EARLY AGE COMPRESSIVE AND TENSILE STRENGTH DEVELOPMENT

OBJECTIVEDetermine how SCC strategies…

high paste content VMA (thickeners) smaller aggregate & controlled gradation HRWR, SP (CAE) Mineral fillers & additives

…change hardened properties… segregation shrinkage and creep entrained air system permeability strength

…and affect performance. early age cracking long term durability surface scaling freeze-thaw resistance abrasion resistance

EXPERIMENTS

Mechanical Performance of Self Consolidating ConcreteM. D. D'Ambrosia, D. A. Lange , A. J. Brinks

University of Illinois at Urbana-Champaign

UIUC SCC DATABASE

SCC proportions differ from ordinary concrete

Selected control mixtures from literature explore various strategies of SCC mixture proportioning

VMA

Mineral Fillers

GradedAggregate

High Paste

Average w/cm = 0.41, w/p = 0.35 33% contain limestone powder (LSP) 52% contain fly ash, 37% contain slag 30% contain slag and fly ash 0% contain both LSP and pozzolans 45% contain VMA 1% contain both VMA and LSP Only 3% contain 3 different size

aggregates

Slump flow of SCC mixtures was measured for quality control

A separate rheological study was conducted (L. Shen, L. Struble, J. Hidalgo)

SCC mixtures tend to have low w/c ratio and high paste%, and thus higher strength than most ordinary concrete. When compared to OPC with same w/c and paste%, strength is similar, indicating that SCC admixtures had little effect of strength

SG Unit OPC1 SCC1 SCC2 SCC3 SCC4

Cement (Type I) 3.15 lb/yd3 726 661 601 685 679

kg/m3 431 392 357 407 403

Fly Ash (Class C) 2.65 lb/yd3 0 157 325 0 151

kg/m3 0 93 193 0 90

Coarse Aggregate, 3/4" (20mm) 2.70 lb/yd3 1853 367 1365 1627 579

kg/m3 1099 218 810 965 343

Coarse Aggregate, 3/8" (10mm) 2.70 lb/yd3 0 1075 0 0 1018

kg/m3 0 638 0 0 604

Fine Aggregate (FM = 2.57) 2.64 lb/yd3 1192 1403 1336 1389 1389

kg/m3 707 832 792 824 824

Water 1.00 lb/yd3 290 311 301 278 267

kg/m3 172 185 179 165 158

Superplasticizer (CAE) 1.06 fl oz/yd3 22 63 29 49 36

l/m3 0.84 2.44 1.12 1.89 1.38

VMA 1.00 fl oz/yd3 22

l/m3 0.84

Paste content by Volume % 32% 37% 40% 33% 34%

FA/CA ratio 0.64 0.97 0.98 0.85 0.87

Slump flow (standard slump for OPC1) in 5 30 28 26 27

mm 100 750 700 650 700

Air Content % 2.5% 1.0% 2.5% 3.5% 3.0%

w/cm 0.40 0.38 0.33 0.41 0.32

0.0

0.5

1.0

1.5

2.0

2.5

50% 55% 60% 65% 70% 75% 80% 85% 90% 95% 100%

Aggregate Content (%)

FA

/CA

Rat

io

SCC Database

Mixtures studied

SCC4

OPC1

SCC3 SCC2

SCC1

Typical non-SCC materials, according to ACI mixture proportioning method

0

2000

4000

6000

8000

10000

12000

14000

0 10 20 30 40 50 60

Concrete Age (Days)

Com

pres

sive

Str

engt

h (p

si)

OPC1, w/c = 0.40

SCC1, w/c = 0.39

SCC2, w/c = 0.33

SCC3, w/c = 0.41

SCC4, w/c = 0.34

0

200

400

600

800

1000

1200

1400

0 2 4 6 8 10

Age (days)

Ten

sile

Str

engt

h (p

si)

OPC1, w/c = 0.40

SCC1, w/c = 0.39

SCC2, w/c = 0.33

SCC3, w/c = 0.41

SCC4, w/c = 0.34

AUTOGENOUS SHRINKAGE AND TOTAL SHRINKAGE DURING DRYING

-200

-150

-100

-50

0

0 5 10 15 20 25 30

Age (d)

Aut

ogen

ous

Shr

inka

ge (

10-6

m/m

)

OPC1, w/c = 0.40

SCC1, w/c = 0.39

SCC2, w/c = 0.33

SCC3, w/c = 0.41

-1000

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

0 5 10 15 20 25 30

Age (days)

Fre

e S

hrin

kage

(x1

0-6

)

OPC1, w/c = 0.40

SCC1, w/c = 0.39

SCC2, w/c = 0.33

SCC3, w/c = 0.41

SCC4, w/c = 0.34

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 2 4 6 8 10

Age (d)

Str

ess-

Str

engt

h R

atio

OPC1, w/c = 0.40

SCC1, w/c = 0.39

SCC3, w/c = 0.41

SCC4, w/c = 0.34

RESTRAINED STRESS DEVELOPMENT, STRESS STRENGTH RATIO AND RELAXATION BY TENSILE CREEP

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6 8 10

Age (days)

Spe

cific

Cre

ep

(x1

0-6 m

/m/p

si)

OPC1, w/c = 0.40

SCC1, w/c = 0.39

SCC3, w/c = 0.41

SCC4, w/c = 0.34

-300

-250

-200

-150

-100

-50

0

50

100

150

200

0 1 2 3 4 5 6 7Time (days)

Str

ain

(

)

0

1

2

3

4

5

6

7

8

9

10

Applied L

oad (kN

)

Restrained Specimen

Free Specimen

Load (kN)

Creep

Cumulative Shrinkage + Creep

-300

-250

-200

-150

-100

-50

0

50

100

150

200

0 1 2 3 4 5 6 7Time (days)

Str

ain

(

)

0

1

2

3

4

5

6

7

8

9

10

Applied L

oad (kN

)

Restrained Specimen

Free Specimen

Load (kN)

CreepCreep

Cumulative Shrinkage + Creep

Cumulative Shrinkage + Creep

Free Shrinkage Load

Restrained

Drying began at concrete age of 1 day Environment was 50% RH and 23°C An LVDT extensometer was used to

measure deformation Sealed barrier of aluminum foil applied

to impose symmetric drying

Feedback controlled closed loop system applies a restraining force with servo hydraulic actuator

Instron Controller operated by a Restraint Simulation Program (RSP) (LabView)

Restraint Simulation Program keeps strain virtually constant over time (to within 0.005mm )

Summation of restrained deformation allows for creep calculation

Autogenous shrinkage of sealed specimens was measured from time of casting

Internal relative humidity in sealed prism was also measured to asses the driving force for autogenous shrinkage

0

50

100

150

200

250

300

350

400

450

0 2 4 6 8 10

Age (days)

Sh

rin

kag

e S

tre

ss (

psi

)

OPC1, w/c = 0.40

SCC1, w/c = 0.39

SCC3, w/c = 0.41

SCC4, w/c = 0.34

SUMMARY An investigation of the EA mechanical

behavior of SCC has revealed a potentially high risk for cracking in some mixtures

Mechanical properties can be influenced by higher paste content and low w/c, BUT it is best not to treat SCC as a group of materials with similar mechanical behavior.

Autogenous shrinkage may cause significant stress at early age

SCC Mix Design should minimize cement paste content and use a w/cm that avoids significant autogenous shrinkage while achieving necessary flow characteristics

Providing external water during curing in field applications will delay shrinkage stress development at early age and reduce the overall magnitude of shrinkage and cracking risk.

Low w/c drives autogenous shrinkage, which can then be a major contributor to total shrinkage at early age. High early age shrinkage leads to tensile stress and cracking when concrete is restrained

Stress measurements indicate that to reduce the risk of early age cracking in SCC, using a w/cm ratio of 0.40 to 0.42 can prevent autogenous shrinkage from causing significant stress, while at the same time minimizing drying shrinkage.

Stress-strength ratios demonstrate that microcracking and damage may be occurring as early as one or two days after drying at early age. Creep capacity is directly proportional to paste content and is inversely proportional to w/cm ratio. The high stress-strength ratio of SCC1

induced microcracking damage High cracking risk In SCC4 the stress develops rapidly due to the lack of relaxation by creep and damage occurs rapidly right before failure Also high cracking risk