APPLICATION CASES WITH CRC AND JOINTCAST

An innovative 'superglue' for fusing elements on-site

CRC JOINTCAST

In his latest post, our Development Manager, Bendt Aarup, introduced the origin and concept behind JointCast—Hi-Con's special 'superglue' for fusing elements on-site into monolithic structures.

In this post, Bendt uses a few application cases to explain some of the challenges and considerations that need to be addressed to successfully use the JointCast material at the project level.

The Catharina Bridge in Leiden and the staircase at Per Aarsleff are good examples here.

They exemplify both the advantages and challenges of using JointCast and the creativity required to achieve a good result, structurally and, perhaps even more importantly, in terms of installation and aesthetics.

In both cases, the main reason for using JointCast is the large size of the final structure. Since Ultra High Performance Concrete (UHPC) CRC i2® cannot be cast on-site, element handling and transportation are limited to the size of individual elements. This is compensated for by using JointCast to fuse the elements so they structurally function as if they had been a monolithic piece. This is the greatest advantage of JointCast.

What is Ultra High Performance Concrete?

THE PER AARSLEFF STAIRCASE

Installation process of the Per Aarsleff staircase, showing the elements after setup, scaffolding, and joint casting (left), and one of the joints after the scaffolding was removed (right).

The challenges encountered in the above projects include:

Aesthetics - color and texture differences between JointCast and "normal" UHPC CRC i2®
Tolerances in the joints - managing both structurally and aesthetically acceptable tolerances
Curing time required - temperature-dependent
Execution - difficult to cast and a very critical part of the structural integrity
Scaffolding - often complex shapes from the joints

As a comment on Bendt's blog post, it was pointed out that a limitation of using JointCast is that its use needs to be considered early in the design phase—something we fully agree with, given the list of topics above. This is one of the reasons why Hi-Con primarily uses the material for our own elements.

SO HOW ARE THE CHALLENGES ADDRESSED?

Several challenges are related to the same task: casting the joint.

When joining two elements with normal installation tolerances, there will be misalignment. The width of the joint makes it somewhat easier to visually smooth out, but it will never be a perfect shape, just as formwork on-site can shift and cause small deviations from the intended form.

To minimize the necessary effort and finish the joints, the JointCast section is often cast 5 mm to be flush with the adjacent elements. This allows a painter to use repair filler, injection mortar, or similar to shape the transition between the elements without needing to sand away excess JointCast material—saving time.

The formwork is typically customized for each joint and ordered along with the formwork parts. When the above method is used for finishing, the formwork is attached with self-tapping screws drilled directly into the edges of the adjacent elements.

Of course, this means that the entire surface must be sanded down and painted to cover holes, patches of repair material, polished sections, etc. This is often the chosen option to handle the aesthetic issue—as was the case with the parapet of the Per Aarsleff staircase. The steps are not connected with JointCast, so the top surface remains the smooth, cast, soap-washed CRC i2® surface.

THE CATHARINA BRIDGE

An alternative approach is to use the elements themselves to conceal the JointCast as much as possible.

This was the case in the Catharina Bridge where an extended part of the edge profile's "shell" was used as permanent formwork for the JointCast in the structural cross-section of the bridge profile.

This "shell" solution was originally developed for another staircase, Copenhagen Plant Science Center. In this case, JointCast was poured from above through a 5 cm slot and could be inspected from below after casting to ensure that the joints were properly filled. Below is a video showing the entire underside of the staircase, with both finished joints, after the scaffolding was removed.

Video: Copenhagen Plant Science Center JointCast connections

In this case, the underside was not painted, but a render was added to create a uniform texture. To achieve the same color, a semi-transparent silicate paint was applied. This allowed much of the raw concrete "feel" of the staircase to be preserved while softening the color variation—before and after images are shown below.

The same approach was used on the Catharina Bridge, except that the joint was open to the top, allowing inspection during the casting of the joint. In this case, for architectural reasons, a colored finishing layer was added to cover the rough surface of both the elements and the JointCast.

In addition to the structural and aesthetic considerations in the final phase, the schedule must also be taken into account. JointCast develops strength very quickly in warm weather, and there have been instances where a structure was put into use only two days after JointCast was cast.

However, the material is quite temperature-sensitive, and in cold weather, temporary supports may need to remain in place for several weeks while the on-site temperature is monitored and used to calculate the ongoing accumulated maturity.

All the aforementioned aspects, from the design of element geometry, divisions, and reinforcement, to the casting and finishing of the joints, and the surface finish to the execution schedule, must be considered during the project preparation phase. This is typically an iterative process between the client, consultants, contractor, and Hi-Con—culminating in a formulated and agreed-upon execution plan, usually even before an order for the building is placed.

But don't let that scare you—at Hi-Con, we have more than 15 years of experience using the material and will guide you through the process—so continue to challenge us: If you can imagine it, we'll find an innovative way to realize it!