UHPC – Constructability meets Durability
All too often in construction when we gather around a set of plans, we immediately come up with some really good ideas to improve constructability. This can be detailing alterations which may make things easier on the laborer in the field, providing a safer alternative approach, or realizing a time savings alternative where components can be made in advance and connected simply, quickly to get out of a hazardous project or even meet a demanding schedule. However, even in our best, most well thought out approaches to simplify processes, durability is always a primary concern. We need longevity and resiliency in our structures, and it is the responsibility of all parties to ensure that we meet these goals.
Ultra High Performance Concrete (UHPC) offers a unique opportunity to improve constructability and provide even greater durability than the current methods of doing business.
Let’s explore one example. We do a lot of precast, prestressed marine concrete piers and wharves on concrete or steel piling. Typically, this is comprised of precast, prestressed caps spanning multiple piles and closures made between the ends of the concrete section. These piers can be several hundred feet long and the precast elements anywhere from 30 ft to 80 ft depending on crane capacity. The reinforcement projecting out of the ends of the members vary from #8’s up to #11’s. In some challenging projects, we have used headed #18 bars. The normal concrete closure is typically 4 ft to 6 ft long as illustrated in Figure 1.
A lot of these projects are on sites not easily accessible as the sites by land are typically congested and require special clearance and water even more complex yet each one of these closures only amounts to 2 or 3 yards of concrete. Filling these joints are also a critical path item where waiting for a large placement by a pump truck may or may not be possible and in some cases this all has to be coordinated with the tide. (Figure 2)
Enter UHPC as the alternative. Pre-bagged material may be shipped in advance and batching may commence onsite as needed for this critical connection. Studies are underway where bars up to #11’s may only require a 12” UHPC closure to fully develop the bar. This greatly reduces the volume required resulting in less loose material delivered to the job site as well. More importantly, this delays the requirement of normal concrete delivered to the job site until after the deck planks are set. Therefore, traditional concreting occurs only when there is a large platform for crews to walk on, safely and more efficiently.
The same value added occurs in bridges as well. If a precast, prestressed cap is used which is large or heavy, then a UHPC closure provides the connection thereby delaying the delivery of ready mix concrete until after the girders and stay in place forms are set.
Future of UHPC Caps in a Marine Environment
An entire new level of utilizing UHPC in marine environments is a shell structure as utilized in Malaysia (Figure 4). These are much lighter allowing for a smaller crane and enough capacity for the connection to the pile and dead load of the deck and infill concrete. The infill concrete can be placed from safely above doing away with the need for any extra formwork in the field where continuity reinforcement is placed inside the shell. A discrete corbel at the outside of the shell may support transverse caps or a continuous ledge may support deck slabs greatly simplifying the construction process and taking UHPC from predominately a connection material to a cost effective, value added component.
Enhancing safety while encouraging constructability, UHPC provides a rare opportunity to think outside the box with inherent durability in the material or the connection thus changing the approach to construction.