PERFORMANCE OF CARDBOARD CARTON FORMS
Cardboard carton forms (void boxes) are commonly used to form the void space between the bottom of grade beams and slabs over expansive soils. However, other than laboratory compression tests and 100% humidity tests, there is little documentation for the actual performance of these boxes in place. This paper summarizes field tests done in an attempt to simulate actual conditions to answer the following questions: •
Do the boxes deteriorate at an acceptable rate?
Does the wax coating or impregnation and a polyethylene vapor barrier affect deterioration? •
Will the boxes fail under heave conditions?
Are trapezoidal boxes appropriate and do they perform in an acceptable manner? •
Which type of boxes are appropriate in each situation? •
What are some of the construction considerations necessary to insure proper function of the boxes?
Two configurations of boxes were used: Vertical Cell and Diagonal Type
II. Initial Test 1 procedure:
It was initially decided to pour a test slab over 4 sets of void boxes. Each box was 4 ft x 4ft x 8 in deep. Concrete was poured on top and all around each box to simulate actual conditions. Four 1 ft x 1 ft steel forms were placed on top of each box for the purpose of loading each box at different intervals to determine if they had deteriorated. All boxes for the first test were vertical cell. Each box was placed with different conditions: • Void box with no poly anywhere
• Void box with poly wrapped all around
• Void box with poly on top only
• Void box with poly on bottom only
Test set up with boxes and loading pads Placing concrete over boxes.
The procedure was to pour the concrete and test the capacity of the boxes once a week to determine the rate of deterioration.
Description of results:
• Week 1:
Each box was loaded with 200 psf and no deflection was observed • Week 2:
Each box was loaded with 400 psf with no deflection
• Week 3:
Each box was loaded with 400 psf with no deflection. The box with poly on bottom was flooded with water to accelerate deterioration • Week 4:
Each box was loaded with 400 psf. The box that had been flooded with water had 3/8” deflection. All the other boxes had no deflection • Week 5:
The test pad on the boxes with no poly and the one with poly on bottom were removed to view the boxes. It was discovered that the top of the cardboard had stuck to the concrete causing the cardboard to span across the opening thus giving erroneous information. Both of the boxes appeared to have significant strength but were destroyed in the removal process and could not be tested.
• Week 6:
The two remaining boxes, the one with poly all around and the one with poly on top had the concrete removed to expose the boxes. The boxes were tested with over 400 psf and still had plenty of capacity with little deterioration. The box with poly wrapped all around had a little water in the box, maybe a cupful, but this did not deteriorate the box. The source of the water is unknown, possibly from when the adjacent box was flooded with water.
III. Test 2 Procedure:
1. Two vertical cell boxes with no wax were buried in sand and uncovered after one week to view the rate of deterioration: The boxes and loaded with a 50 pound weight and they immediately. 2. A Vertical Cells beam box with wax and a Diagonal Type Beam Box with wax were buried in sand for one week. The buried boxes were tested by standing on them, there was nominal deflection which indicates the wax has a definite impact on the deterioration rate.
3. Two vertical boxes with wax were buried in sand for three weeks. One was completely covered in poly and the other had no poly. Both boxes showed no appreciable deterioration after three weeks. Since the boxes with no wax failed...
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