South Pasadena, Los Angeles,
Restoration of this groundbreaking concrete structure involved the use of specialty patching mortars to reproduce the board-formed concrete look and feel.
Back in 1906 they were still learning about how to build with this 'new-fangled' material, and there were numerous locations where spall damage due to the placement of steel reinforcement too close to the surface resulted in damage. Here, the 4" thick (!) parapet is being removed to alleviate failure in the concrete. Note the early twisted steel reinforcing bar.
Spall damage was evident at many locations beside the parapet. In every case, proximity of the steel reinforcing or support mechanism to the surface allowed the exposure to moisture penetrating from the exterior.
Careful cutting and removal of the failed historic material was achieved using light-impact pneumatic hammers and diamond blades.
In each case of repair, the standard removal pattern for board-formed cast-in-place concrete was carefully followed. All cuts were made perpendicular or horizontally to the axis of the old board form marks (In those days they didn't have plywood). This way the new patch would visually blend better. This was especially important since the finished concrete surfaces would not be painted or coated in any way.
Since the repair mortar would have minimal polymer additives to assist in breathability, a mechanical anchoring system was installed to assist in the adhesion of repair mortar. Here the worker is drilling holes for the installation of stainless steel anchor pins.
Here the stainless steel masonry screw is further tied into the original masonry with stainless steel wire to provide yet more retention.
Note here the use of boards to form a section of parapet to be cast in place. Plywood is the norm for this kind of work these days, but it was imperative to maintain the same type of form marks as the original construction, in order to match the original pattern of the concrete surface.
Here at a different section of parapet the boards of the form have been coated with a 'retarder', a chemical that eliminates the smooth surface normally seen with newly-poured concrete. Instead this treatment, after simple pressure washing, yields an open pored surface looking as though it were old and weathered.
The spall repairs were done in two separate lifts (applications) to minimize shrinkage. Here the scratch coat has been applied and further roughened to receive the second finish coat.
Repairs were completed on all surfaces of the bridge using fixed and mobile access. The black surface on the left underside of the bridge is the result of coal smoke and exhaust from the steam engins that ran under this span for years before the line was shut down.
Several decorative spires at opposing ends of the walkway were restored as well.
Special micro-abrasive cleaning techniques were used to remove graffiti without having to resort to chemical alternatives.
The lovely arc of the succesive spans is a sight to behold, and well worth the visit if you are ever in South Pasadena.