Important Facts About PC Bar Used in Spun Pile

The PC Bar Verification Test

The ability of PC bar to withstand tensile stresses can be checked by putting the samples of Pre-stressed Concrete Steel Bar through tensile strength testing. It is a fundamental mechanical test to determine and verify the ultimate tensile strength of PC bar that is newly procured prior to using to make spun piles (or even any other concrete products).

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In spun pile industry, tensile test is conducted by first forming the button heads; these are formed at both ends of the PC bar as to simulate the actual application scenario. In actual application, the button heads are formed and locked into countersunk holes at end plate (at both end of the spun piles) in order to anchor the PC bar before applying the pre-stressing force.

Similarly, in tensile testing machine, PC bar sample with button head is first anchored to button grips at both ends, then the PC Bar is pulled by stressing until it breaks. This would give us the ultimate tensile capacity and the failure point of the sampled PC Bar.

During the testing stage while load is being applied, the PC bar will actually go through two (2) different phase, namely the elastic deformation and plastic deformation. Elastic (also known as reversible deformation) occurs when a PC bar is still able to return to its original dimension once the load is released.

While the plastic (also known as irreversible deformation) occurs when PC bar passed beyond the elastic limit state and will not be able to return to its original dimension when load is relaxed. At this point the PC Bar hasn’t reached the failure point yet; prior to failure there would be permanent elongation point at PC bar where you can notice the particular failure point will be thinning.

While the plastic (also known as irreversible deformation) occurs when PC bar passed beyond the elastic limit state and will not be able to return to its original dimension when load is relaxed. At this point the PC Bar hasn’t reached the failure point yet; prior to failure there would be permanent elongation point at PC bar where you can notice the particular failure point will be thinning.

Stress-Strain Graph of PC Bar Testing

The stress-strain curve defines the property of PC bar when subjected to tensile load. Generally, there are three categories of tensile property as per stress-strain graph above. At point A, PC Bar reached Yield Strength which it can withstand the tensile load without permanent deformation. 

The highest or maximum tensile load that PC Bar able to withstand is called Ultimate Tensile Strength (point B). Under continuing loading, PC bar continues to plastically deform. Small elongation of PC Bar sample begins to appear thus will result in necking. The PC Bar will break at point C; called Breaking Strength where it snapped at necking section. According to JIS G: , the minimum yield strength of PC bar used in spun shall be N/mm² and the minimum tensile strength shall be N/mm².

How to Determine Good Quality PC Bar

The quality of the PC Bars can also be determined by observing the failure pattern of the PC Bar at the ultimate load. Due its extremely high tensile property, fundamentally the PC Bar would be brittle. Usually, when the PC Bar fails at ultimate load; two kind of failure pattern can be expected.

The first type of failure (which is desirable) would the “necking cup and cone shape” pattern. When the PC Bar is being continuously loaded to its maximum capacity, as mentioned earlier it will go through an elongation stage prior to failure. At this stage, the internal molecular of steel is being altered and eventually it will snap at the elongated section creating “cup and cone” shaped failure section. This is highly desirables kind of failure.

In second type of failure, the PC Bar will have a sudden brittle failure, without allowing the steel to go through the plastic and elongation stage. The failure pattern at this stage would look like the steel has been chopped into two section with flat surface at both end of failure point.

The second type of failure is not desirable, because it will also cause the structure that using it also to have a sudden brittle failure without any warnings.

The Basic Chemical Composition of PC Bar

The chemical composition of the raw steel/iron bar will affect the ductility and tensile property of PC bar produced. Thus, the main chemical composition that needs to be checked are the maximum content of Phosphorus; P (0.030% max.), Sulfur; S (0.035% max.), and Copper; Cu (0.30% max.). 

For the commercial reason purpose (due to high testing cost), the chemical composition test will be carried out by the PC bar manufacturers at independent laboratories, and a copy test certificate will be issued to the end users upon delivery of the products (the PC Bar).

How the PC Bar Used in Making Spun Pile

The PC bar will be welded together with spiral wire to form a reinforcement cage for spun pile. It is very important to ensure the incoming PC bar is in good quality as to avoid any wastages in manufacturing of spun pile. 

The PC Bars shall be free from rust, as this would weaken the strength of welding points, and eventually the PC Bar will disintegrate from the spiral wires. The PC Bars delivered also must be checked for any visible cracks as these will affect the tensile characteristic of the PC bar.

The stock of the PC Bar delivered to the plant shall be subjected to physical checking as well as mechanical testing to ensure its quality. And this process shall be repeated prior to actually using it in making the steel cages for the spun piles. This is to ensure that all the PC bar used in the production of spun piles are in good condition, so that it can be safely pre-stressed.

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Summary

In Spun Pile manufacturing process, Pre-Stressed Concrete Steel Bar plays extremely important role in making sure the piles meet the intended design criteria. Therefore, the PC Bars selected and used shall be off optimum quality as well. 

In the realm of construction and civil engineering, the utilization of reinforced concrete structures is a common practice to enhance the strength and durability of buildings, bridges, and various infrastructure projects. Pre-stressed concrete (PC) technology takes this concept further by introducing tensioned steel elements that counteract the forces of compression in concrete. Two crucial components in pre-stressed concrete are PC wire and PC strand, each serving distinct purposes in reinforcing concrete structures. However, they differ in terms of construction, applications, and functionalities.

PC Wire: A Brief Overview

PC wire, short for pre-stressed concrete wire, is a high-strength steel wire with a circular cross-section. It is typically available in various diameters, and its primary function is to provide pre-stressing force to concrete elements. This is achieved by tensioning the wire before pouring concrete around it. Once the concrete sets and hardens, the released tension in the wire creates compression within the concrete, counteracting the potential tensile forces that the structure may experience during its lifetime. PC wires are often used in applications where single-wire pre-stressing is sufficient.

HDPE Coated PC Wire - Steel Wire for Greenhouse Framework

PC Strand: A Closer Look

On the other hand, PC strand, short for pre-stressed concrete strand, consists of multiple PC wires twisted together to form a helical arrangement. This twisted structure enhances the load-bearing capacity and provides better distribution of pre-stressing forces within the concrete element. PC strands are available in various configurations, with common strand types being 7-wire and 19-wire strands. The strands are generally used in applications that require higher pre-stressing forces and where distributing these forces across multiple wires is advantageous.

PC Strand 

Key Differences Between PC Wire and PC Strand

Construction

PC Wire: Consists of a single high-strength steel wire with a circular cross-section. 

PC Strand: Comprises multiple PC wires twisted together to form a helical arrangement. 

Load-Bearing Capacity

PC Wire: Suited for applications where moderate pre-stressing forces are required. 

PC Strand: Designed for applications demanding higher pre-stressing forces and improved load-bearing capacity. 

Applications

PC Wire: Used in applications where single-wire pre-stressing is sufficient, such as smaller concrete elements or where lower forces are anticipated. 

PC Strand: Applied in larger concrete elements and projects that necessitate higher pre-stressing forces and improved structural performance. 

Flexibility: 

PC Wire: Offers more flexibility due to its single-wire construction. 

PC Strand: Generally less flexible due to the twisting of multiple wires. 

Force Distribution

PC Wire: Applies pre-stressing force along a single axis. 

PC Strand: Distributes pre-stressing force more evenly across multiple wires, enhancing load distribution.

Conclusion

In summary, both PC wire and PC strand play crucial roles in enhancing the structural integrity of concrete elements through pre-stressed concrete technology. PC wire offers simplicity and flexibility for applications requiring moderate pre-stressing forces, while PC strand provides enhanced load-bearing capacity and more even distribution of forces for larger and more demanding projects. Choosing between PC wire and PC strand depends on the specific requirements of the construction project, including the desired pre-stressing force, structural performance, and load-bearing capabilities.