A Chip Off the Old Block: Restoration of Concrete Masonry Units


Image details

By Christa J. Gaffigan, AIA, LEED-AP BD+C and Anne E. Weber, FAIA, FAPT

Concrete block developed in the early 20th century as an inexpensive yet durable material for vernacular construction. It was used extensively in industrial and commercial construction, and was also marketed heavily for agricultural and residential construction. By the 1950s, block was in wide use for schools and similar structures, and was available in many sizes, face finishes, and shapes.

Bath House after restoration. Credit: Brian Rose

In the 1920s, concrete block had a strong presence in the do-it-yourself market, with many publications offering instructions to farmers and homeowners for constructing farm buildings, garages, and even houses. Several companies including Sears Roebuck sold mixers and machines which would allow individuals to make their own blocks for small projects such as garages or garden walls. Many of these machines produced blocks with decorative faces. Popular Mechanics’ Famous Concrete Block House of 1949 included a section on making your own block.1Lightweight block was developed commercially in the 1930s, with Haydite and Waylite being two of the prominent brands. Block production surged during and after World War II, with 1.6 billion concrete masonry units (CMU,) as they are called technically, produced in the US in 1951.2As manufacturing increased, ASTM Standards C129-14a and C90-15 were developed to cover non-loadbearing and loadbearing concrete masonry units.

While conservation and restoration of this material follows the same procedures as that of stone or other masonry materials, there are some issues particular to concrete block to be aware of. The restoration of two modern works constructed of concrete block – the Trenton Bath House in Ewing, NJ and the Paepcke Memorial Building at the Aspen Institute – illustrate a few of these issues.

In 1955, the Jewish Community Center of Trenton commissioned Louis I. Kahn to design its new suburban facility in Ewing, NJ. The master plan included a pool complex with two pools and changing rooms; a day camp facility; a community center; and playing fields. The pool complex, now known as the Trenton Bath House, was the first piece to be constructed, opening in the summer of 1955. Kahn selected cost-effective materials including CMU, wood roof framing, and asphalt shingle roofing for the four structures in the nine-square grid of the plan.

After 50 years of exposure to the weather of central New Jersey, the CMU was severely deteriorated in several locations. The monumental pyramidal roofs on the pavilions had no gutters, and rainwater ran directly over wall surfaces in many locations, causing erosion of the block surface. This erosion created a very hospitable surface for organic growth and build-up of soiling. At two locations, the eave of the roof is directly above the single wythe CMU wall. Saturation of this wall caused cracking, displacement, and severe erosion of the block, along with excessive organic growth. At many locations one could see light through the block construction, and the walls were deemed by the design team to be structurally unsound.

 Bath House before with covered mural. Credit: Brian Rose

The restoration challenges faced were cleaning and removal of paint, mastic, and organic growth at existing CMU; and sourcing replacement block. Replacement block would be needed to reconstruct the two structurally compromised walls, to build some new partitions in the changing rooms, and for spot replacement of damaged blocks.

Most of the soiling and organic growth on the CMU could be removed with a biocide and low-pressure washing. Low pressure was used because at some locations, the block had become quite fragile due to the surface erosion. In other places, higher pressure could be used for more stubborn staining.

Paint and mastic were found on the block after a snack bar added in the late 1950s was demolished. Trials of chemical cleaners and solvents had mixed results, so mechanical methods were considered. Various blasting media were tested in a Torbo blasting machine, and walnut shells were selected as effective, but minimally damaging to the block. Various other media were more effective, particularly on the mastic, but did an unacceptable amount of damage to the CMU. Ghosting remains from the mastic because of the penetration of the black material into the block, and where it was particularly objectionable, some blocks were replaced.

The first round of samples of replacement block did not yield a good match. The new block had a much denser and smoother texture than did the largely unweathered block at walls protected by the large roof structures. During this process, the Owner discovered Kahn’s specifications in the files of the Jewish Community Center, and the block was identified as Waylite, one of the most widely used lightweight block formulas. Trenton Block was still manufacturing Waylite in Trenton, and their second round of samples was much more successful. In the third round, a local aggregate was used, leading to a good match for the unweathered block. The new block was used to rebuild the two courtyard walls and for large areas of patching. Unweathered original block mined from plumbing chases was used for smaller areas of infill.

The original highly porous surface of the Waylite block contributed to the level of deterioration found at the Bath House after 55 years; the material was quite absorbent, and the open surface provided lots of nooks and crannies for soiling and organic growth, especially when the block was wet. A 1939 catalogue for Waylite SuperPoke recommends that the exterior be waterproofed with several coats of cement paint, which likely would have improved the durability of the surface, but would not have conformed to Kahn’s philosophy of expressing materials naturally.3

Both the replacement and restoration of CMU at the Bath House were successful, but will require ongoing maintenance to retain their appearance.

 Bath House after with restored mural. Credit: M+Sa Staff

The Paepcke Memorial Building, designed by Herbert Bayer, is located on the campus of the Aspen Institute in Aspen, Colorado. The building remained largely unchanged 40+ years after its completion in 1962. Bayer conceived a building of simple forms using load-bearing concrete block walls and wood-framed roofs. The building houses the Institute’s administrative offices and auditorium, which hosts a range of programs including scholarly lectures, films, recitals, and children’s theater.

During the design phase of the restoration project, it was determined that the concrete block was most adversely affected by water infiltration at the auditorium portion of the building. Water from ice, snow, and the landscape sprinkler system in the spring and summer was saturating the wall. Moisture was trapped behind the paint, damaging the coating.

The design team, which included conservators from The Collaborative Inc. (tick) and Conservation Solutions Inc. (CSi), performed a materials conservation investigation to determine the best method for removal of existing paint from both the interior and exterior walls of the auditorium concrete block. Microabrasive cleaning was tested along with three chemical paint strippers.

Testing determined that the microabrasive technique with crushed glass media was the most effective and comprehensive method of paint removal, and left the existing block with a near new appearance. For chemical strippers, Zip Strip provided the best results among the strippers, removing the most paint without clogging the pores of the concrete block.

Mineral paint (Silin AZ) was then applied to three areas in an effort to evaluate how effective new paint would be in covering the remaining paint while still retaining open pores of the concrete block for acoustical and vapor transmission properties. These areas were then tested with Rilem tubes to determine the effect of the newly overlaid paint on the vapor transmission coefficient of both the existing painted areas and raw concrete block.

It was determined that moisture was not able to pass through the newly coated exterior concrete block under normal weather conditions, and the newly coated interior wythe of concrete block did not inhibit the passage of moisture, which was the desired outcome. In addition, the mineral paint on the exterior wythe allowed water vapor to escape to the exterior while inhibiting water moisture intrusion. Therefore, the goal of preventing moisture buildup within the concrete block could be achieved.

 Image Left: Paepcke Memorial Building interior showing CMU deterioration. Credit: M+Sa Staff. Image Right: Paepcke Memorial Building interior showing CMU after restoration. Credit: Michael Brands

During construction, the implemented methods varied slightly from what was determined to be most effective during design. This was due both to cost as well as the remote location of Aspen, which made it more difficult to find trained technicians. The contractor used a chemical stripper only to remove the majority of the existing exterior paint, and did not use the microabrasive method. At the interior, the existing single layer of paint was covered by a lightly applied spray coating of the mineral paint, which allowed the color to be updated without negatively affecting either the acoustical properties or vapor transmission of the wall. The contractor worked to match the mineral paint to the color and sheen of the regular latex paint applied beyond the auditorium, both at the exterior concrete block and interior gypsum board walls.

The Paepcke Memorial Building project resulted in the successful restoration of the original 1960s concrete block walls of this iconic building, while maintaining the necessary physical properties of the wall construction.

The established principles of masonry restoration developed primarily for stone and brick are currently being applied to concrete block, with little specialized information available in the literature. These methods were applied successfully at the Paepcke Memorial Building and the Trenton Bath House. As more significant CMU buildings of the second half of the twentieth century undergo restoration and are documented, the literature and state of the art will continue to improve.

 Paepcke Memorial Building after restoration. Credit: Michael Brands

Christa J. Gaffigan, AIA, LEED-AP BD+C, Senior Associate at Mills + Schnoering Architects, is a preservation architect with expertise in a range of institutional, educational, and cultural projects. She was the Project Architect for renovations to the Paepcke Memorial Building, and for the preservation plan for the Trenton Bath House and Day Camp.

Anne E. Weber, FAIA, FAPT, is a Partner at Mills + Schnoering Architects where she specializes in the management of large-scale preservation projects. In addition to the restoration of the Trenton Bath House, she has performed work at the Statue of Liberty, the Essex County (NJ) Courthouse, and the Gateway Arch.


1. Ward, James R. Popular Mechanics Famous Concrete Block House. Popular Mechanics Press, 1949.

2. Simpson, Pamela H.; Hunderman, Harry J.; and Slaton, Deborah, “Concrete Block” in Twentieth-Century Building Materials: History and Conservation, Thomas C. Jester, editor. 2nd edition, Getty Conservation Institute, 2014.

3. The Waylite Company, Waylite-Superock: the modern building material: plan book, 1939. p. 5.