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.
Journault writes:
What about the vibration absorption of railroad ties? Is that significant? I presume wood would be superior in this regard. Also, does that affect noise?
NPCA Technical Services engineers answered:
Typically, after the ties are positioned, a rubber pad is placed on the tie where the rail will sit. This pad absorbs impact and provides electrical insulation between the rail and the concrete. Sound mitigation should be roughly the same for both rail tie materials.
Precast concrete ties have many advantages. Precast ties provide superior load capacity and stability. A track using precast concrete has greater vertical and lateral stiffness. In concert with the consistent, non-shifting gauge, this drastically reduces maintenance for loose rails and fasteners, and track-related derailments are less likely. Unlike wood and steel, concrete ties do not deteriorate or corrode from exposure to weather. Since they are not coated with creosote like wood ties and do not have sharp edges like steel ties, they are safer for employees and better for the environment. They are also noncombustible and reduce the chance of track fires.
CHK writes:
Can anyone elaborate on segregation, especially while using self-consolidating concrete?
NPCA Technical Services engineers answered:
Segregation is defined as concrete constituent materials separating prior to concrete set. This is not a phenomenon limited to SCC. It can happen with any type of concrete. For example, if you vibrate conventional concrete too long, you risk having the heavier aggregates sink and the water rise. This causes the aggregates to segregate toward the bottom of the structure.
Since we place SCC differently and let it flow under its own energy, there’s an increased risk of segregation, especially of aggregates if they are not kept in suspension during flow. As such, segregation is one of the most frequently reported issues with fresh SCC. Resistance to segregation is a factor that contributes to stability. Stability includes dynamic stability and static stability. When the concrete is flowing, we are talking about dynamic stability. Once it reaches its final placement, then we are dealing with static stability. Resistance to segregation depends on many factors, including aggregate size and gradation and viscosity of the mix. Viscosity modifiers are sometimes used to reduce segregation.
For example, if you notice immediate segregation, this usually points to a viscosity issue and can indicate excess water. Excess aggregate moisture can be the culprit as it can significantly raise the water-cement ratio. A delayed segregation, or segregation that occurs once the SCC reaches its final position, can be caused by adding too much high-range water reducer.
If you’re having issues with segregation, a good place to start is to look at your moisture contents. Are you adequately checking your aggregate moistures? If you’re doing that with probes, have those probes been calibrated? It’s also a good idea to consult with your admixture supplier.
Jun writes:
Our design mix is supposed to be for 80-millimeter slump. The water-cement content ratio is 0.35. The slump we got was 40 millimeters. How are we going to adjust the slump to 80 millimeters without altering the water-cement ratio? Is batch adjustment used for wet aggregates, dry or both?
NPCA Technical Services engineers answered:
If the aggregates are dry, they will absorb the mix water and reduce your slump and w/c ratio. Prior to batching, you should check your aggregate’s moisture content and compare it to how the mix design was proportioned. If your mix design assumes you’ll be using aggregates in a saturated surface dry state, but the aggregates you use are dryer than SSD, the aggregates will absorb mix water. Dry aggregates can also absorb any liquid admixtures in the mix, which can greatly reduce effectiveness. Similarly, if your aggregates are wet, they will add water to your mix. This results in greater slump and in a greater w/c ratio.
One effective way to get a higher slump while maintaining the w/c ratio is by using water reducers. You would use it as a plasticizer since you’re looking to increase workability while maintaining current water content. Talk to your admixture supplier about getting to 80 millimeters and staying at 0.35.
ACI 211.1, “Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete,” provides further information.
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