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SEMINARIO INTERNACIONAL Diseo y Durabilidad del Shotcrete
Design & Modeling
Plan
Design of a wet-mix shotcrete mixture Strength and durability Pumping:
Basic rules for shotcrete pumping State-of-the-art approach
Modeling of shotcrete placement
Strength & Durability
Shotcrete is a method to place concrete so (almost) everything we know about concrete
applies to shotcrete
The main difference is in the consolidation or compaction Concrete: vibration Shotcrete: high velocity of the material
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Strength & Durability
The single most important factor for strength and durability is the water/cement ratio!! Mas importante: razon agua/cemento! because the w/c controls the porosity
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Strength & Durability
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Strength & Durability
Unfortunately, this statement is incomplete: water/cement ratio is the most important parameter, as long as:
the fresh concrete is stable and homogeneous placement and consolidation is complete curing (hydration) takes place
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Concrete Mix Design
The challenge is the same everywhere around the globe: Produce the cheapest ($) concrete while reaching the
specifications of the engineer It is a simple calculation:
1 ton of cement: 180 $-220 $ 1 ton of aggregate: 10-50 $ 1 litre of water: ??? $ Admixtures: 5 - 60 $ (& ++) For a concrete (m3) : 130-200 $/m3 (& ++)
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Concrete Mix Design
The lowest cost means less cement and more water (w/cm ) and a lot of aggregates...
The specifications of the engineer (strength & durability) usually translate into a low w/cm
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Concrete Mix Design
A good mix design is therefore a compromise where we look for: Maximum amount of aggregate A low w/cm for durable and strong paste while maintaining adequate workability for
placement and consolidation (minimum amount of paste in the system + admixtures)
and preventing instability (segregation or bleeding)
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How to Mix design ACI 211
Select a w/c based on exposure conditions AND compressive strength (< value)
Based on aggregate* size, air content and target slump, choose water content and calculate cement content
* gradation per ASTM C33
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How to Mix design ACI 211
Based on sand Fineness Modulus, select
coarse aggregate content Fill the rest of the space with sand Adjust water content for Saturated Surface
Dry (SSD) conditions of aggregates Try it !
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PUMPING CONCRETE
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Concrete Pumping Why ?
Concrete pumping is widely used for: Hard to reach places Multi story buildings Floors Slabs on grade Wet-mix shotcrete
Blockage Stops productivity $$$ Demoralizes the troops Excessive wear on equipment
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How to Mix design for the PUMP ACI 304.2R
Select a w/c based on exposure conditions AND compressive strength (< value)
Based on aggregate* size, air content and target slump , choose water content and calculate cement content TARGET SLUMP: 5-15 cm 2-6
* gradation per ASTM C33 and more
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Sand gradation -(304.2R)
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Target FM: 2.68 max variation: 0.2
Sieve No.50: 15%-30% No.100: 5%-10%
Combined aggregate -(304.2R)
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ACI 304.2R
No zig-zags!
Combined aggregates
Usually results in coarse agg./total agg.
Aggregates Gradation
Interesting Quotes (304R-00)
For gradation purposes, the fine and coarse aggregate should be considered as one
The use of extra quantities of cementitious materials as the only means to correct pumping difficulties is shortsighted and uneconomical. Correcting any deficiencies in the aggregate gradation is more important.
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Literature review
Other publications (on concrete) all agree on the importance of: Coarse aggregate/Total aggregate content (
IF WE GO A LITTLE FURTHER
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Pumping Concrete
Pumpability (definition) The ability of confined concrete to flow
under pressure while maintaining its initial properties
which led to types of studies: Stability under pressure Mobility under pressure
Gray, 1962; Beaupr, 1994
Pumping Concrete Mobility: Ede (1967) observed that the
concrete flow in a pipe respected the laws of hydraulics
Flow is independent of pressure Head loss is linear
Flow vs. friction Several studies are available, but results vary
considerably: Only consider slump, omit viscosity
Browne & Bamforth, 1977; Tattersall & Banfill, 1983;
Kaplan, 2000; Chapdelaine, 2006
Slump & Viscosity ? Rheology
Study of fluids in motion Bingham rheological
model
= 0 +
Yield stress (0) Plastic viscosity ()
No se puede mostrar la imagen en este momento.
Friction in the pipe ?
Tribology Study of the interaction of surfaces in relative motion
Chapdelaine, 2006
Behavior Bilinear model
Low speed tribology Interface prop.
Higher speeds rheology and tribology
Interface & flow prop.
(Kaplan, 2000; Chapdelaine, 2006)
Key pumping parameters
Chapdelaine, 2006
PIZZA EFFECT!
Pipe : 15 cm (6) Lubricating layer: 2 mm Paste in layer: 2.6%
Pipe : 5 cm (2) Lubricating layer: 2 mm Paste in layer: 7.8%
Reducers
= 15 cm (6)
= 5 cm (2) Vin = 1
Vout = 9
The MIX must therefore allow for a rapid reorganization of the aggregates to allow the migration of paste to the exterior wall, while accelerating, and while maintaining stability (not segregation) !
SHOTCRETE
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Wet-mix Process
The concrete is pumped through a hose and air is added at the nozzle to accelerate the mixture to a high velocity
Wet-mix Process
Wet-mix Shotcrete 2 in. hose High Output
Wet-mix Shotcrete
1.5 in. hose Low Output
What is different with shotcrete?
Two supplemental requirements: We have to pump in smaller hose diameter:
Pumping concrete in general: = 10-15cm (4-6) Shotcrete: = 3.7-6 cm (1.5-2.5)
We need it to stick on the receiving surface!
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Literature review
ACI 506R-05 Guide to shotcrete suggests: the use of its Gradation #2 with 10 mm nominal
max. aggregates Max. aggregate size = 1/3 of hose diameter the use of 390-450 kg/m3 of cement (and adds) a W/CM between 0.4 and 0.5
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ACI 506R-05 Guide to shotcrete
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Challenges in wet-mix shotcrete
Limit hose blockages
Limit (eliminate!) fall-outs from shotcreted surface
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How ?
Limit hose blockages Increase workability while maintaining stability
(improve mix design (!), use of AEA and SP) Target: slump of 5 - 12.5 cm (2-5) Challenges & Dangers:
Difficulties in controlling quality and uniformity of mix components (usually aggregate gradation)
Overuse of SP, creating stability problems: if it does not pump at 15 cm slump, there is a problem!!
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How ?
Eliminate fall-outs from shotcreted surface Use set-accelerator at the nozzle Dangers:
If the previous slide is not working and the mix pumped is unstable or just too workable (slump > 15 cm), then one needs more set-accelerator to prevent fall-outs
which may (will) result in reduced strength and durability
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Examples of mix design #1 -Mortar #2 - Concrete #3 SF Concrete
Cement (kg/m3) 540 450 395
Silica fume (kg/m3) - - 32
Sand (0-5 mm) (kg/m3)
1600 1630 1090
Coarse agg. (2.5-12 mm) (kg/m3)
- 235 600
Water (kg/m3)
230 (w/b = 0.42)
170 (w/b = 0.38)
210 (w/b = 0.49)
AEA (ml/kg binder) 12.5 - -
SP (ml/kg binder) 1.3 - -
Air content (%) 10.3 5.0 4.6
Slump (cm) 12 4.5 5
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Summary
We have identified: the aggregate gradations (sand & combined),
FM, (Coarse Agg./Total Agg) the water/cement ratio
for strength and durability
as the control parameter of our shotcrete production.
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LETS GO EVEN FURTHER
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Research Project
Objective Improve and understand wet-mix shotcrete
Special attention is given to properties relating
to: Pumpability of concrete
To pump or not to pump? Shootability of shotcrete
To stick or not to stick?
Results Pumpability:
% Paste Pumping pressures Rheology
&Tribology Shootability
Air flow Rebound In-place proportions Hardened
properties
Pumpability
Pre-bagged material More than 25 mixes
W/B = 0.4 Variable paste content Variable air content (3-
13%) > 40 m3 of concrete
Example