Questions from the Field

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 on precast.org. 

If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/technical-services.

Kara writes:

We are currently revising our engineer-stamped drawings of all precast tanks manufactured at our plant to be submitted at the state level. I am wondering if there are some guidelines or examples of what should be shown and noted on the drawings. Should the mix design name be noted? Should specific standards be called out on the drawings?

NPCA Technical Services engineers answered:
Requirements for tanks and their drawings vary by state, municipality or city. However, at a minimum, drawings should include:

• Structure’s description
• All tank dimensions with units, including exterior length, width and height, wall thickness, lid thickness, inlet and outlet height, lid opening dimensions, liquid level, etc
• Reinforcement information, including type of reinforcement (ASTM A706, A1064, etc.), number of bars, length of bars, bar spacing, spacing and positioning tolerances, and area of steel per linear foot
• Information about splices or welds
• Positioning, size and capacity of lifting devices
• Information about riser size and type
• Number of compartments and position of baffle
• Details of inlet and outlet piping
• Inlet and outlet height
• Liquid capacity
• Maximum depth of cover and condition of soil (saturated or unsaturated)

Other information to consider including is whether the tank assembly is traffic-rated; the structure’s weight and the lid’s weight; inlet and outlet connection (type of boot or gasket); and whether it complies with ASTM C1227. Any other applicable codes or standards should also be noted.

Photo courtesy of Dalmaray Precast Concrete

Clark writes:

When I was an active civil engineering technician, I learned organics would retard the set of concrete. However, I stumbled across information that there are some organic accelerators. How can both be true? Can you offer some details?

NPCA Technical Services engineers answered:

Thank you for your inquiry about the effect of organics on concrete set time. You are correct that, in general, the concrete industry notes that organics will retard the set time of concrete. Organic compounds present in soils will have such an effect. The study, “The effect of organic matter on the setting of soil-cement mixtures,” reports on conducted experiments where the type of organic compounds found in soils were added to clean sand to observe the effect on concrete set time. All behaved as retarders.

You touch on a concept that is continuously studied on the micro level, since organic matter exists in various forms. In the 2015 November-December Precast Inc. article, “Material Matters: Accelerators,” we do reference organic soluble accelerators, specifically triethanolamines. However, the type of organic compounds found naturally in soils should not be confused with the chemicals created using organic matter as an ingredient. Naturally occurring organic compounds (simple or complex) will have a retarding effect and, as a production practice, those materials should not be introduced during the batching process at any precast concrete plant. Chemicals created using organic matter will behave differently and may accelerate set time.

The short answer is – you were trained correctly. In the field, naturally occurring organic matter will retard the set time of concrete if batched into an otherwise sound mix.