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FAQs
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Q:
Could RCC dams be founded on non-rock foundation?
Q: What are
the different types of facing systems used for RCC dams?
Q. What
is the difference between Roller-Compacted Concrete (RCC) and Soil
Cement (SC)?
Q: What are
the control and required densities for RCC?
Q: What
values of coefficient of roughness “n” are
typically used in the Manning’s formula when designing open
channels and spillways with soil-cement (SC) or roller-compacted
concrete (RCC)?
Q: What methods
are available for edge compaction of RCC?
Q: How
are RCC mixtures for water resource applications proportioned using
the soils spproach?
Q: Should RCC
lifts be bonded?
Roller-compacted concrete (RCC) is a no slump
concrete that must be dry enough to support the weight of large
vibratory compaction rollers yet wet enough to permit adequate distribution
of paste. RCC dams are typically constructed in 1-ft (0.3-m) lifts
and, because of the dry nature of the mix, subsequent lifts placed
after the initial set time of previous placed lifts do not necessarily
bond without the introduction of bedding mortar or conventional
concrete. The material within each lift is referred to as “parent”
RCC and the horizontal joints between layers are referred to as
lift joints. Bonding of lift joints is important for two reasons;
structural stability and seepage control.
Structural designs of gravity dams must address structural stability
in terms of overturning, sliding, and internal stresses. Consequently,
when considering internal stresses, both the strength properties
of the parent RCC and lift joints must be evaluated. In most cases,
the lift joint is weaker than the parent RCC except where bedding
mortar or bedding conventional concrete is utilized across the entire
lift surfaces. Proper bonding of RCC lifts could result in lift
joint strength equal to or greater than the strength of the parent
RCC.
Although seepage normally is not a structural concern, excessive
seepage may produce excessive internal uplift pressure not accounted
for in the design, permit the loss of large amounts of water, create
a maintenance problem involving collection and disposal, cause internal
erosion of the structure, or create freeze-thaw problems at the
downstream face. Seepage along horizontal lift joints should be
distinguished from other sources of seepage such as through the
parent RCC, control joints, cracks and foundation drains. Bedding
mortar/concrete will reduce permeability at the lift joints but
would not have a significant effect on the seepage from other sources.
When an RCC lift is covered with the next lift before the preceding
lift reaches initial set, the joint at the interface is considered
a fresh joint and should provide a strong watertight joint for reasonably
workable RCC mixes. If the lower lift reaches initial set prior
to covering with another lift, a cold joint begins to develop, resulting
in loss of bond strength and potential increase in permeability.
Generally a cold joint occurs in non-retarded RCC within 4 to 6
hours and within as little as 1-1/2 hours when ambient temperatures
are at or above 90 degrees F. Once a cold joint develops, a bedding
layer treatment may be needed to achieve the required bond strength
as well as water tightness. Contaminated surfaces should always
be cleaned prior to placement of subsequent lift regardless of the
joint maturity. Also some projects require placement of bedding
mortar for a specified width along the upstream face at all horizontal
lift joints to limit seepage at these joints.
 Bedding
mixtures generally have consisted of either bedding mortar or bedding
concrete with bedding mortar being more common due to its greater
ease in manually spreading across the lift surface. Bedding mortars
typically contain sand; a cementitious content ranging from 400
to 600 pcy (235 to 355 kg/m3); water; water reducer; and set retarder.
The bedding mortar typically has a slump ranging from 7 to 9 in.
(180 to 230 mm) with a minimum set time of 3 hours at 95 degrees
F (35 degrees C). The primary difference between bedding mortar
and concrete is that bedding concrete contains a small percentage
of coarse aggregate and the concrete has a slump range from 5 to
7 in. (125 to 180 mm).
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Q: What are the control
and required densities for RCC?
A: Compressive strength and density test results from numerous
roller-compacted concrete (RCC) projects were analyzed and the data
indicated that the density has a direct effect on the strength of
RCC. Sufficient density of a good RCC mix can be achieved if proper
compaction means and methods are implemented.
The required density of RCC is often referred to as specified minimum
density and is typically defined in the project specifications.
Prior to production placement of RCC, a control (or reference) density
is established. At least four different approaches have been used
to determine the control density. These are:
- Soil compaction techniques
- Laboratory tests to determine the density of compacted specimens
using a vibratory table or hammers
- Calculation of theoretical air-free density
- Proof rolling of test lift(s) to determine the maximum achievable
density
Once a control density is established, the required minimum density
can be calculated based on the project specifications. Typically,
engineers specify the required minimum wet density to be 96 to 98
percent of the wet control density. Calibrated nuclear density gauges
are used to determine the density of RCC in the field. Based on
the field test results, engineers can determine if the achieved
density meets the density requirements of the project.
Detailed information on density of RCC and descriptions of test
methods are described in PCA’s publications, Roller-Compacted
Concrete Density: Principles and Practices ( IS541) and
Guide for Developing
RCC Specifications and Commentary (EB214).
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