Most of us in the construction industry is very familiar with pilings. Two of the most common types of piles are concrete piles and steel piles. While these two types of piles are very different, each offers certain advantages over the other.
With the introduction of new and innovative concrete technology—Ultra-High Performance Concrete (UHPC)—many of the advantages that steel may have enjoyed need to be re-examined.
We are even able to predict that UHPC piles will usher in the demise of steel pile foundations.
The head-to-head: Steel versus concrete piles
Again, most of us are familiar with the advantages of steel piles:
- Often seen as producing the strongest piles in the industry because of their impressive tensile and compressive strengths (typically in the range of 50,000 pounds per square inch [psi]).
- Lighter than traditional concrete piles, making them easier to ship and install.
- Easier to splice than standard concrete piles, via welding.
- Used for supporting heavier structures, such as skyscrapers and large bridges.
- Can penetrate hard layers in the ground.
The disadvantages of steel piles:
- More expensive than concrete.
- Corrode and deteriorate more easily than concrete; this is typically the result of moisture in the environment and reactive soils.
There are two types of concrete piles to consider—precast concrete piles and cast-in-place concrete piles. The main distinction between the two is that precast piles can be built in various shapes and sizes to meet the needs of your project.
Advantages of concrete piles:
- Less expensive than steel.
- The quality of the concrete can be checked before driving the piles into the ground.
- More stable in softer soils; local buckling is less of a factor.
Disadvantages of concrete piles:
- May cause a lot of soil displacement.
- May be damaged during the driving process.
- Can deteriorate over time due to adverse environmental conditions, such as moisture, seawater, and freeze/thaw cycles.
- May require extensive reinforcement such as carbon fiber or stainless steel to improve durability, thus adding cost and challenges during construction.
UHPC concrete piles: Combining the best of both worlds
Ultra-High Performance Concrete (UHPC) is an innovative concrete material that is stronger and more durable than traditional concrete. UHPC overcomes the typical challenges with steel piles—but it also delivers the same strength advantages of steel, making it the best choice for any structure requiring pile foundations.
Concrete piles made from UHPC are stronger and last longer, increasing the quality and safety of the structures they support. Research has proven the advantages of using UHPC for piles over other materials.
Testing was done in Iowa for UHPC piles for bridges. The results showed that the UHPC piles demonstrated strong durability and excellent performance under very difficult driving conditions; even without the presence of a cushion. Piles made of UHPC could also be made shorter (or fewer could be used given the added strength of the material), creating efficiencies in the construction process.
A recent publication from PCI also reported the many benefits of UHPC pilings, including the advantages over steel. UHPC piles demonstrated little to no shrinkage and had a high modulus of elasticity. Given UHPC’s self-leveling quality and steel fiber reinforcements, piles made from UHPC have reduced cross-section requirements and exhibit weights and stiffnesses on par with those of steel piles. UHPC piles have been proven to withstand hard-driving and show superior durability.
When compared to steel piles, UHPC piles deliver many of the same advantages (at virtually the same price point), while overcoming the disadvantages:
- Strength—UHPC has a compressive strength of 30,000 psi once fully cured, providing an opportunity for precompression by pre-tensioning. This improves drivability and does not significantly impact the service or ultimate axial capacity. Some UHPC mix designs can even achieve 50,000 psi, matching that of steel.
- Resistance to environmental degradation—Testing of UHPC showed that samples were impervious to water, had negligible surface damage, and had resisted more than 1000 freeze/thaw cycles and storms.
- Permeability—UHPC has a higher density than regular concrete. This makes it virtually impossible for ordinary water and chloride ingress to penetrate the surface of UHPC.
Other advantages of Ultra-High Performance Concrete for pile design include:
- Reduced material requirements—Less material is required to build UHPC concrete piles, saving money on the cost of materials.
- Reduced maintenance—The many benefits of UHPC directly translate to reduced maintenance requirements. Structures last longer and require less upkeep.
- Cost savings—This new concrete technology delivers a much lower lifecycle cost. Because less material is required for construction projects, footing and support requirements are reduced. The use of UHPC concrete piles can reduce the total number of piles needed for a bridge foundation. The extended longevity of UHPC virtually eliminates maintenance costs and drastically extends the life of the product. When the cost of UHPC is spread out across the life of the product, it has extensive cost benefits over steel.
Why use Cor-Tuf for UHPC concrete piles
Contractors and construction companies should be aware that there are a number of UHPC products on the market, but they are not all created equal. Many conventional UHPC mixes are incredibly strong and durable, but present other challenges when it comes to usage and consistency.
For example, most conventional UHPC products cannot be mixed, poured, or transported using standard concrete machinery. They cannot be mass-produced either, which creates unwanted downtime on the site and can create inconsistencies in the end product.
Cor-Tuf UHPC, however, overcomes all of these barriers, making it the right choice for any project, especially those involving concrete piles.
Cor-Tuf UHPC can be mass-produced using standard machinery. You can get a consistent product with a uniform mix throughout the piling. In fact, tests have proven Cor-Tuf UHPC’s consistency, showing no line of demarcation between batches. With conventional UHPC (and even with traditional concrete), there is often a cold joint between pours.
This occurs when one batch begins to set before the next is poured, and it creates points of weakness in the structure. Tests of Cor-Tuf UHPC have demonstrated that no cold joints form between batches, providing one consistent piece that is stronger and more durable.
Pourability is one of the main benefits of using Cor-Tuf UHPC over conventional mixes for concrete piles. Cor-Tuf UHPC pours in the same manner as traditional concrete and is even pumpable. This makes it easy to use for large structures such as pilings (whereas other UHPC mixes are more difficult to work with).
Another benefit of Cor-Tuf UHPC is that it can be customized to meet the needs of your concrete piling project. The proprietary mix can be modified to increase or decrease working time. Even the strength can be customized, making sure you get all of the support needed for a given project.
Specific testing has even been done on concrete piles made of Cor-Tuf UHPC with excellent results. In these tests, one 100-foot pre-tensioned piling and two 30-foot pre-tensioned pilings were produced using Cor-Tuf UHPC in a large, mass-production batch plant.
Tests confirmed the precast pilings were free of voids and showed no signs of inward shrinking during the curing process. The 30-foot piling was able to withstand 84,000 psi of downward flexural pressure and 4.5” of deflection before failing. When the piling cracked, its failure point was consolidated to one area with no additional radiating cracks along the sides of the piling.
The 100-foot Cor-Tuf UHPC piling was subjected to pressure from a D36 and a D62 hammer. Regardless of which hammer was used, or how forcefully or rapidly we attempted to hit the piling, our data from the event showed that the structural integrity of the piling never dipped below 100 percent.
While steel piles offer a number of advantages to contractors and engineers, the arrival of concrete piles made from UHPC is ushering in a new era. UHPC concrete piles deliver several advantages over steel piles and at virtually the same price point, making them the smartest choice and wisest investment for your future piling projects.
We are currently working on some UHPC H-piling projects in the Mid-Atlantic region. Stay tuned to this space for full test results and an upcoming video from our most recent flexural testing sessions with FDOT in Tallahassee, Florida.