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.
Lauren writes:
ASTM A706, “weldable rebar,” should not have to be used if welding practices are good with ASTM A615. Is this correct?
NPCA Technical Services engineers answered:
According to the NPCA Quality Control Manual for Precast Concrete Plants, “Reinforcing steel used for structural purposes may be welded as long as it is accomplished in compliance with standards set forth in the American Concrete Institute’s ‘Building Code Requirements for Reinforced Concrete’ (ACI 318) and The American Welding Society’s ‘Structural Welding Code – Reinforcing Steel’ (AWS D1.4).”
It also states, “Welding of ASTM A615 reinforcing steel is not generally an acceptable practice. According to AWS D1.4, the carbon equivalent for bars to be welded should be less than 0.45 percent for bars larger than #7 and less than 0.55 percent for #6 bars and smaller.”
Therefore, if ASTM A615 steel is welded, the carbon equivalent must be calculated and documented and the bars must be preheated, if necessary. Preheating guidelines are also set forth by AWS. You are correct if you calculated the specific weldability of the ASTM A615 rebar being used. If it meets the requirements of the carbon threshold, then you can weld it.
For more information about welding reinforcement, read the article “Hot Topic: Welding Reinforcement” from the March-April 2011 issue of Precast Inc. or “Welding Reinforcement Steel: AWS D1.4/D1.4M:2011” from Structure Magazine.
Umair writes:
Why does fly ash contribute to concrete strength after 28 days?
NPCA Technical Services engineers answered:
Fly ash helps increase strength development after 28 days – and in some cases months to years beyond 28 days – because of its reactions. When ordinary portland cement reacts with water, the products include calcium silicate hydrate and lime. CSH is the main contributor to concrete strength, while lime’s strength contributions are negligible until it reacts with another material to produce CSH. Fly ash is a pozzolan and it reacts with the free lime to produce additional CSH. In the presence of fly ash, the lime can react to produce additional CSH. This can continue as long as the reactants are available in suitable quantities and other environmental factors are favorable.
Randy writes:
I am looking for the dimensions of the “bell” length and taper and taper length on concrete pipes. I would think that it would be a standard, but maybe not, and that it varies by manufacturer of the pipe? I am in the process of designing a unique lifting device for which the intent is to help reduce the amount of time that workers would be down in the trench while installing the pipe.
NPCA Technical Services engineers answered:
Unfortunately, uniform or standard precast concrete joint dimensions do not exist within the North American market. As the industry grew, each individual company, and in fact even plant branches, purchased and used the joint forming equipment (headers for spigots, tongue and pallets for bells and grooves, etc.) they thought best. However, this has not been a large problem out in the field because it is rare for competing pipe manufacturer joints to be coupled together. The respective ASTM joint standards provide performance requirements along with design guidelines related to rubber gasket deformation limits, maximum bell taper angles, etc. However, there are no requirements stating joint length or internal diameter dimensions.
For precast product joints, there are two distinctive types: a bulk or formed butyl, or bitumen sealant material within the annular space between the spigot and bell. These joints typically have a half-inch wide annular space and the taper of the bells and spigots can be large, 5 to 10 degrees. The depth, or length, of the bell socket typically varies from 3 to 5 inches. This type of joint is characteristically found on precast rectangular box culvert sections or non-circular pipe (elliptical or pipe-arch) culverts.
For circular concrete pipe, most manufacturers are using a rubber gasket joint configuration. The main difference is the annular space between the spigot and bell is a critical dimension. The high tolerances are required to ensure the proper gasket compression occurs within the design limits for both the centered and joint shear position. Additionally, the bell taper is typically limited to a 2-degree maximum in the event the pipe is not homed all the way and the gasket compression is not drastically reduced as the joint is opened. Because the compressed gasket exerts radial forces into the bell, the bell dimensions for gasketed joints need to be reinforced and thick enough to resist these forces. Consequently, gasketed joints typically have projecting bell profiles larger than the pipe wall outside diameter. That is until the pipe wall of larger pipe can be divided to incorporate a reinforced bell socket resistant to the various installed joint forces.
It would appear if your device is intended to have a reasonably tight fit into a precast concrete pipe or box section, it will need to be adaptable to many unique dimensions or be custom made. It should be noted that each manufacturer should have detailed dimensional drawings of their joint equipment being used.
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