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

ICH Capacitacin

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

ICH Capacitacin

Strength & Durability

ICH Capacitacin

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

ICH Capacitacin

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 (& ++)

ICH Capacitacin

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

ICH Capacitacin

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)

ICH Capacitacin

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

ICH Capacitacin

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 !

ICH Capacitacin

PUMPING CONCRETE

ICH Capacitacin

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

ICH Capacitacin

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

ICH Capacitacin

Sand gradation -(304.2R)

ICH Capacitacin

Target FM: 2.68 max variation: 0.2

Sieve No.50: 15%-30% No.100: 5%-10%

Combined aggregate -(304.2R)

ICH Capacitacin

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.

ICH Capacitacin

Literature review

Other publications (on concrete) all agree on the importance of: Coarse aggregate/Total aggregate content (

IF WE GO A LITTLE FURTHER

ICH Capacitacin

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

ICH Capacitacin

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!

ICH Capacitacin

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

ICH Capacitacin

ACI 506R-05 Guide to shotcrete

ICH Capacitacin

Challenges in wet-mix shotcrete

Limit hose blockages

Limit (eliminate!) fall-outs from shotcreted surface

ICH Capacitacin

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!!

ICH Capacitacin

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

ICH Capacitacin

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

ICH Capacitacin

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.

ICH Capacitacin

LETS GO EVEN FURTHER

ICH Capacitacin

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