Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and comments on blogs 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.
Troy writes:
We deliver precast concrete and no matter how careful we are, we battle with the age-old issue of marks, road dirt, chips and anything else associated with damage – no matter how large or small. Is there an industry standard that says visual inspection?
NPCA Technical Services engineers answered:
Two specific references come to mind regarding architectural surface acceptance by mockups. In ACI 303R-04, “Guide to Cast-in-Place Architectural Concrete Practice,” Section 2.1.1 uses a distance of 20 feet for visual acceptance. PCI MNL-117, “Quality Control for Plants and Production of Architectural Precast Concrete Products,” references a distance of 20 feet or more as agreed upon by the architect, owner, contractor, etc., in Section 1.5.4 and again in Section 2.10
Douglas writes:
I have a question regarding joints in precast concrete wet well sections. We have been reviewing precast drawings that show the use of molds to increase the height of a section. The lower portion of the section is poured to a depth of +/- 72 inches with the reinforcing exposed above the pour to allow setting an upper mold to increase the section depth to +/- 113 inches. The reinforcing steel meets lapping requirements but the joint has a vertical, ribbed PVC water stop. ASTM standards C478, C913, C433 and C990 all call for keyed joints with flexible compressible sealants. But this appears cast sections. Per these ASTM standards, are water stops allowed during the precasting process?
Precast concrete lift station mechanicals
NPCA Technical Services engineers answered:
Our understanding is that the precaster has an existing 6-foot-tall form and wishes to make a single integral section of 9 feet, 5 inches. Because this section will consist of two separate pours, a PVC water stop will be cast into the junction position of the sections. Typically, the need for a taller or extended manhole section is due to a pipe connection that cannot be avoided to hit a standard manhole vertical joint. Sometimes, precasters (with contractor consent) will furnish a deepened or sump manhole section to pull the conflicting manhole joint under the pipe connection. However, in some cases, based on the pipe diameters and the respective elevations, a manhole joint cannot be avoided, and the other option is to cast a taller section by a form extension.
To answer your question specifically, what you are describing is a unique structure that isn’t specifically covered within ASTM C478, Section 14: Riser and Conical Tops. This construction joint also isn’t covered by the traditional bell and spigot joint requirements within ASTM C443 or C990. However, this isn’t necessarily noncompliance. What you describe from the precaster – using a properly placed water stop in the secondary pour area – is good practice. And there must be some valid reason they are going to such lengths to get the extra-tall riser section.
This could then relate to ASTM C478, Section 5.2: Modified or Special Design, which states: “Manufacturers are not prohibited from submitting to the owner, for approval prior to manufacture, designs other than those prescribed in the specific section for a product. If such approval is obtained, then the product shall meet all the tests and performance requirements specified by the owner in accordance with the appropriate sections on manufacture and physical requirements.”
The ACI C301 and ACI C350 codes, which permit the use of PVC water stops on cast joints, oer an excellent guide reference on this topic.
Greg writes:
How does one formulate a concrete mix to make concrete panels of light weight and high strength? What special additives are needed?
NPCA Technical Services engineers answered:
Lightweight concrete is essentially standard portland cement concrete that uses a lighter aggregate (expanded shale, clays or volcanic aggregate) in lieu of normal coarse aggregate. The best reference to obtain mix design information is ACI 213R-14, “Guide for Structural Lightweight-Aggregate Concrete.” This document provides history, physical characteristics, design parameters, etc. Chapter 4, “Specifying, Proportioning, Mixing, and Handling,” would be of interest.
Another resource regarding lightweight concrete is the Expanded Shale, Clay and Slate Institute. For more information, visit escsi.org and search “lightweight concrete.”
There are no secret additives in the concrete industry to enhance strength. The basic formula of a lower water-cementitious material ratio is still the primary factor for concrete strength. There are a variety of admixtures available to reduce your water content while still providing the workability needed to cast what is required. Moisture control is a critical component to be managed.
Another method to lighten precast concrete panels and provide improved insulation capacity (R values) is by producing a sandwich panel. Just as it sounds, these panels are constructed by setting up forms, placing the required reinforcing steel, pouring the concrete to the specified thickness, placing a high-density foam layer atop the first layer of cast concrete and then completing the panel by casting additional concrete over the insulating material. This provides a strong and possibly architecturally pleasing finished panel with improved thermal properties. More on sandwich panels can be found at precast.org/sandwichpanels.
Another means to lighten a precast panel would be to reduce the thickness. Today, with advanced concrete properties, structural sections are being reduced through the use of ultra high performance concrete, enhanced tensile properties with steel reinforcement, or both. UHPC has been used on amazing architectural building components with stunning results. Further information about UHPC can be found at precast.org/uhpcwp and precast.org/uhpcendless.
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