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Demolition & Panel Preparation

Demolition

Demolition – Wrecking Ball Method

Wilder Construction Company believed that an improvement to the efficiency of the demolition operations could be achieved by quickly destroying the concrete panels by striking the panel with a wrecking ball. This would save them the labor-intensive task of having to saw-cut the panels into sections and install eyebolts into the sections so that they could be lifted out by an excavator. A test slab was poured in Wilder’s yard which served to model the panels that were to be replaced for the project. The 12 foot by 15 foot test slab was further sawcut to mimic the relief cuts that would be used when using the wrecking-ball demolition method in actual practice.

Test slab used to model the wrecking ball demolition method. Source: WSDOT

Wrecking ball demolition method in action. Source: WSDOT

The 5000lb wrecking ball was dropped repeatedly by an excavator from a height of 4 to 5 feet. WSDOT officials would only permit the wrecking ball demolition method if the panel could be destroyed with no more than four drops resulting in sufficient destruction of the panel to allow the rubble to be removed by an excavator. The contractor’s method worked well during the trial on the test panel and the concrete slab was destroyed with four drops of the wrecking ball. The use of an impact hammer was required however, to further rubblize the concrete so that an excavator could collect the debris.

The wrecking ball method was attempted during a nighttime closure on April 16, 2008. 42 drops of the wrecking ball were required before appreciable destruction was done to the panel. Moreover, once the panel was destroyed, an excavator with a hoe ram attachment was needed to further break apart the panel so that an excavator bucket could scoop out the concrete rubble. It was apparent that the concrete panel was much stronger than the test slab used to model the pavement. Based on core samples taken along sections of I-5, the compressive strength of the existing concrete pavement has been estimated to be close to 10,000 psi (to be confirmed).

Wrecking ball demolition method in first action during a nighttime closure. Source: WSDOT

Excavator with hoeram attachment further separates the concrete debris for removal. Source: WSDOT

Demolition – Saw-Cut and Lift

In order to continue work and not lose momentum on the project, the contractor decided to abandon the use of the wrecking ball and utilize saw cuts to divide the panels into sections in order to lift them out and transfer the panels to end-dump trucks for removal. Relief cuts were made 12 inches in from the isolation cuts that were made around the perimeter of the panel. The purpose of the isolation cuts were to mitigate any damage as a result of aggregate interlock with adjacent panels during the removal process. The relief cuts further served as additional protection and were designed to eliminate the transfer of breakage forces during removal of the concrete panels. In order to allow for the piecewise removal of the concrete panels, one longitudinal cut was made along the centerline of the panel. One transverse sawcut was also made which divided the panel into four sections - two smaller sections (5ft x 6ft) and two larger sections (10ft x 6 ft). A concrete drill was used to install eyebolts in the smaller sections which were coupled to a shackle and chained to the bucket of an excavator. The excavator was then able to lift out the smaller sections and deposit them into the bed of an end dump truck.

Walk-behind sawcutting machine used to make isolation and relief cuts in preparation for panel removal. Source: WSDOT

Removal of a panel section by lifting with chain. Source: WSDOT

An excavator with a thumb attachment connected to the bucket was used to remove the larger sections of the concrete panel. This attachment allowed for the excavator to pick up and transfer the individual panel sections directly into the bed of a dump truck.

Using a excavator with a bucket and thumb to pick a panel section up for removal.

Placing the panel in the bed of a end-dump truck.

All demolition debris was removed from the site and stockpiled at the contractor’s yard. This debris can be recycled and used as a high-quality fill for future projects. Wilder Construction has been known to supply their own aggregate and fill.

Stockpile of concrete debris maintained by the contractor.

Panel Preparation

The condition of the subgrade was assessed each time a panel was removed. If the subgrade was in poor condition, it was overexcavated, replaced with fill and a crushed stone base course and them compacted per specification. Depending on the size of the overexcavated area, a jumping jack compactor or a small steel-wheeled roller was used to compact the base course.

To further prepare the panel for the concrete pour tie bars and dowel bars were drilled into the longitudinal and transverse edges, respectively, of the adjacent concrete panels. The holes were drilled using a pneumatic impact driver.

Using a pneumatic impact drill to allow tie bars to be inserted into adjacent panels.

Drilling to permit dowel bar to be inserted.

Smooth, epoxy-coated dowel bars were located 12 inches on center and required only in the location of vehicle wheel paths. Dowel bar cages were installed for panel sections with a combined length greater than 15 feet. In other words, if a section was the equivalent of multiple panels then dowel bar cages were placed at 15-foot intervals and at the location of the sawcut to create the transverse joint that would serve to separate the panels.

Dowel bar baskets were used for combined panel sections.

Dowel bar baskets set aside for later use.

Before insertion of the dowel bars into the adjacent panels, they were lubricated with lithium grease to eliminate the tendency of the bars to bond to the concrete. It is important that the dowel bars are allowed freedom of movement so that they can serve their function as a load transfer mechanism.

Laborers applying lithium grease to dowel bars.

Close-up of lithium grease used to coat dowel bars. Source: WSDOT.

Per the contract specifications, polyethylene sheeting was glued to each edge of the adjacent slabs using a spray adhesive. The polyethylene sheeting is designed to minimize the amount of aggregate interlock between the panels. It was observed that the contractor also sprayed on a bondbreaker compound (from MASCO Masons Supply Company) over the sheeting. The contractor requested the use of the bondbreaker compound in lieu of the polyethylene sheeting but was not permitted to use this compound as a direct replacement per the WSDOT standard specifications for PCC pavement design and construction.

Polyethylene bondbreaker applied to edges of adjacent panels.

Bondbreaker compound was also sprayed on as an additional measure.