As a Marketing Coordinator, I have visited many concrete pours for building construction projects throughout my 20 years in the industry. Recently, I had the opportunity to visit a concrete pour for a multi-span bridge deck. It was quickly obvious to me how much more intricate highway concrete is. While any concrete slab must be functional, the smallest error in road construction can lead to cracked concrete, lost camber and compromised integrity of the bridge – leaving it unfit for travel.
To better understand how Kinsley delivers multi-span bridge projects, I sat down with two of our Highway and Bridge team members, Tim Ferrence and Chris Kotchish. They explained that multi-span bridges are comprised of two abutments with at least one pier in between, as shown in Figure 1. The space in between the piers is known as the span; a two-span bridge has one pier, a three-span has two piers, and so on. Once these abutments and pier(s) are in place, steel or concrete beams stretch atop the structures with an arch (or camber).
Understanding the Camber
On average, a bridge span can measure anywhere from 30 to 160 feet. Compared to a large building slab, like a warehouse, this doesn’t seem like much space, but what makes bridge decks so much more complex is the camber. When pouring a bridge deck, the camber flattens out as the concrete is poured, which must be done in a specific sequence to ensure the concrete remains pliable while the beams settle and flatten.
To get this right, engineers must consider a multitude of factors when designing the pour sequence, including the bridge’s length and the weight of the load for which it’s created. The parts of the beam with the greatest camber, known as the positive space, must be paved first. Only after concrete is placed over the positive space can it then be poured over the flatter areas of the beam, known as the negative space (see Figure 2). This ensures the beam has room to flatten and avoids cracks in the concrete when it cures. The parapet and median are poured last.
Throughout the design process, our Highway and Bridge experts work closely with the engineer to potentially revise the proposed pour sequence, ensuring it is the most cost-effective approach. This helps save time and money without diminishing the quality or stability of the bridge structure. We also conduct pre-pour meetings seven days prior to the first pour to ensure the team is familiar with all specifications and details.
The Importance of Concrete
One of the major factors in a bridge’s quality is the concrete – it requires the perfect mix with the correct air content, adequate strength, low chloride permeability and low drying-shrinkage potential. To ensure the concrete is mixed properly for the job, the concrete supplier tests it at their plant while our concrete technician subsequently tests it when it arrives on site. If the concrete isn’t the correct mixture when it arrives for the pour, it’s sent back and a new mixture is created. Each batch of concrete is tested for:
- Slump to ensure it has the correct water content
- Air entrainment which tests whether the concrete has room to expand during freezing temperatures to avoid ruptures in the surface
- Comprehensive strength to verify the weight of the load its designed for, such as vehicular or pedestrian traffic
- Evaporation rate to ensure it won’t dry too quickly before it gains enough tensile strength to resist cracking
Finishing the Deck
Once the team verifies the correct concrete mixture was delivered to the site, it is time to begin the actual pour. Kinsley usually uses a Bid-Well paving machine to grade, consolidate, finish and texture the concrete. After the pre-pour meeting, we perform a dry run with the Bid-Well. By setting up the machine after rebar has been placed but prior to the concrete pour, we are able to verify the thickness of the concrete deck and proper rebar clearance – yet another quality control measure.
Immediately after the concrete is placed, we cover the deck with wet burlap and then continuously spray with water for 14 days. This is necessary to protect the deck from the elements and maintain hydration through the early curing process, ensuring the deck doesn’t cure too quickly and become prone to surface cracking and scaling.
After the concrete has gone through its 28-day hydration process, we then mechanically groove the surface to add the necessary texture and traction to the road’s surface. Throughout the process, we work with government inspectors who verify the concrete deck depth during the pour and ensure all specified conditions are met.
Each step and quality test is vital to deliver the bridge without cracks, loss of camber or surface scaling – all of which can leave the structure unsuitable for use. The attention to detail and meticulous measures required throughout the process allows us to construct strong concrete bridges that are ready for travel.
Proud to Work for Kinsley
A bridge deck is not your average slab pour. I’ve driven on roads and bridges that damaged my tires or the body of my car and made me feel unsafe. Now I understand the intricate and meticulous process that must be followed to avoid this. I am impressed by the knowledge of my team and with the processes and quality control efforts my company employs. Most of all, I am proud of the company I work for and the bridges we build.