Concrete versus Limestone

posted by Elizabeth Fama

You've got to hand it to the SAVE THE POINT group: they had a brilliant marketing campaign. Look at this (horrifically misleading) ad they put in the Hyde Park Herald in April of 2003, when they knew full well that the stretch of all-concrete revetment between 51st Street and 54th Street was no longer what the City was offering us:

Apparently, all's fair in love and marketing.

Implicit in this visual message is not only a critique of design, but also of material. The concrete is photographed to look cold and sterile (at the sight line of a rat), and the limestone is photographed from above, with warm human beings delighting in their surroundings.

In typical bureaucratic fashion, it took the City too long to come up with its own visual campaign of what it was really offering:

Some of our readers have commented that they do prefer the look of limestone, and they have wondered all along why the City can't just re-build Promontory Point out of limestone. After all, the limestone blocks from the 1930s are fully intact, while some of the cement sidewalks the City poured just last year in Hyde Park aren't.

I'm not a structural engineer, but I'll tell you what I know about concrete versus limestone.

1) Yes, individual limestone blocks are hard and durable. But the Army Corps of Engineers will not build a revetment for us that does not have a steel and concrete core. Why? Because limestone blocks can't be anchored against wave action and the shifts and separations that freezing and thawing cause.

2) If good-quality concrete is poured in the right way and at the right temperature and humidity, it's durable -- very durable. The contractual requirement for the lakeshore project is that the concrete must resist 5,000 lbs of pressure per square inch. As the contractors pour the concrete, they pour sample cylinders on site for the Army Corps to test after the concrete has cured.

3) The Army Corps and the City are willing to re-use all of the existing limestone, by (a) making the top two steps of the revetment out of limestone blocks, (b) building limestone steps into the water for swim access, and (c) covering the steel pilings with a tumbled limestone toe berm. These non-structural uses of limestone will at least allow us to retain some of the beauty of the stones, to sit on them, and to wade on them. The Illinois Historic Preservation Agency has said that this mixed-material design satisfies its requirements for historic preservation. The famous Memorandum of Agreement (a document that I'll recap in a future post) does not specify building materials, nor does Barack Obama's "Scope of Work" document.

Here's a series of photos that I find revealing about the strengths and weaknesses of limestone:

In the foreground of this shot you can see how the wood pilings and steel rails have failed, and the limestone blocks have shifted and fallen. (The old sub-structure is a "crib" or cage of wood pilings, steel rails, and tie-rods, which is filled with "stone aggregate," or coarse, crushed stone.) In the background of this photo you can see the "concrete coffins" at the tip of the Point -- a concrete repair of the promenade that took place around 30 years after the Point was built and is now itself around 45 years old. The concrete coffins are still very much intact.


Thus, concrete as a material is sturdy enough to handle 45 years of crashing northeast waves, and 45 winters of freezing and thawing. But that's only part of the story. Since the sub-structure at the tip of the Point was never repaired, the wave action is still eroding the stone aggregate underneath the concrete promenade, and causing the limestone steps behind the coffins to buckle:

The stone aggregate must be almost gone under this section. But even beyond that, the erosion of the waves is sucking dirt and sand from under the grassy meadow-side of the revetment as well:


When the Point was built, the soil level reached the top of the top step of the revetment. Now my friend Gerald can stand 3 - 4 feet deep in the ditch behind the top step.

And just look at these caves:

(My kids are not allowed to walk above this section.)

The new revetment won't have a "crib" by the old definition. The contractor will drive strong, stable steel piling, and then backfill with crushed stone before pouring the concrete promenade. The concrete is reinforced with rust-resistant rebar (rust would cause the rebar to expand, which would crack the concrete around it). The steel piling does most of the structural work, and the concrete core protects the backfill from erosion because it doesn't allow water to penetrate.


In the end, the longevity of the core of the new revetment will depend on sound engineering and quality control of inputs and labor, not on whether it's built of limestone, or concrete, or kryptonite.