V28-2 The case of the Rosehill Cemetery walls

Vol. XXVIII No. 2 - SPRING 2017

By George W. Seegebrecht

Editors note: The following article was sent to us by a real scientist who studies concrete. He took it upon himself to study the concrete walls around Rosehill Cemetery, which have lasted a long time. He started with the question: How can these walls have survived so well for so long?

To understand this you must first be aware that these are not conventional concrete walls. They are constructed of a kind of concrete called Pervious concrete. Perhaps you have noticed some deterioration problems as you drive by on Peterson Avenue on the north side of the cemetery.

Conventional concrete is made up of cement, water, stone, sand and air. We have all seen the huge concrete mixers making concrete but I bet you never thought of air as being part of the mixture. Once conventional concrete is mixed and put in place the intent is to have a well compacted strong and durable concrete that resists the penetration of water.

Now what about the air. A special admixture generates bubbles in the paste and mortar of the mix which are evenly distributed throughout the mix but make up 9% of the concrete by volume. These bubbles provide a pressure relief valve to dissipate the moisture collected in the freezing and thawing of a tough Chicago winter. When the ice expands it fills in the bubbles and when it melts it evaporates and runs off. This discovery was made in 1950s and greatly improved the quality of the concrete in our environment.

The concrete of the Rosehill Cemetery walls is a different concrete, called popcorn concrete or more technically pervious concrete. Here’s the article from George Seegebrect of CCE in Westchester, Ill.

Reinforcing Steel in Pervious Concrete

While pervious concrete (also known as no-fines, porous or popcorn concrete) has recently received much attention as a pavement material, this type of concrete is not new and its applications haven’t been limited to pavements. In fact, records show that walls constructed using no-fines concrete (two houses and a seawall in the United Kingdom) date as early as 1852. Further, no-fines concrete has been used in construction of single and multi-story buildings in the United Kingdom and other European countries since 1930.

Pervious concrete has also been used to construct walls in the United States, including concrete walls and various building elements at the 63rd Street Beach House and the perimeter boundary walls of the Rosehill (Fig. 1) and St. Boniface Cemeteries in Chicago, IL. Over the past 18 years, periodic observations were made of the condition of these walls. When possible, concrete samples from spalled locations were examined. This article summarizes the observations and discusses the surprising performance of this reinforced pervious concrete.

History

The Rosehill Cemetery was chartered in 1859, and the first plat comprised about 60 acres (24 ha). From time to time, the Rosehill Cemetery Company acquired additional land and the cemetery grew to about 320 acres (130 ha). According to court records regarding a disputed further expansion of the cemetery in the 1930s, the concrete wall was constructed in about 1928.

In recent years, some of the wall segments and pilasters on the Rosehill Cemetery wall have been replaced or removed due to damage or changes in use. The walls on the western boundary of the cemetery, for example, were replaced with wrought iron fencing around 1990, mainly because the cap stones on the wall’s pilasters were falling (information provided by the late Albert Litvin, while at CTLGroup, Skokie, IL).

During one of the replacement projects, square twisted reinforcing bars, were exposed. This type of bar was patented by E.L. Ransome in 1884, and produced by Carnegie Steel in Pittsburgh, PA, and Ryerson Steel in Chicago, IL. This bar type would be expected to be used in a wall constructed in 1928.

The St. Boniface Cemetery, which was consecrated in 1863, is just a few blocks southeast of Rosehill Cemetery. While the construction date for the pervious concrete walls at the St. Boniface Cemetery is unknown, the walls are quite similar to those at Rosehill. Although significant portions of the St. Boniface walls were demolished and replaced with fencing in 2012 and 2013, many sections of the original walls remain.

Decades of Service

Pervious concrete has been shown to perform well under freezing-and-thawing cycles, largely because it drains rapidly and therefore can’t become saturated. Placing reinforcing steel in pervious concrete would, however, seem to present durability issues. An advancing carbonation front (the change in the steel that occurs when it encounters moisture) would be expected to quickly reach the steel, resulting in relatively early initiation of the corrosion process (with sufficient moisture conditions). Expansive pressure exerted by the formation of corrosion products could then be expected to eventually cause visible corrosion related damage (spalling which may be chipping or flaking). Yet, despite these seemingly negative characteristics, significant damage of the walls at Rosehill Cemetery occurred only after many years of service. Why have the walls performed well for over eight decades? We begin to answer this question simply by examining the potential corrosion mechanisms for steel embedded in pervious concrete.

Concrete Qualities

During one of the recent replacement projects at the Rosehill Cemetery, samples of the pervious concrete were obtained. The concrete contains 5/8 in. (15.9 mm) crushed limestone, but it is not strictly a no-fines mixture. Based on laboratory examinations, approximately 5% of the total mass of the mixture comprised fine aggregate.

The pervious concrete walls drained water well and exhibited no widespread freezing-and-thawing deterioration except in isolated areas where drainage appeared restricted.

The vertical orientation of the walls, the mostly unobstructed exposure to air flow on both wall faces, and the interconnected void system of the mixture almost certainly helped to avoid critical saturation, thereby reducing the probable damage due to freezing and thawing.

Over the past 18 years, increasing incidences of visible distress have been noted. The distress commonly occurs above the interface of the pervious concrete wall and the mortar-coated porous concrete foundation (Fig. 6).

While the majority of the visible sections of newly exposed steel bars were in excellent condition with almost clean bar surfaces, those located in zones with reduced drainage near the wall/foundation interface (where repairs have been made using conventional mortar) exhibited heavy corrosion or complete section loss.

It can be difficult to match repair materials with existing concrete, but aesthetic and durability requirements frequently call for repair mixtures similar to the base material. The mortar repairs observed on the pervious concrete wall, however, don’t meet that simple objective. The relatively non-pervious mortar used in the wall repairs would be expected to clog the pervious concrete void structure, trapping moisture within the member and impeding drainage from the wall. This can be expected to contribute to both corrosion and freezing-and-thawing damage. To restore appearance and improve performance, it would be prudent to conduct trial mixtures as Gaudette and Stanton did for the 63rd Street Beach House evaluation.

Some of the walls are, however, located near busy city streets. It’s possible that road salt would not have affected the pervious walls for at least the first two decades of their existence. Deicing chemicals, particularly rock salt, did not come into heavy use on U.S. streets and highways until the 1950s. When road salt did become more common in the 1960s and 1970s, however, spray from passing vehicles would have transferred chlorides to the walls, and the pervious concrete would have allowed the chlorides to penetrate and possibly reach the reinforcing bars. While rain water would be expected to remove some of those chlorides, poor drainage in the lower portions of walls would be expected to result in increasingly higher chloride concentrations at the interface with the conventional concrete foundation.

For more on the factors that effect the deterioration of pervious concrete please read the complete article on the EHS website: www.edgewaterhistory.org/concrete

Moisture

The cemetery walls are exposed to atmosphere on both faces. Wind-driven rain can therefore be expected to enter the no-fines concrete and increase the relative humidity within the wall. However, air movement also allows relatively quick drying of the walls. Long periods of dry weather could lower interior relative humidity levels, with the net effect of reduced rates of corrosion until the next rainstorm.

Because the foundation concrete actually consists of the same porous concrete but covered with a uniform mortar coating approximately 1/8 in. (3 mm) thick, this coating resists rain penetration and capillary rise from below quite well until the mortar coating is breached. It’s likely that the concrete for the walls was mixed in the field and placed in shallow lifts.

Decades of Success

The pervious concrete walls at the perimeter of Chicago’s Rosehill Cemetery have been in place for at least 85 years. While the embedded reinforcing steel has exhibited corrosion in recent years, the walls have performed surprisingly well. Corrosion has been predominantly limited to the zone near the interface between the pervious concrete wall panels and the mortar-coated porous concrete foundation.

Unfortunately, distress suggests that drying has apparently been prevented at the interface between the pervious concrete wall and the mortar coated pervious concrete footing. The footing may also be allowing an accumulation of chlorides from road salts used on nearby streets. Corrosion damage is causing spalling near the base of the pervious concrete wall panels while inappropriate selection of repair materials may be compounding the problem.

George W. Seegebrecht, FACI, is Principal of Concrete Consulting Engineers, LLC, in Westchester, IL, specializing in troubleshooting of concrete and shotcrete. He is a licensed civil engineer in Illinois and other states, with 35 years of experience in the construction industry. He is an associate member of ACI Committee 506, Shotcreting, and a member of ACI Committees 522, Pervious Concrete; C610, Field Technician Certification; and C630, Construction Inspector Certification. He is also Chair of Joint ACI-CRSI Committee C680, Adhesive Anchor Installer Certification. A member of the American Shotcrete Association and the International Concrete Repair Institute, he is an educational seminar speaker for the Portland Cement Association and the University of Wisconsin.