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.
Scott writes:
The state of West Virginia’s Material Control, Soils and Testing Division is now requiring us to cure our cylinders in the exact environment as our products. We have used the lime bath method for many years without any issues, so the change came as a surprise. My question is, can we eliminate the lime bath and cure cylinders using the ambient weather and if we do so, will we still remain in compliance with the National Precast Concrete Association?
NPCA Technical Services engineers answered:
NPCA Technical Services engineers answered: Yes, you can still remain in compliance with the National Precast Concrete Association Plant Certification Program requirements if you stop using the lime bath to cure your compressive strength cylinders. Section 5.3.5.1 of the NPCA Quality Control Manual for Precast Concrete Plants states, “Compressive strength cylinders shall be made in accordance with ASTM C31, ‘Standard Practice for Making and Curing Concrete Test Specimens in the Field.’ Specimens shall be cured in a manner similar to the curing of the concrete products represented by the specimens, unless otherwise required by the project.”
Therefore, curing your compressive strength cylinders in a manner similar to the products represented by those cylinders should not pose an issue regarding NPCA Plant Certification compliance.
However, curing your cylinders in an ambient environment without a lime bath or without added moisture could produce lower compressive strength values when you break your cylinders. The cylinders cured without the lime bath will represent your products and their curing environment more accurately, but you’ll have to monitor your quality control data carefully to ensure you’re still meeting stripping and shipping strengths. Also, if you stop using the bath altogether, you will lose all correlation to previous 28-day breaks.
Even though the state of West Virginia will require these field-cured cylinders for approval, you may consider maintaining some level of testing frequency of the lime-bath cured cylinders. The field-cured cylinders introduce many additional variables that can affect cylinder strengths. Consequently, this can increase the complexity of trying to troubleshoot a concrete mix issue or track concrete mix- and/or batching-process trends. Therefore, you may need to add another set of cylinders for 28-day breaks that you’ll store with the products to comply with state requirements.
Cylinders for 1-day, overnight and stripping-strength breaks are cured with the precast products. Shipping-strength cylinders are also cured with the products. Typically, the cylinders used for 28-day breaks are cured in the bath; however, some owners require cylinders be kept with the product as well.
Jon writes:
Can you explain the difference between the 6-foot and 4-foot heights for walking/working surfaces and why both pertain to the precast industry? And what is considered the plant area? Is this plan approved by the Occupational Safety and Health Administration and applicable to the septic tank precast industry?
NPCA Technical Services engineers answered:
OSHA has different fall protection requirements for different industries. Manufacturing precast concrete products in a plant environment falls under the general industry standards (29 CFR 1910) and the 4-foot rule applies here. Construction or performing construction-related activities is governed under the construction standards (29 CFR 1926) and the 6-foot rule applies in those instances. The best way to look at it is, anything that occurs at your plant, including your production area, storage yard or office, would fall under the 4-foot rule. If you go out and install precast products on-site or place in an excavation/final destination at a construction site, then the 6-foot rule governs.
Phillip writes:
What is the purpose for a gravel layer under a manhole? We have a project where the contractor is excavating down to hard sandstone and placing a manhole structure. The municipal standard called for 9 inches of gravel/stone to be placed under manhole structures. He was wondering about the intent of this stone layer.
NPCA Technical Services engineers answered:
Its intent is to be a leveling layer to provide uniform bearing across the precast base section and also to provide more precise elevation control when placing precast sections. ASTM C1821, “Standard Practice for Installation of Underground Circular Precast Concrete Manholes Structures,” states this layer is only 3 inches minimum and is not intended to be a structural-soil-support component in the presence of a soft subbase. In that case, additional soil testing would be necessary to design an appropriate aggregate base to be placed to support the manhole load. If the municipal standard is 9 inches minimum, it would appear they included some additional thickness to stiffen the base layer and resist settlement.
For this application with the structure resting on excavated rock/sandstone, the aggregate leveling course is very important to reduce the possibility of point loads on the bottom of the precast base section by either high spots on the rock or an imperfect casting surface.
Jim writes:
We have a client who requires the precast product to be within 10 degrees Fahrenheit of the ambient temperature before the form can be stripped. Is that typical?
NPCA Technical Services engineers answered:
The short answer is we have never heard of this temperature requirement until now. For some prestressed bridge components or other critical sections we know of a requirement to include thermocouples within the mix to verify the product’s temperature is similar to the concrete cylinders’ temperature. This is used to verify field strength. Also, thermocouples are used to verify strength through a process called the maturity method.
It certainly would be beneficial to see if there is a reason this requirement was developed. The specifying agency may have had some type of concern where accelerated curing was taking place and the ramp up and/or down phases of the concrete temperature were not followed.
Unfortunately, sometimes the “problem” is more perceived than real. It is possible that a staff member read about the importance of the curing cycle and misinterpreted it. The only other reason would be if the agency experienced some type of thermal shock of the concrete due to a large temperature range during casting, stripping and stocking. Within ACI 306R, “Cold Weather Concreting,” Figure 7.1 shows a permissible temperature difference between the concrete and air temperature to avoid cracking. The temperatures are in the 20-to-35 degree range.
Graphical determination of safe differential temperature for walls (Mustard and Ghosh, 1979).
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