Toward a sustainable SCC through the

use of high volume Fly ash and Slag to

reduce cement in SCC and the effect on

its cracking potential

Salah Altoubat

University of Sharjah, UAE

Middle East Conference on

Sustainable Building Materials

February 23rd, 2011

• SCC Characteristics: – Does not need vibration– Excellent deformability

– High resistance to segregation

• Attractive choice in the construction industry

• SCC has gained wide recognition in the Gulf– its use is on the rise

– Research on local materials is demanded

Background

• Carbon footprint of concrete is contributed by the following: – Material production (cement, reinforcement, agg…etc)

– Concrete production

– Repair during service life

– Demolition and recycling

• Sustainable concrete demands reduction of cement use– One ton of cement produce around 0.9 ton of CO2

• Sustainable concrete demands durable concrete– Reduce Repair and increase service life

Sustainability Concern: Construction Industry contribute around 20 to 25% of CO2

• Durability is the other face of Sustainability

• Reduction of Cement

– Fly ash and GGBF Slag to replace cement

– High volume of replacement of cement

• Fly ash and GGBF slag are byproducts of steel

production and thermal power generation

– Reduce risk of environmental issues by using them in concrete

Sustainability Requirements:

• Significant Reduction of Cement

– use of supplementary cementitious materials

– High volume of slag and fly ash

• Keep eyes on durability issues

– Premature cracking

– Early age cracking

– Corrosion

– Quality control

• Early age shrinkage cracking is critical factor for

durability

– Shape the durability of concrete during service life

Scenarios to achieve sustainable concrete:

• Durability Issue:

– SCC exhibits high potential for shrinkage cracking.

– Results on drying shrinkage of SCC are conflicting

– Lack of local results in the Gulf

Motivation

SCC is typically characterized by:

SCC has high potential for shrinkage

• Low content of coarse aggregate

• Low water to cement ratio

• High content of binder

• High content of fine aggregate

Shrinkage cracking of SCC is a concern !!

Important Factors:

Shrinkage Cracking

• Shrinkage potential

• Tensile creep

• Curing conditions

• Restraint conditions

Restrained Shrinkage Test should be

performed !!

Material Parameters

External Parameters

Passive Restraint Test Active Restraint Test

Restrained Shrinkage Test: Linear Test

Restrained Shrinkage Test: Ring Test

Ring Test is a viable test to perform

Restrained Shrinkage

AASHTO STANDARD RINGSASTM STANDARD RINGS

Dimensions ASTM Standard Ring AASHTO Standard Ring

Steel Thickness 12 mm 12mm

Steel Height 150mm 150mm

Concrete Thickness 38mm 75mm

Concrete Height 150mm 150mm

Degree of Restraint Depends on type of Ring

ASTM ring: 60 to 70 % AASHTO ring: 40 to 50 %

• Studying the restrained shrinkage & cracking potential for SCC made from

local materials in UAE.

• Investigating the effect of following parameters on cracking potential of SCC– Type & proportion of supplementary cementitious materials( Fly ash, GGBS , Micro silica)

– Degree of Restraint

– Curing Regime

Objectives

• Perform restrained shrinkage ring

tests for SCC mixes with different

supplementary materials

• SCM includes:

– Fly Ash

– GGBS

– Micorsilica

– Control

Experimental program: Methodology

• External Parameters

– Degree of Restraint• High degree (ASTM ring) ~ 60 to 70 %

• Moderate degree (AASHTO ring) ~ 40 to 50 %

– Curing Conditions• Air drying

• Moist curing for 3 days

• Moist curing for 7 daya

• Materials parameters

– High strength SCC mix with w/c of 0.36

– Compressive strength = 60 MPa

– Proportion of fly ash (0, 20%, 35%, and 50%)

– Proportion of GGBS (35%, 50% and 70%)

– Proportion of micro silica (5, 7, and 10%)

– Combination of fly ash or GGBS with micro silica

Experimental program: Study Parameters

– Ring test

• Monitor steel strain

• Age at cracking

• Net time to cracking

– Compressive Strength

• At age of 3, 7, 14, 28, 56, 100 and 120 days

– Free shrinkage

• From demolding to age of 120 days

– Tensile strength

• At age of 3, 7, 14, and 28 days

Experimental program: Measurement

Mix

No.

Type

of

SCMs

Fly Ash

(SCMs)

(%)

Restrained Shrinkage Test (Ring Test)

ASTM Standard Test AASHTO Standard Test

Air

Drying

3Days

Moist

7Days

Moist

Air

Drying

3Days

Moist

7Days

Moist

1 Control - Х Х Х Х Х Х

2 Fly Ash 20 Х Х Х Х

3 Fly Ash 35 Х Х Х Х

4 Fly Ash 50 Х Х

7% micro silica was added to mixes 3 and 4 in addition

to fly ash and cured for 3 days

Experimental program: Test Matrix

Additional tests

Primary tests

Mix w/c Cementitious Materials

Kg/m3

Aggregate

Kg/m3

HRWRA

L/m3

VE

Kg/m3

Cement Fly Ash TotalCoarse

Agg.

Fine

Agg.

Fine Agg.

/Total Agg.

Control 0.36 450 - 450 736 1101 0.60 6.5 0.6

Fly Ash-20% 0.36 360 90 450 735 1090 0.60 6.5 0.6

Fly Ash-35% 0.36 292 185 450 723 1056 0.59 5.5 0.6

Fly Ash-50% 0.36 225 225 450 723 1032 0.59 5.5 0.6

Experimental program: Concrete Mix Proportion

with Fly Ash

Maximum aggregate size was 10 mm

Wash sand, crushed sand and dune sand as fine aggregate

Class F fly ash

Experimental program: Concrete Mix Proportion

with Slag

Maximum aggregate size was 10 mm

Wash sand, crushed sand and dune sand as fine aggregate

Class F fly ash

Mix w/c Cementitious Materials

Kg/m3

Aggregate

Kg/m3

HRWRA

L/m3

VE

Kg/m3

Cement GGBS Total Coarse Agg. Fine Agg.Fine Agg.

/Total Agg.

GGBS-35% 0.36 292 158 450 747 1096 0.59 5.5 -

GGBS-50% 0.36 225 225 450 748 1094 0.59 6.0 0.5

GGBS-70% 0.36 135 315 450 738 1083 0.59 5.5 0.6

Mix Fresh Properties

Slump Flow

(mm)

Flow Rate

(s)

L-Box

Ratio

Control 620 10 0.70

20%FA 720 5 0.85

35%FA 600 6 0.80

50%FA 680 5 0.95

L- Box Test≥0.7

Slum p Flow

(600-750mm)

Flow Rate

(3 – 10 sec)

Experimental program: Targeted Fresh Properties

Binder Materials

Type Specific Gravity Source

Cement 3.15 Union Cement Factory (UAE)

Class F Fly Ash 2.3 Available in UAE Market. It is originally from a

India.

Aggregates

Type Specific Gravity Absorption Source

10 mm Agg. 2.96 0.9 Siji

Crushed Sand 2.78 1.5 Ras Al Kaimah

Washed Sand 2.74 1.4 Siji

Dune Sand 2.63 0.8 Al Ain

Admixtures

CHRYSO® Fluid Optima 230 Poly Carboxelated based High range water reducing admixture

Feyplast SUB-AQUA Viscosity Enhancer Admixture in powder form

Materials Properties

Experimental program: Ring test set up and instrumentation

Data acquisition system

Experimental program: Curing and Strain Monitoring

Age of first cracking is sensitive to fly ash proportion and

curing condition

Test results: ASTM Steel Ring Strain Evolution

Test results: AASHTO Steel Ring Strain Evolution

Cracking sensitivity to fly ash proportion and curing

condition is more pronounced in AASHTO ring test

Curing is

effective

Test results: Summary of Net Time to Cracking

35% of fly ash perform well under high degree of

restraint, provided moist curing is adopted for 3 days

Test results: Summary of Net Time to Cracking

Addition of 7% SF combined with fly ash significantly

improved the crack resistance of the mix under high

degree of restraint.

Test results: Summary of Net Time to Cracking

50% of fly ash perform well under low degree of restraint,

provided moist curing is adopted for 7 days

Age of first cracking is sensitive to GGBS proportion and

curing condition

Test results (GGBS): ASTM Steel Ring Strain Evolution

Test results (GGBS): AASHTO Steel Ring Strain Evolution

Cracking sensitivity to GGBS proportion and curing

condition is more pronounced in AASHTO ring test

Test results: Summary of Net Time to Cracking

50% of GGBS perform well under high degree of

restraint, provided moist curing is adopted for 3 days

Test results: Summary of Net Time to Cracking

70% of GGBS perform well under low degree of restraint,

provided moist curing is adopted for 7 days

Air Drying 3Days Moist 7Days Moist

Test results: Crack Resistance Under High Degree

of Restraint

35% fly ash and 50% slag increase the

crack resistance relative to control

provided moist curing is adopted

Significant

improvement with

7% SF and fly ash.

Test results: Crack Resistance Under Low Degree

of Restraint

Fly ash and Slag increase the

crack resistance relative to control

provided moist curing is adopted

Fly Ash can be added by

up to 50% and Slag by

up to 70 % in low degree

of restraint application .

1. Tensile Strength

2. Shrinkage Potential

3. Induced Tensile Stress

4. Degree of Relaxation

Analysis: Key Parameters affecting cracking

Analysis: Free Shrinkage

Fly ash did not affect long term drying shrinkage

Early shrinkage potential decreased as the fly ash proportion increased

Analysis: Early Age Free Shrinkage

Analysis: Degree of Relaxation

Fly ash improves early age stress relaxation

Analysis: Degree of Relaxation

Slag improves early age stress relaxation

• Highlighted critical parameters to achieve sustainable SCC– % of Fly Ash

– % of GGBF Slag

– Curing Condition

– Degree of Restraint

• Fly ash and slag can be used to reduce cement in SCC– Fly ash can replace cement by up to 50%

– GGBF slag can replace cement by up to 70 %

• Fly Ash and GGBF typically IMPROVES crack resistance of SCC provided that moist curing is adopted

Summary and Conclusions

• Durability consideration:

– Fly ash can be added by up to 35% in HIGH DEGREE of restraint structure provided that moist curing for at least 3days is adopted.Beyond this proportion 7% micro silica should be added.

– For LOW DEGREE of restraint structure, fly ash can be added by up to 50% provided that moist curing for at least 3days is adopted, preferably 7days.

– GGBF Slag can be added by up to 50% in HIGH DEGREE of restraint structure provided that moist curing for at least 3days is adopted.

– For LOW DEGREE of restraint structure, Slag can be added by up to 70% provided that moist curing for at least 3days is adopted, preferably 7days.

Summary and Conclusions