Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and comments on blog posts or magazine articles posted on precast.org.
If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/plant-resources/technical-services.
ASTM C478 includes information for allowable wall thicknesses for a round, 6-foot diameter pump station.
Stanley asks: Is there an ASTM chart that shows the allowable wall thicknesses for a round, 6-foot diameter pump station?
NPCA technical experts answer:
ASTM C478, “Standard Specification for Circular Precast Reinforced Concrete Manhole Sections” outlines requirements for the manufacture of precast concrete manhole sections.
Section 14.3 states, “The minimum wall thickness shall be one-twelfth of the largest internal diameter of the riser or conical top.”
Additionally, Section 14.7.2 states, “The wall thickness of risers and conical tops shall be not less than that prescribed in the design by more than 5% or plus/minus 3/16 inches (plus/minus 5 mm), whichever is greater. A wall thickness greater than that prescribed in the design shall not be cause for rejection.”
NPCA provides a free crystalline silica safety document that is available in the Technical and Product Resources section at precast.org.
Oliver asks: What kind of mask is required for concrete buffing work and concrete grinding?
NPCA technical experts answer:
The type of mask required for concrete buffing and grinding work depends on a variety of factors, including whether the work is being done indoors or outside, how long the work is being performed and what type of filter the buffing or grinding equipment has attached to it, among other factors.
To find the requirements that apply to your specific scenario, check out Table 1, “Specified Exposure Control Methods When Working With Materials Containing Crystalline Silica” in OSHA 1926.1153(c)(1) on Respirable
Crystalline Silica, available here: https://www.osha.gov/laws-regs/regulations/standardnumber/1926/1926.1153
Also, the NPCA Exposure Control Plan for Respirable Crystalline Silica is free to members. This document provides information on the hazards associated with silica dust and outlines the steps to take to ensure employees who work with, or around, silica are not exposed to hazardous levels of silica dust. It also provides procedures for common silica-related work duties to minimize exposure in accordance with the OSHA Air Contaminants standard.
Visit the NPCA Shop to download it.
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