Methodology

Mixing the Materials

The concrete mix was composed of the following materials:

· White portland cement meeting ASTM C-150 Type I Standards
· Inorganic masonry pigment, chromium oxide green as manufactured by Mineral Pigments Corporation
· Coarse aggregate (gravel) 3.8 to #4 “Eau Claire”, dry and bagged
· Fine aggregate (concrete sand), “Eau Claire” passing a #16 screen, dry and bagged
· Air-entraining agent (liquid admixture of the vinsol resin type conforming to ASTM 260-77)
· Water-reducing agent, liquid type conforming to ASTM C-494, “Chemical Admixture for Concrete,” Type A (Pozzolith 122-N)
· Water, clean, clear, potable with a pH reading between 5-8 and the conductivity not exceeding 100 MILLIMHOS on the 0-200 scale.

These materials were mixed in a paddle mixer large enough to combine the materials so that each batch would not exceed the number of molds that could be vibrated without tempering the mix with water. Each day, before the start of mixing, a minimum of 2 cubic feet of design mix “butter” was run through the cleaned mixer and dumped. This procedure seasoned the mixer. If another batch was mixed within 30 minutes, the remnants of the previous batch was used as the “butter”. The mixer was cleaned at the end of each day.

Finding the exact concrete mix to replicate the color and appearance of historic stone is usually a matter of some experimentation. However, in this case, by using the successful formula derived in part from the work on the Renwick Gallery, the only major obstacle was finding the proper pigment color that would match the serpentine stone, and would not fade. After a series of test stones were manufactured and fully cured, a close color match was established. The mix that produced a color closest to that of the serpentine stone, and also meeting all the required strength tests, was composed of the following materials in the quantities indicated (the quantities shown are in pounds for ½ cubic yards):

· White portland cement- 376 lbs.
· Pigment- 12 lbs.
· Fine aggregate- 700 lbs.
· Coarse aggregate- 775 lbs.
· Water- 150 lbs.
· Vinsol resin- 4 oz.
· Pozzolith 122N- 18 oz.

These materials were combined in the following sequence:

· Fine and coarse aggregate
· Maximum of 2/3rds of the water
· White portland cement
· Pigment
· Admixtures
· Remaining water
The materials were combined in the mixer until there was a consolidated, homogeneous concrete of the required slump—usually about two to three minutes and always less than ten minutes.

Quality Control

Once the materials were thoroughly mixed and ready to be poured, measures were taken to ensure that the newly-fabricated pre-cast units met the specified strength criteria. In the manufacturing process adopted for Six Logan Circle, a minimum of 6 standard testing cylinders (6”x12”) were filled from a selected batch once a day, and sent to a testing laboratory. Two of the cylinders were tested for compressive strength at 7 days and at 28 days. The remaining cylinders were retained for 56 days for additional testing before disposal. A minimum of 2 standard 6”x12” cylinders cast from all the other batches was collected and also tested for compliance with the design criteria of 5,000 psi at 28 days.

Pouring the Concrete

Figure 7. The workmen shown here are cutting the air bubbles from the sides of the mold using a cake spatula during the first 10 seconds of the vibration process. Photo: James Smith

The consolidated and homogeneous concrete mix was poured from the mixer into the specified mold which had already been placed on the vibration table. The vibrating process forces the entrained air and surface air out of the mold, eliminating in most cases the possibility of surface imperfections from air bubbles. Proper vibration of the newly poured concrete was extremely important to obtain a high quality pre-cast concrete. The vibration table operated at a rate of 2,600 vibrations per minute and could accommodate all the castings from a batch within thirty minutes after mixing. Small cast units required approximately sixty seconds of vibration and the larger ones required approximately ninety seconds. A cake spatula was effective in cutting many of the air bubbles from the surface of the sides of the mold during the first ten seconds of the vibration process (see figure 7).

Air Curing, Mold Removal and Inspection

After the vibration process was completed for each stone, the molds were placed on heated benches for sixteen hours, after which the molds could be gently removed without damaging the stone. Each stone was then inspected for any visual inconsistencies or other imperfections from the mixing, pouring and vibrating process. The stones were evaluated for problems such as uneven or non-matching coloring, chips, exposed aggregate, air bubbles (pin holes), or depressions. During the manufacture of the new pre-cast stone, more than 400 newly cast stones were rejected due to surface deficiencies.

Figure 8. After soaking for 6 hours to remove soluble salts in a polyethylene-lined wood water tank, the pre-cast units were allowed to dry for another 6 hours. This process was repeated 3 times for each pre-cast unit. Photo: James Smith

Removing Soluble Salts

The final step in the manufacturing process was essential to the successful outcome of the product. In this step, the stones were submerged in a water tank for a period of six hours, then removed and dried for six hours (see figure 8). The drying process forced soluble salts inherent in the concrete mix to the surface of the stone. An application of a weak solution of muriatic acid with a thorough water rinse was used to wash away the leaching salts. The procedure of soaking and drying was repeated three times for every stone. After completing this process, each stone was transferred to the basement for storage on wooden skids.

Installation

Before any of the original stone was removed, the corresponding section of pre-cast material was arranged in the “staging area” in the basement. Using the measured drawings and numbering system prepared earlier, the blocks of cast stone were assembled on the basement floor on top of 4” of sand. Assembling them in the staging area allowed for final inspection for the blocks and also allowed the master mason to arrange the new stones during a “dry run” prior to the actual installation on the building, thereby minimizing problems once the work on the building began (see figure 9).

Figure 9. Using the measured drawings as a guide, the pre-cast units were assembled in the staging area on top of 4 inches of sand prior to their installation on the façade. Photo: Robert Powers

Figure 10. The new pre-cast units were hoisted up the scaffolding on wide belt slings with an electric winch, and installed in place using a portland cement-based mortar appropriate for cast-concrete. The original bricks were relaid with a more traditional lime-based, dark colored mortar. Photo: James Smith

Once all the new pre-cast concrete blocks were manufactured, inspected and fully cured, the process of dismantling the existing deteriorated façade and installing the new material began. Since the serpentine stone was used as a veneer covering structural brickwork and was not itself a structural element, sections of the veneer could be removed without jeopardizing the stability of the building.

With the front elevation fully scaffolded, sections of the serpentine stone and ornamental brick were removed. Because the brick trim was in excellent overall condition and was to be reused in the new façade, it was carefully removed with hand tools, inventoried, and stored for reinstallation. With the serpentine and face brick removed, necessary repairs were made to the structural brick, and stainless steel anchors for the new pre-cast veneer were installed. The new cast stones were hoisted up the scaffolding on wide belt slings with an electric winch and installed in place (see figure 10).

Since the new pre-cast concrete blocks are much harder than the original serpentine stone, a mortar mix containing a large proportion of white portland cement was used. However, other than the composition, the new mortar joints match the light-colored, raised mortar joints used on the original stone façade. The walls were then backfilled with a parching material.

The original bricks were individually cleaned with bristle brushes and water before being reset into the new façade. The new brick mortar was based on an analysis of the original, dark colored brick mortar and matched it in composition, color, texture, joint width and profile (see figure 11).

Figure 11. Close up views of the newly installed pre-cast concrete units and the reinstalled original brick. Photo: James Smith