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.
Most concrete volume changes seem to relate to the time when the concrete is curing. After the concrete is set (let’s say for more than one year), will concrete volume increase due to exposure to and absorption of surface water? Or will the volume be relatively fixed and water will just occupy internal porosity within the concrete?
NPCA Technical Services engineers answered:
A significant amount of concrete volume changes will occur in the first 24 hours of placing concrete. Other types of volume change can occur after the concrete has hardened, and may take place for years or even throughout the concrete’s service life. Early-age volume changes can affect long-term volume changes.
Concrete that has been designed and manufactured to specific quality standards should have a low porosity and low permeability. When air-entraining admixtures are used, very small air bubbles ranging from about 10 to 1,000 micrometers in diameter are intentionally incorporated into the concrete matrix. When moisture is absorbed into hardened concrete, these evenly distributed microscopic air bubbles provide an area for the water to expand as it freezes, which helps prevent cracking and other issues which can be associated with freeze/thaw cycles.
Entrapped air depends somewhat on certain raw material characteristics and is largely attributed to mixing, placing and poor consolidation practices. The resulting air bubbles are usually 1 millimeter in diameter and larger. When concrete cures, these entrapped air bubbles turn into void spaces, which results in greater porosity, greater permeability, lower strength and lower durability. They could technically provide a space for absorbed water to expand as it freezes; however, the voids are sporadically spaced which makes that behavior unlikely. Entrapped air results in so many detrimental behaviors and characteristics that it shouldn’t be rationalized as a means of attempting to manage water expansion.
Drying shrinkage can occur in hardened concrete for years after placement. The amount concrete shrinks, however, is affected by the curing methods when the concrete was first placed. Moist curing methods, higher relative humidity and other favorable curing methods can help reduce the amount of shrinkage.
Temperatures also affect hardened concrete volume. Like most elements, concrete contracts in lower temperatures and expands in higher temperatures. The raw materials used in the concrete have a significant impact on concrete’s thermal expansion and contraction potential.
Curling and warping can also occur. These behaviors are most common in slabs on grade rather than precast concrete applications. The degree to which a specimen may curl or warp is highly dependent upon the raw materials and curing procedures used, and can be minimized when appropriate precautions and considerations are taken during material selection, design, placing and curing.
Creep is another potential avenue for hardened concrete volume change. Creep is deformation caused by long-term application of loads. When loads are applied, this deformation begins to take place immediately and continues over time, but at a decreasing rate. The severity of creep depends on the applied load, the age and strength of concrete to which the load is applied, the duration the load is applied to the concrete, as well as the concrete’s raw materials and curing conditions. Creep in “younger” concrete is more likely to result in a higher degree of permanent deformation, while creep in more mature concrete is less likely to result in permanent deformation.
How does your Manhole Sizing Guide correspond to square junction boxes? Is it recommended to leave a 6-inch structural leg between the pipe outside diameter and the interior corner of the box, or does the wall thickness of the box count as the structural leg? For example, would a 36-inch reinforced concrete pipe with a 4-inch wall thickness fit in a 48-inch box, or would it need a 60-inch box to maintain a 6-inch minimum structural leg on each side?
NPCA Technical Services engineers answered:
The short answer is no. The parameters described for circular manholes do not apply to square or rectangular structures. The tricky part with pipe connections for circular manholes is that the opening of these intersecting cylinders isn’t really a circle, but a large ellipse. Consequently, to get enough structure between those openings, the manhole structure needs to be much larger than anticipated. Additionally, if resilient rubber connectors are being used, each manufacturer may have additional structural considerations that could lead to manhole diameter upsizing.
The benefit of square or rectangular shapes is that the pipe interface can be flush with the flat wall and consequently a circular opening can be cast or cut to accommodate the pipe outside diameter. This is true for pipes that are straight through or at 90-degree angles. The next consideration is to provide a box section that is structurally adequate to accommodate the potentially large opening.
Regarding the need to have some minimal wall along the pipe entrance surface, the question is whether a structure with a 48-inch inside box width dimension would accommodate a 36-inch pipe with an 8-inch wall. Or, would it require a 60-inch inside width dimension?
The answer is the 48-inch width could be perfectly adequate if it is designed appropriately. There have been many structures that were precast with a base section and two freestanding opposite side walls cast integrally to the base section. The structure is set into the trench with the incoming and outgoing pipes set to grade – typically designed to set on the base section. The minimal void between the pipe’s outside diameter and the inside of the box wall is then formed and grouted into place flush to the top of pipe, or in this case also the top of the wall. After appropriate setup and curing time for the grout, the inside plywood forms are removed from the top of structure, mastic rope sealant is applied, and the flattop or casting frame is applied atop the structure.
Again, the key to success is providing a structure that can handle the anticipated loading during installation – which is often the most severe – and after pipe placement, grouting, backfill and possibly live loads. The type of pipe used, whether rigid or flexible, would have an influence on some of the resisting loads of the installed structure. Additionally, when pipe angles are at 90 degrees, the leg of concrete in the corner will behave more as a column than a wall and can be analyzed as such.
The use of square or rectangular structures when designed appropriately can provide the benefit of structure size reduction. However, the use and size availability varies throughout the country. Designers and contractors need to contact their local precast plant to determine what is available for their location.
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