural environment;

 

The mechanism of FRP aging caused by all current environmental effects is discussed in detail, and suggestions for future research on the durability of FRP pultruded profiles are given; the largest and most comprehensive durability database to date is collected and organized, including more than 1,900 data points in 134 documents, which can be further used as the basis for developing FRP durability prediction models.

 

Note: The data in this paper refer to the data of pultruded products (pultruded).

 

Learn about Composite Pultruded Profiles

 

The so-called fiber reinforced composite materials (Fiber Reinforced-Polymer Composites, referred to as FRP or composite materials) are composite materials formed by winding, molding or pultrusion of reinforcing fiber materials (glass fiber, carbon fiber, aramid fiber, etc.) and matrix materials. FRP pultruded profiles are long strips of FRP products produced continuously by pultrusion.

 

 

FRP Pultruded Profiles

 

FRP is divided into many types according to the fiber and matrix materials. Although the performance varies, they are generally light and strong, and have considerable advantages in the fields of construction, transportation, manufacturing, etc.

 

The application history of FRP in the field of civil engineering is relatively short. Existing studies have shown that the mechanical properties of FRP will show different degrees of decline under long-term environmental effects. Therefore, the study of its durability will directly affect the design and use of FRP, and is also of great significance for the future promotion of FRP materials.

 

Civil engineering structures often have a design service life of up to decades, while the durability test of FRP in the laboratory environment can only be carried out for dozens of months. Therefore, in order to test the durability of FRP, accelerated aging tests are often used in the test laboratory for a limited time to simulate the performance of FRP after decades. Factors such as wind and rain in nature play a decisive role in the aging of FRP, including: immersion/humidity, alkali solution, acid solution, high/low temperature, ultraviolet radiation, freeze-thaw cycle, dry-wet cycle and their combination.

 

Since the 1970s, there have been many related tests. To sum up, the main aspects are as follows:

 

• Solution Conditions:

FRP ages seriously under immersion conditions. Water intrusion will cause the interface between fiber and resin to be damaged, thus affecting the mechanical properties of FRP; the increase in immersion time, alkaline environment and high temperature environment will aggravate the corrosion of the composite material.

 

• Water Dissolution:

The dissolution of water on FRP is affected by many factors, such as temperature, porosity, etc.

 

• External Stress Effect:

In the short term, the fiber is straightened under external stress, which is conducive to the enhancement of FRP strength; but in the long term, the external stress will increase the water absorption rate of FRP, leading to accelerated aging of the material.

 

• High Temperature:

High temperature has a significant effect on the compressive strength, mass loss and impact resistance of FRP.

 

• Low Temperature:

The effect of low temperature alone is small, but in a humid environment, the combined effect of low temperature and water intrusion will cause cracks inside FRP and separation of the interface between fiber and resin, which is not conducive to the durability of the material.

Natural Environment:

Under natural conditions, FRP often faces the combined effects of multiple environmental conditions. Under the combined effects of temperature, water and ultraviolet rays, the post-curing effect of FRP will lead to an increase in elastic modulus; at the same time, the corrosion problem of FRP in seawater is often more serious than on land.

Test Factors:

FRP durability tests around the world have a large degree of dispersion, so it is difficult to directly draw a general conclusion from the test. The dispersion of FRP materials is mainly reflected in:

 

1. The aging mechanism is complex, and the corrosion of FRP may occur in the resin, the interface between resin and fiber, and the fiber;

 

2. There are many types of fibers and resins. Different FRPs are often composed of fibers and resins with different combinations and proportions, so the test results will be different;

 

3. Different FRP production processes will lead to differences in the fiber content and organizational structure of the material, which will affect the mechanical properties and durability of the material.

 

So, what is the durability of FRP pultruded profiles that everyone is concerned about?

 

Aging Mechanism

 

Since FRP will age under the influence of the environment, how does aging specifically occur?

 

There are both external and internal causes for aging, and the aging mechanism varies under the influence of different factors.

 

 

Aging Mechanism under Immersion/Humidity Conditions

 

 

Aging Mechanism of FRP in Water Immersion or High Humidity Environment

 

• Fiber:

Inorganic fibers such as carbon fiber and glass fiber do not absorb water, but the microcracks caused by the resin absorbing water will expand at the interface between the resin and the fiber, eventually leading to fiber cracking. Water absorption by organic fibers will directly lead to fiber expansion and cracking.

Resin Matrix:

Water in the environment mainly invades FRP through penetration and capillary phenomena. Water invasion will cause the resin to expand and produce microcracks. In addition, water invasion causes the material to plasticize and hydrolyze, resulting in material softening. When dried, the plasticization of the material is partially reversible, but the hydrolysis is irreversible, which will cause permanent damage to the material.

Fiber-Resin Matrix Interface:

The interface provides a convenient channel for water to enter, and the channel effect is more obvious for FRP materials with poor interface quality. Water absorbed along the fiber-matrix interface will cause swelling of the interface and the spread of microcracks. In addition, the fiber-matrix will debond, and part of the matrix material may dissolve in the water invading the interface, directly reducing the interlaminar shear strength of the FRP material.

Effect of Water Intrusion:

Water intrusion into the resin matrix and fiber-matrix interface may further widen the initial cracks and create new cracks and voids, allowing additional water to be absorbed into the FRP material, exacerbating the degradation mechanism. Water intrusion affects the fiber-based properties, namely tensile properties, and significantly reduces the matrix and interface-based properties, namely bending and shear properties.

 

Aging Mechanisms under Acid-base Solution Conditions

Alkaline and acidic solutions age FRP materials in a manner similar to water intrusion. Both solutions can enter the material by penetration, then cause swelling in the matrix and fiber-matrix interface, leading to the formation and propagation of microcracks, thereby reducing the strength and elastic modulus of the material. According to existing research data, alkaline solutions generally have a greater impact on the mechanical properties of FRP materials.

 

Aging Mechanisms under High Temperature Conditions

 

 

Degradation Mechanism of High Temperature

 

When high temperature and solution act together, it can accelerate the aging effect caused by water, alkali and acid solution; and high temperature alone will also affect the mechanical properties of FRP.

 

High temperature affects the viscoelasticity of the resin matrix of FRP. When the ambient temperature is close to or higher than the glass transition temperature of FRP, the resin matrix softens and cannot transfer stress between the fiber and the matrix, resulting in a decrease in the matrix elastic modulus and degradation of the fiber-matrix interface. Therefore, at high temperatures, the failure mode of FRP materials is controlled by the fiber, which manifests as a sudden brittle fracture of the fiber bundle.

 

Aging Mechanism Under Ultraviolet Radiation

FRP is prone to chemical degradation under ultraviolet radiation, and aging starts from the “skin”. Ultraviolet radiation causes oxidation of the resin matrix surface, destroys the chemical bonds between molecules and affects the surface gloss of the material. However, the reduction in mechanical properties due to ultraviolet radiation is limited to a depth of 10μm within the material, and the effect on mechanical properties is almost negligible.

 

Aging Mechanism under Freeze-thaw Cycle Conditions

The coupling effect of water and freeze-thaw cycle can cause aging of immersed materials, but the aging of dry materials under freeze-thaw cycle can often be ignored. Experiments have shown that the bending stiffness of GFRP materials under freeze-thaw cycle does not decrease but increases, which is due to the hardening behavior of the resin matrix at low temperature. Therefore, freeze-thaw cycle has no obvious adverse effect on the mechanical properties of FRP materials.

 

Aging Mechanism under Dry-wet Cycle Conditions

The aging mechanism of FRP caused by dry-wet cycle is similar to that caused by aqueous solution. In essence, water or water-based solution triggers the aging process.

 

Aging Mechanism under Natural Conditions

The aging of FRP under natural conditions is often the combined effect of the above mechanisms.

 

In summary, the long-term environmental effects that cause FRP aging are mainly the above. These external conditions act on the fiber, matrix, and interface of FRP. After qualitatively analyzing the aging mechanism, let’s take a look at the specific test data.

 

Let the Data Speak

Based on the eight most concerned environmental conditions mentioned above and the data collected from the test, we can quantitatively analyze the effects of the environment on the tensile, compression, bending and shear properties of FRP. Due to space limitations, only part of the data and analysis results are shown here. For more comprehensive and detailed data and analysis results, please click the full text at the end of the article.

 

Aging Analysis under Immersion/Humid Conditions

 

 

Time and temperature statistics of FRP tests under immersion/humidity, acid and alkali solution conditions

 

According to current test data, the duration of most durability tests under solution conditions (including immersion in water, alkali solution, and acid solution) is less than one year, and the test temperatures are usually 20, 40, 60, and 80°C. For immersion/humidity conditions, the temperature ranges from 20 (room temperature) to 80°C, which is sufficient to simulate the service temperature of FRP structures in actual environments. However, shorter test times may affect the reliability of durability test results.

 

Since the post-curing effect of the resin matrix and the effects of the fiber-matrix interface and the slow degradation of the matrix in water require a longer test time to fully reflect, this paper recommends the shortest test time, i.e. 18 months, based on the results of existing studies and the recommendations of researchers from many countries.

 

 

Normalized residual tensile strength and test time under immersion/humidity conditions

 

The solid and dotted lines in the figure are approximate trend lines of residual tensile strength at different temperatures. As can be seen from the above figure, the data from different tests are relatively scattered, so only the aging trend of the material can be observed, that is, moisture will cause the residual tensile strength of FRP pultrusion profiles to decrease, and this decrease behavior will become more obvious with the increase of exposure time and temperature.

 

Aging Analysis under Alkaline Solution Conditions

The test results of alkaline solution are similar to those under immersion/humidity conditions: most of the test time is within one year, and the exposure temperature in these tests is usually 20, 40, 60 and 80°C. Except for a few tests that were conducted for 1.5 to 2.5 years, the test time of almost all tests is short.

 

As can be seen from the figure below, the test data has a large dispersion and it is impossible to obtain conclusions that can guide the design; moreover, in the existing test data, there are serious contradictory results, which is caused by the observed aging effect not being fully completed in the test. Therefore, it is recommended to conduct longer tests in the future to fully observe the aging behavior of FRP in alkaline solution.

 

 

Relationship between normalized residual tensile strength and test time under alkaline solution conditions

 

This paper divides the pH value of alkaline solutions into two categories:

 

1) Weak Alkaline Solutions with a pH Value of about 8, simulating seawater or saltwater environments;

 

2) Strong Alkaline Solutions with a pH Value of about 13, simulating concrete pore environments or other harsh alkaline environments. The general trend is that the mechanical properties of FRP pultruded profiles in alkaline solutions decrease with increasing test time and temperature. In addition, compared with weak alkaline environments, strong alkaline environments may cause more severe aging of FRP materials.

 

Aging Analysis Under High/Low Temperature Conditions

Typical test conditions involve exposing FRP materials to high or low temperature air, with test temperatures ranging from -100 to 700°C, and most tests are conducted at temperatures below 200°C.

 

 

Experimental Statistics to Explore the Effect of Temperature

 

Lower temperatures may harden FRP materials, leading to improved mechanical properties. Conversely, high temperatures may soften the resin matrix, thereby destroying its ability to transfer stress between the fiber and the matrix. In general, the residual tensile strength and temperature show a linear negative correlation.

 

 

Relationship between normalized residual tensile strength and test temperature

 

Aging Analysis Under Natural Conditions

 

 

Experimental Statistics to Explore the Influence of Natural Conditions

 

Natural conditions are a combination of multiple conditions that synergistically affect the mechanical properties of FRP pultruded profiles. In the existing tests, the shortest exposure time is 100 days, while the longest exposure time is 8 years. Natural conditions include natural environments simulated under laboratory conditions, seasonal conditions in Switzerland, urban environments in Portugal, and arid environments in Saudi Arabia. Environments around the world have different effects on the mechanical properties of FRP pultruded profiles. Therefore, it is recommended to select and conduct accelerated aging tests based on the actual use environment of the FRP structure.

 

Existing Standards

In the past decade, application standards for FRP pultruded profiles are being established in different countries and/or regions around the world. For example, the national standard GB/T 31539 “Fiber-reinforced composite pultruded profiles for structures” edited by the author considers four durability properties.

 

 

 

Standard Screenshot

 

However, the existing engineering design standards and specifications of various countries have not yet covered all environmental factors that may affect the performance of FRP materials. Therefore, this paper recommends improving the existing design methods and further considering the impact of all types of environmental effects on FRP pultruded profiles.

 

Suggestions and Prospects

 

FRP pultruded profiles have excellent durability and are in urgent need of extensive and in-depth research. The current theoretical prediction model, however, is limited by limited test data and cannot be widely used in the prediction of FRP durability; moreover, although many durability tests have been conducted in the past few decades, the results are often not comparable to each other, so as to draw a unified conclusion. Based on the summary and analysis of relevant research data in the past 20 years, this paper recommends:

 

1. For all types of accelerated aging tests, a longer test time is recommended.

 

2. It is recommended to strengthen the aging analysis of the lateral performance and compressive resistance of pultruded profiles.

 

3. When evaluating and predicting aging behavior, it is recommended to consider the effects of fiber and matrix type, fiber content and specimen thickness.

 

4. It is recommended to consider each environmental effect separately in the aging test (it is not recommended to conduct aging tests with composite environmental effects in the future) in order to truly understand the aging mechanism of the material and guide the design and use of FRP structures.

 

5. It is recommended to improve and unify the standard test methods and develop more accurate durability prediction models in the future.

 

The research on FRP durability is like a puzzle. The experiments of every scholar at home and abroad are adding an important piece to it. We believe that with the joint efforts of the academic community and the strong support of all sectors of society, the research on FRP durability will eventually synthesize a beautiful picture and vigorously promote the widespread application of FRP materials in the future.

 

Looking back on history: my country’s scientific and technological workers conducted a systematic study on the durability of FRP as early as 44 years ago. This article is just a tribute to the older generation of scientific and technological workers.