February, 2021: Crouched between bushes at the top of a steep hill, listening and watching for activity.
First sighting from inside the perimeter: a huge hare, bounding between stacks of scattered concrete beams, beams left like a child’s building blocks to gradually erode and gather moss and become part of the Irish landscape.
Alone, then. Hares aren’t known for liking human activity. We scramble down, and in.
Part 1: exploration
On a walk around the town we spotted a confusing grey cliff face in the distance. Cliffs in a rolling green landscape where cliffs don’t belong, exposed grey stone in a suspiciously sharp line. Mapping and satellite views narrowed it down to a quarry, and street view showed it abandoned, fenced and warning-signed, but huge… big enough that fencing the entire perimeter would likely be cost-prohibitive, and remote enough that the casually interested wouldn’t bother to test it. We made plans for a long walk.
The main entrance is aggressively fenced, inaccessible and in plain view along a main road. A few minutes down the road, though, brush and brambles sweep up a hill, and a few trodden areas indicate other explorations. During a break in traffic, up we go.
A long, single-story building stretches across the edge of the site, the doors at the ends open to the elements, rust gathering inside. A huge space, long and narrow, with strange flat beds running almost the full length. Walkways along the sides of the beds, and tracks above with huge hooks. Switches and warning lights, prominently placed. Huge plates at each end of the beds, some less-valued equipment left in a corner, but the expensive working things removed, sold to other places, other plants. At the end of one bed, a corroding, cryptic warning sign about stressing procedures:
So many questions about the meaning of the warnings, and the purpose of the building, but that comes later. First, more outdoor space to explore!
The area is a moonscape, vast stretches of mounded grey stone, different sizes, different purposes. Puddles, green with algae as bright as cartoon chemical spills, best avoided just in case. A tall structure looms in the distance through a lunar valley. Creaking sounds interspersed with the odd shriek, bone-chilling at first until a monocular helps identify the source as a ragged bit of sheet metal, blowing in the Irish breeze. The hare reappears as if to reassure us, and we’re off.
Silos, storage tanks, pipes, conveyor belts, and space for trucks to be filled from above. Hoppers full of uniform grey stone, labeled by size (10mm). Huge vertical tanks with warning signs proclaiming “Penetration Grade Bitumen” stored at high temperatures and apparently liable to explode. Heavily rusted tanks for oil and possibly water, patterns in the rust like maps of other planets. Open pipes from the bitumen tanks leading to a hardened black lava-like spill, prompting Pierre to proclaim it his “first industrial bitumen spill site” and me to avoid stepping on it, unfamiliar with the term and potential dangers involved. Unnecessary caution, it turned out, but one can never be too careful around unfamiliar industrial processes.
Expansive views from the top of the loading area, where trucks once poured pre-sized gravel through grates into hoppers bound for the production zone. Signage that could be informational, or cautionary, or locational (“please send my letters to 17 Tar Dust Street”). We plot the next move, toward the satellite-spotted water-filled quarry itself.
Around the bend the sheer walls of the quarry sweep upwards, edged with angled remnants of earlier layers of mining. The pit is vast and the water a beautiful turquoise: still, glassy, deep, and terribly dangerous. Signs announce the presence of surveillance cameras near the water, likely due to several swimming deaths in local quarry pools a few years back. A good percentage of the lakes in this part of Ireland are remains of quarries, deceptively welcoming on the surface but cold and very, very deep, with limited or no exit points. Cameras or not, we stay far away from the edge, well aware of the danger and content to admire the view from a safe distance.
Away from the mine is a working area, where the stone would have been ground down to size, stacked in holding areas, then trucked away to be processed onsite or sold elsewhere. Lots of art here, and crumbling metal, and a confusing half-tunnel ending in a stone wall.
From there we go back to the starting point, past another tall structure (for mixing concrete, in retrospect?), catch one more sighting of our friend the hare, and make a quick dash down to the road.
Part 2: research
Further investigation at home revealed that bitumen is what I know as asphalt – unpleasant to breathe, but not deadly. And that lava spill? Just road-building tar. I also learned more than I ever expected to know about the process of pre-stressing concrete, and now the warning sign in the long building makes perfect sense.
Pre-stressing concrete is a method for making it stronger and better able to support weight without cracking. It’s used in bridges, silos and storage tanks, nuclear facilities, and tall buildings. The idea has been around since the late 1800’s, but sufficiently strong cables and concrete didn’t exist until about the late 1920’s, so the process developed later.
First, a set of steel cables are stretched horizontally the length of the concrete casting bed (which is long enough to cast multiple segments at once), and attached to an anchor at one end. The other end gets attached to a barrel & wedge that’s used to firmly secure the cable and keep it from slipping free. Then this part is attached to a hydraulic jack. The cables may be a single row, or they may form a pattern (e.g. a row at the bottom of the concrete beam and a few placed nearer the top to balance it out).
Then the cables are tightened until they’re straight and taut, and checked for any flaws and tangles. Chains are strapped across the cables at regular intervals, to help minimize disastrous recoils if they should snap. The casting mold is set into place (e.g. for a bridge girder), and additional elements are added (for example, vertical pieces of regular steel rebar to strengthen the finished segment against sideways shearing forces like wind). Then the area is cleared of humans and the jack is engaged, tightening and stretching the cables with an enormous amount of force. This can be up to 80% of the total strength of the steel cables used, but the exact tension depends on the specifications for the project and its expected working load.
Once the appropriate load is reached without any cables snapping, workers re-enter the space and (very carefully) pour concrete into the mold, over and around the stretched cables. The concrete hardens and cures (sometimes heated at this stage) until it’s completely set and adhered to the cables. Then the jack is loosened, which transfers the tension from the stretched cables into the concrete, compressing it tightly. These forces are so powerful the concrete segment can actually bend under the pressure, creating a slight arch. This is useful as an additional measure to counteract weight applied from above (e.g. when a truck rolls over the bridge) and also helps prevent the segment from visibly sagging the other direction (which makes people nervous). The cable-ends are cut at the end of each segment, and the concrete is ready for use.
Prestressed concrete is much stronger than normal reinforced concrete, because the tension means the entire concrete segment can support the load it’s under. In normal reinforced concrete, only the portion of the segment between the load and the axis is actually resisting the load. That means pre-stressing (and post-stressing, which is a similar but opposite process) the concrete leads to a better weight-to-strength ratio, which allows segments to cross longer unsupported spans. It’s also more resistant to cracking and more durable. Pretty neat!
- What is Prestressed Concrete?
- 9-minute video from Practical Engineering, with lab-sized demonstrations of techniques
- Why Prestressed?
- Short overview and history from the National Precast Concrete Association
- How does a prestressed, precast concrete bridge beam work?
- 7-minute video from an Irish precast concrete company
- British Precast Code of Practice for the Safe Stressing of Prestressed Concrete Products
- An highly detailed step-by-step guide to the process, with illustrations
- A Good, Hard Look at Prestressed Concrete by Kristina Panos
- Hackaday blog post
- Pre-Stressing Company Policy Statement from FP McCann
- Safety practices with pictures; a nice supplement to the warning sign above
- Prestressed Concrete
- Where would we be without Wikipedia.
- Production of precast prestressed elements on casting bed
- 8-minute video of the process in action in a facility like the one pictured above, in Spanish with subtitles
- Full-Scale Testing of Prestressed Concrete Structures by Susanne Herta Buchner
- PhD thesis detailing techniques for measuring the stresses in prestressed concrete structures after they’re built