high strenght concrete

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High Strength Concrete - Durability Investiga-

tions by Using the CDF-Test - First Results

Robert Krumbach1, Katrin Seyfarth2, Wolfgang Erfurt2, Karen Friedemann1

SUMMERY

Some unusual observations with respect to the durability of some high-strength

concretes (HSC) caused investigations on the influence of typical properties of

HSCs like hydration behaviour and strength development, especially with regard

to the frost de-icing salt resistance (FDSR). The research project is a co-

operation with the F. A. Finger-Institute of the Weimar-University. In this pa-

per first results are presented.

1 INTRODUCTION

Resulting from their high density, high-strength concretes differ from normal

strength concretes in a higher durability, e.g. a high frost de-icing resistance and a

high resistance to chemical substances. Some unusual observations on HSCs

were made since the development and practical utilization of HSC which caused

objections regarding the durability. Tests on building members produced of HSCshowed, that a decrease of the compressive strength and a formation of mi-

crocracks can occur gradually. This happened primarily when concretes were

exposed to high temperatures during the hydration such as inside building mem-

bers. It was also observed that their resistance to freeze de-icing cycles can be

considerably lower than expected [1-4].

The guideline of the DAfStb (German Committee for Reinforced Concrete) [5]

reflects these problems: The utilization of concretes with a strength class higher

1Dipl.-Ing., Institut fr Massivbau und Baustofftechnologie, University of Leipzig2 Dipl.-Ing., F. A.-Finger Institut fr Baustoffkunde, Bauhaus-University of Weimar


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LACER No. 3, 1998

58

than B 95 for exterior building members and the use of superplasticisers is lim-

ited.

Causes of the peculiarities of HSCs and the risk of a possible microcrack forma-tion mechanism are conceivable e.g. the entering of moisture into building

components owing to capillary suction - which induce or intensify building dam-

age:

Temporary temperatures up to 70 C inside the structural component owing to

fast heat development during the hydration [6, 7]

Stronger formation of monosulphate/AFm,

At the same time entering of more water e.g. through cracks, makes the

chemical reaction Monosulphate/AFm

Ettringite/AFt possible (thevolume growing 2,3 times)

Incomplete hydration connected with inside drying-out owing to extreme

low w-c ratios

At the same time entering of more water, e.g. through microcracks makes

the swelling of the cement gel and the formation of new phases possible

being the reason for a considerable volume growing (e.g. C3A, C4AF,

C3AH6, Monosulphate/AFm Ettringite/AFt).

No capillary pores content, dense mortar matrix

At the same time entering of more water e.g. through microcracks spacefor expansion there is no for especially:

De-icing water at freezing under hydrostatic pressure (volume growing

connected with blast effects)

Development of new phases under imposed deformation (microcracksinto the concrete structure or flakings can occur)

A reduced durability of HSCs cannot be excluded as mentioned above. Further-

more, unfavourable additional effects caused by high superplasticizer content (up

to 70 g/ml per kg cement) can occur depending on the moment of adding.

Our investigations shall clarify, whether and in which extent a reduced durability

must be expected, what is causing this and by which practical steps the risks can

be counteracted.


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High Strength Concrete - Durability Investigations

by Using the CDF-Test - First Results

59

2 GENERAL ASPECTS

In our research project the influence of the typical properties the production of

HSC, like:

extremely low w-c-ratio

high content of superplasticizer at different times

suffering high temperatures during the hydration

on the hydration process and on the strength development, especially on the

FDSR (CDF-test method), shall be investigated.Additionally, the arrangement of the concrete structure as well as possible modi-

fications of the structure will be observed before and after different kinds of cur-

ing and after the CDF-test (ultrasonic). To get a wide spectrum of various

parameters influencing the concrete properties the concrete compositions indi-

cated in table 2 were selected. For comparison, the mixtures will be produced

with two cements with different sulphate resistance: CEM I 42,5 R and CEM I

42,5 HS (sulphate resistance, DIN 1164, tab. 1). The HS-cement showed after 28

days of hydration that is before the frost de-icing salt test slightly more AFm

phases than the ordinary Portland cement containing more C3A.

The concretes have w-c-ratios of 0,40 (no superplasticizer) and of 0,25 (5 M-% of

superplasticizer). The used superplasticizer (FM) is a mixture of melamine and

naphthalinsulphonic resin.

Depending on the concrete mixture microsilica (MS) was used (content: 0 and

10 M-%, tab. 2) in the form of a slurry with 50 M-% solid content.

As aggregates we use sand (0/2) from the river Main and gravel (2/8; 8/16) from

the river Rhine.


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LACER No. 3, 1998

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Table 1: Analysis of cements used

CEM I 42,5 R CEM 42,5 HS

Blaine fineness, cm/g 3581 4449

Initial setting, start, min. 2:42 3:32

Required amount of water, M.-% 28,3 29,7

Compressive strength, N/mm

(2d; 28d) 31,9; 59,0 31,4; 57,7

SiO2 20,5 19,5

Al2O3 5,3 4,0

Fe2O3 2,6 6,7

CaO 65,3 64,2

MgO 1,9 1,6

K2O 1,1 0,7

SO3 3,2 3,1

3 FIRST TEST RESULTS

3.1 Consistence and development of durability

The determination of the consistence of any concrete mixture is performed ac-

cording to DIN 1048.

Clear differences were found between the concrete series I (w/c = 0.40; no FM)and series II (w/c = 0,25; 5 M-% FM added during mixing).

In spite of the extremely low w-c-ratio of the concretes of series II slumps be-

tween 43 cm and 55 cm are achieved by adding 5 M-% FM (II/III: 4,5 M-%;

II/IV: 4 M-%).

In contrast the concretes of series I were very sticky. The working quality was

unsatisfactory and the concrete had to be compacted on average for 2-3 min.. The

slumps achieved sizes below 35 cm.


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The consistencies of the concretes of series III (2 min. compacted, too) were also

unsatisfactory. The aim of these series was to test concretes with a w-c-ratio of

0,25 concerning their utilization as ready mixed concrete. We had planned toinvestigate the hydration by delayed FM addition. It was the intention to add

3 M-% at the beginning of the mixing process and the second dose after 45 min.

(time of transport!). Between the beginning and 45 min the mixer should work

periodically (5 min.) 1 min. each.

However, the tests did not show the expected results because the consistencies of

the mixtures with only 3 M-% were so sticky, except for mixture III/I, that we

feared an early development of solidification. Therefore 4 M-% or 5 M-% FM

had to be added at the beginning of mixing, already. The slumps of the compara-

ble mixtures of series II, however, (see tab. 3) were not achieved also after 45min. On the other hand the slumps of mixture III and IV from series III (10 M-%

silica) showed improved results.

The compressive strength tests according to DIN 1048 were carried out after 7 d,

28 d, 56 d and 182 d to estimate the strength development over a longer period.

The specimens were moist-cured in accordance with ENV206.

Table 2: Slumps and development of compressive strength

Compressive strength W.200 inN/mm

Series FM-additionin %

Slumpsin cm

Bulk densityin kg/m

7d 28 d 56d 182d

I/I - 29 2,37 51 62 73

I/II - 29 2,39 54 69 73

I/III - 32 2,31 48 71 76

I/IV - 29 2,34 60 76 80 82

II/I 5 43 2,37 60 76 78 80

II/II 5 55 2,40 66 80 89 88

II/III 4,5 45 2,38 74 96 105 107

II/IV 4 51 2,36 70 95 97 98

III/I 3+2* 35 2,39 59 75 78 78

III/II 5+1* 29 2,39 71 84 83 87

III/III 4+1* 40 2,38 70 88 93

III/IV 4+1* 38 2,40 85 107 110

* after 45 min. added FM


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LACER No. 3, 1998

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All concretes (tab. 2) met the 28 d-compressive strength-criterion of HSC of the

guideline of the DAfStb (w.200 > 55 N/mm, see tab. 2). Its noticeable that the

tested compressive strengths of the concretes with HS-cement (concretes II andIV) are higher than those of concretes with R-cement (concretes I and III; comp.

tab. 2, fig. 1).

As expected the concrete mixtures with 10 M-% silica (concretes III and IV of

series I III) yielded higher compressive strengths than the concretes without

silica (concretes I and II of series I III, tab. 2).

Mixtures with FM (w/c = 0,25) produce more favourable properties of green

concrete (slumps > 40 cm). The concretes can be compacted better and higher

compressive strengths are reached compared to concretes with w/c = 0,4 (withoutFM, comp. tab. 2).

The bulk densities amount to 2,31 - 2,40 kg/cm (tab. 2).

Fig. 1: Compressive strengths w.200 of series I w/c = 0,4; without FM

Compressive strength w200 of series I, w/z = 0,4 without FM

40

45

50

55

60

65

70

75

80

85

90

0 182

Concrete age [d]

Compressivestrengthw

200[N/mm]

CEM I 42.5 RCEMI 42.5 HS

CEM I 42.5 HS, 10 M-% silica

CEM I 42.5 R, 10 M-% silica

7 28


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Table 3: Survey of concretes and the different kinds of curing and test methods

Series of concrete/

concrete

Cement Water-

cement

ratio

Content of

silicaslurry

in %

Content of

FM

in %

Curing

DIN 1048I CEM I 42,5 R 0,4 0 0

48 h 60C,.till 7th d water

immersion

DIN 1048II

CEM I 42,5 HS

0,4 0 0

48 h 60C, till 7th d water

immersion

DIN 1048III CEM I 42,5 R 0,4 10 0


immersion

DIN 1048

I

IV CEM I 42,5 HS 0,4 10 0


immersion

DIN 1048I CEM I 42,5 R 0,25 0 5, immedi-

ately

60C und 80C, no curing*

DIN 1048

II

II CEM I 42,5 HS 0,25 0 5, immedi-

ately

60C und 80C, no curing*

63


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Series of concrete/

concrete mix

Cement Water-

cement-

ratio

Content of

silicaslurry

in %

Content of

FM

in %

Curing

CEM I 42,5 R 0,25 10 5, immediately DIN 1048III

60C und 80C, no cur-

ing*

CEM I 42,5 HS 0,25 10 5, immediately DIN 1048

II

IV

60C und 80C, no cur-

ing*

DIN 1048I CEM I 42,5 R O,25 0 5, dosed (3+2)

60C, no curing*

DIN 1048II CEM I 42,5 HS O,25 0 6, dosed (5+1)

60C, no curing*

DIN 1048III CEM I 42,5 R O,25 10 5, dosed (4+1)

60C, no curing*

CEM I 42,5 HS O,25 10 5, dosed (4+1) DIN 1048

III

IV

60C, no curing*

* planned curing and test methods

64


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Compressive strength w200; w/c=0.25; 5 M-% FM (series II)

40

50

60

70

80

90

100

110

120

0 182

Concrete age [d]

Compressivestregthw200[N/mm]



CEM I 42.5 R

CEMI 42.5 HS

287 56

Fig. 2: Compressive strengths w.200 of series II, w/c = 0,25%; 5 M-% FM

40

50

60

70

80

90

100

110

120

0 182

Concrete age [d]

Compressive strength w200 of series III, w/c=0.25;

5 M-% FM, splite


CEM I 42.5 R, 10 M-% silicaCEM I 42.5 HS

CEM I 42.5 R

7 28 56

Fig. 3: Compressive strengths w.200 of series III, w/z = 0,25; 5 M-% FM delayed


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LACER No. 3, 1998

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3.2 The frost de-icing salt resistance of the investigated high strength

concrete

Our investigations base on different questions. On the one hand the influence of

the concrete composition (w/c = 0,4 or 0,25; FM contents = 0 or 5 M-%; silica

contents = 0 or 10 M-%, comp. tab. 2) on the FDSR shall be analysed. On the

other hand the influence of the cement composition itself shall be investigated.

The CDF-Test according to SETZER [8] was carried out on specimens

150x150x75 mm3. All specimens of series I III were exposed to 28 freeze de-

icing cycles, one cycle takes in any case 12 h.

The first FDSR investigations (tab. 2) produced the expected results. HSC with

w/c 0,4 shows a high or very high FDSR. The very dense structure of these

concretes prevents the increase of mass by capillary suction of the 3 %-NaCl-

solution. The amount of the increasing mass was always below 0,35 M-% (tab. 4;

compared with normal concrete: 0,8 1,5 M-%). Especially the concretes with

w/c = 0,25 have a very small mass increase: 0,07 m 0,12 M-% (tab. 4).

The results correspond to the CDFtests: the rates of scaling of concretes with

w/c = 0,4 (28 cycles: 188 603 g/m) are higher than the rates of scaling withw/c = 0,25 (28 cycles: 51 104 g/m).

As expected silica containing specimens (concretes III and IV) have lower rates

of scaling than specimens without silica (concrete I and II, tab. 4, fig. 4-6). This

points to a denser paste matrix by the filling effect caused by silica [9].

An influence of the cement quality on the results can be recognised on concretes

of series I (w/c = 0,4, tab. 4, fig. 4). Concretes produced with CEM I 42,5 HS

yield significant higher rates of scaling (compare 28 cycles, tab. 4) than those

with CEM I 42,5 R. This tendency was also observed in earlier investigations

[10]. Furthermore, the two cement qualities lead to a different formation of the

concrete structure. This connection has to be considered, too.

Compared with that, the concretes of series II and III (w/c = 0,25) have smaller

differences between the rates of scaling with respect to R- and HS-cements. Here,

these small differences correspond with the results of pore radii distribution

(PRV) of the mortar matrix, because the concretes produced with R- and HS-

cements show a nearly identical PRV.

It is known, that CEM I 42,5 R , with w/c 0,4, shows a higher content of capil-

lary pores and a lower content of gel pores as well as microairpores than the HS-


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67

cement [10]. Concerning this an improved FDSR of HS-cement should be ex-

pected. However, the opposite tendency could be observed. Consequently, a valid

physical correlation between CDF-Test result and PRV cannot be derived [11].The causes for that are based on occurring chemical reactions, probably.

Table 4: Frost de-icing salt resistance

Rates of scaling (CDF-test) in g/m

Number of cycles:

Concrete Mass growth by

capillary suction

in %

4 8 14 28

I/I 0,32 31,6 93,2 260,4 603,1

I/II 0,33 35,3 133,5 348,1 921,1

I/III 0,30 - 75,3 105,3 188,1

I/IV 0,21 - 106,4 198,3 347,3

II/I 0,07 20,2 38,1 66,2 96,9

II/II 0,12 16,8 30,2 53,3 104,4

II/III 0,07 8,9 17 31,1 56,8

II/IV 0,07 6,2 9,3 42,2 84,0

III/I 0,09 8,2 25,6 65,8 98,2III/II 0,08 6,5 7,8 37,3 79,5

III/III 0,08 10,8 12,1 26,7 51,1

III/IV 0,08 - 16,1 26,7 56,0

The measured rates of scaling (after 28 cycles) of all series meet the acceptance

criterion of 1500 g/m.


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LACER No. 3, 1998

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Frost de-icind salt resistence (CDF-test)

w/c = 0.4; without FM (Series I )

0

100

200

300

400

500

600

700

800

900

1000

0 28

Number of freezing and thawing cycles

8 14



CEM I 42.5 R

CEMI 42.5 HS

Fig. 4: Frost de-icing salt resistance of series I

Frost de-icing salt resistance (CDF-test) w/c = 0.25; 5 M-% FM

(Series II)

0

20

40

60

80

100

120

0 28

Number of freezing and thawing cycles

Scaling[g/m]

4 8 14



CEM I 42.5 R

CEMI 42.5 HS

Fig. 5: Frost de-icing salt resistance of series II


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Frost de-icing salt resistance (CDF-test) w/c = 0 ,25; FM split (series III)

0

20

40

60

80

100

0 28

Number of freezing and thawing cy cles

CEM I 42.5 R, 10 M -% silica


CEM I 42.5 R

CEMI 42.5 HS

4 8 14

Fig. 6: Frost de-icing resistance of series III

3.3 Results of ultrasonic investigations

All specimens (150x150x75 mm3

) of the series I III (tab. 2) were tested byultrasonic before and after the CDF-Test. The aim of these investigations was to

get an information about possible damage inside the concretes structure or modi-

fications underneath the concrete surface.

To guarantee the required sensitivity high frequency testing heads (eigenfre-

quency in any case 250 kHz) were used. By a broad band receiver the sound

signal is detected. Afterwards, the detected vibration signal is digitalized and

evaluated by a transient recorder. The sound signal is generated by an ultrasonic

generator of the company GEOTRON-ELEKTRONIC. The specimens are cou-

pled to the sound converters in a special measuring device. Thus, an adjustment

of a reproducible coupling pressure is possible. Clay was used for coupling. The

position of the sound converter during the measuring is shown in figure 7.


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LACER No. 3, 1998

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Ultrasonic generator

Scaledsurface

Transient recorder

Specimen

Transmitter

Receiver

A A

B

B

Fig. 7: Position of the sound converter during the measuring

The sound converters are coupled to the specimens as shown in figure 7 (both

positions A A and B B). So, exact localisation of occurring damage near thesurface or inside the concrete structure is possible.

Figure 8 shows the differences between the measured sound velocities before and

after the CDF-test. At the series I (w/c = 0,4) the sound velocities are lower than

the sound velocities of specimens of the series II and III (w/c = 0,25).

The measured sound velocities after the 28 freeze de-icing-cycles are slightly

higher than the sound velocities before the CDF-test, except concretes I/II and

II/I. This fact may be caused by the crystallization of NaCl during the drying of

the specimens after the CDF-test.

The obtained differences of sound velocities of 50 m/s can be neglected due to

low water penetration.

Consistent damage of the material structure could not be observed.


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4400

4500

4600

4700

4800

4900

5000

5100

I/II I/III I/IV II/I II/II II/III II/IV III/I III/II III/III III/IV

Soundvelocity[m/s]

bevore CDF-test after CDF-test

Fig. 8: Sound velocities before and after CDF-test

4 FURTHER INVESTIGATIONS

Up to now, we have also made observations indicating different behaviour of the

investigated HSCs under extreme conditions. Especially if the specimens were

exposed to higher temperatures in the first two days during the curing higher rates

of scaling could be obtained. The further investigations shall simulate the devel-

opment of the hydration heat of 60 C and 80 C. It is also planned, to increase

the number of the freeze de-icing cycles up to 56 (tab. 2).

For the evaluation of the hydration process extensive analytical investigations by

XRD, REM/ESMA or ESEM, DTA/DTG/TG shall be carried out.

REFERENCES

[1] Guse, U.; Hilsdorf, H.: Zum Frost- und Frost- Tausalz- Widerstand

hochfester Betone. Wissenschaftliche Zeitschrift der HAB Weimar 40,

1994


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LACER No. 3, 1998

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[2] Penttala, V.: Effects of delayed dosage of superplasticizer on high per-

formance concrete. Proceedings High Strengh Concrete 1993, Sympo-

sium in Lillehammer, Norwegen, 1993[3] Larsen, E., Lauridsen, J.; Eriksen, K.; Hansen, O.; Molgaard, T.: Betons

holdbarhed. Rapport nr. 6: Rya broen, Forsog med silicabeton Egen-

skabsudvikling 1981 1993. Dnemark 1993

[4] Nasser, K. W.; Ghosh, S.: Durability Properties of High Strength Concrete

Containing Silica Fume and Lignite Fly Ash. In V. M. Malholtra: Dura-

bility of Concrete, ACI SP 145-10, S. 191-214, Nice 1994

[5] DafStb-Richtlinie fr hochfesten Beton, Ergnzung zu DIN 1045 (07.88)

fr die Festigkeitsklassen B 65 bis B 115, Deutscher Ausschu fr Stahl-beton, Berlin, August 1995

[6] De Schutter, G.; Taerwe, L.: Influence of Retards on the Early Age Ther-

mal Cracking in High Strength Concrete. Proceedings 4. Workshop on

High Performance Concrete: Material Properties and Design, Weimar

(Hrsg. F. H. Wittmann, P. Schwesinger), AEDIFICATIO Verlag Freyburg,

1995, S. 23-40

[7] Dilger, H. W.; Wang, C.: Effects of W/C, Superplasticizers and Silika

Fume on the Development of Heat of Hydratation an Strength in HPC.Proceedings 4. Workshop on High Performance Concrete: Material Prop-

erties and Design, Weimar (Hrsg. F. H. Wittmann, P. Schwesinger), AE-

DIFICATIO Verlag Freyburg, 1995, S. 3-22

[8] Setzer, M.J.; Hartmann, V.: Verbesserung der Frost-Tausalz-Widerstandsprfung. Beton und Fertigteiltechnik 1991, Heft 9, S. 73-82

[9] Bechthold, R.; Wagner, J.-P.: Verwendung von Silikatzustzen im Beton.

Beton 46, 1996, H.4

[10] Stark, J.; Chelouah, N.:Freeze-Thaw and Deicing Salt Resistance of High

Performance Concrete. Proceedings 4. Workshop on High Performance

Concrete: Material Properties and Design, Weimar (Hrsg. F. H. Wittmann,

P. Schwesinger), AEDIFICATIO Verlag Freyburg, 1995, S. 205-218

[11] Martschuk, V.; Stark, J.: Hochleistungsbeton mit hoher Dauerhaftigkeit.

Betonwerk und Fertigteiltechnik 4/1998. S. 73

high strenght concrete

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