Questions from the Field

Michael writes:
What requirements do we need to be aware of regarding risers, stack outs and other access ways with respect to confined spaces?

NPCA Technical Services engineers answered:
Whether a manhole riser is a 48-inch diameter or a 120-inch diameter, it would still be a confined space in the eyes of the Occupational Safety and Health Administration. In some cases, even open-ended culverts have been determined to be confined spaces. Consequently, all confined space requirements need to be implemented before entering into an all-service manhole. During construction, some aspects of the confined space rule may not apply, but, nonetheless, specific safety rules and considerations still need to be followed prior to entry.

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A confined space has one or more of the following characteristics:Contains or has the potential to contain a hazardous atmosphere; contains material that has the potential to engulf an entrant.

• Has walls that converge inward or floors that slope downward and taper into a smaller area which could trap or asphyxiate an entrant.
• Contains any other recognized safety or health hazard, such as unguarded machinery, exposed live wires or heat stress.1

Confined spaces such as manholes, crawl spaces and tanks are not designed for continuous occupancy and are difficult to exit in the event of an emergency. People working in confined spaces face life-threatening hazards, including toxic substances, electrocutions, explosions and asphyxiation.2

Resources:

1 https://www.osha.gov/SLTC/confinedspaces/
2 https://www.osha.gov/confinedspaces/index.html

Asif writes:
What is the importance of bond strength, surface texture and geography of steel reinforcement?

NPCA Technical Services engineers answered:

The surface texture of the steel bar is critical to the development of friction and the ability of the concrete to lock in the bar to create the maximum bond between the two. You can easily visualize that a smooth bar embedded in concrete would provide less pull-out resistance than a bar with ribbed protrusions. It would require some concrete shearing before the bar can move. In fact, a bar with a protruding ribbed surface may actually require the concrete to be crushed before the bar will move. Consequently, most design codes provide different factors to be used if the reinforcement is ribbed or smooth.

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There have been many tests and quite a bit of research related to the behavior of traditionally ribbed steel rebar. Consequently, there is a high level of confidence in the bond characteristics that is reflected in design factors. However, there are many surface configurations and composite-yielding characteristics for embedded fiberglass reinforcing bars that require further research before similar confidence and rational uniform design factors can be used.

If the bar geography deals with the placement location of the bar, such as with respect to the concrete cover, then the bond strength of the bar can be affected by a concrete slabbing failure. This can occur when the tensile forces within the bar, typically in a curved shape (arch or pipe) or connection detail (floor or beam to a column), exceed the shear resistance of the concrete, allowing the bar to pull itself through the concrete cover. That drastic condition creates a bond issue. Besides the durability, this is another important reason to maintain the minimum concrete cover required by product standards or keep a minimum development length dimension when terminating using a hook within walls.