Study in Romania

8/13/2019 Study in Romania

1/4

fibSymposium PRAGUE 2011

Session 2B-5: Construction Technology

495

ULTRA-HIGH STRENGTH PERFORMANCE CONCRETE

PROPERTIES

Ofelia

Corbu

Cornelia

Mgureanu

Dumitru

Moldovan

Henriette

Szilgyi

Abstract

The present study relates to an ultra-high strength concrete composition with and without steelfibres, having a 150 MPa and respectively, over 170 MPa strength. The ultra-high strength concretemix containing cement, fine sand powder, silica fume and water at a mixing ratio from 15% to 18%is based on local commercially available materials. The Water/Binder ratio has been chosen toensure workability and high strength.

The object of the present study is twofold: to provide an ultra-high strength mix based oncement, fine quartz sand powder and silica fume mixed at prescribed intervals, for a given

workability and to observe the influence of curing conditions on the compressive strength ofconcrete, respectively

Keywords: Ultra High Strength Concrete, Hybrid Concrete, disperse reinforcement, reactivepowder, slump

1 IntroductionIt is common knowledge that Reinforced Concrete is the main construction material worldwide.The leading position is achieved by continuous research which in turn generates improved superiorperformance concrete, prior to and after setting. Furthermore, structural applications with breath

taking aesthetics are becoming more common.From the early reportby Olariu, Ioani and Poienar (1988) to the more recent researchby

Magureanu (2007), the object of interest shifted from beam-column joints for seismic areas toachieving green concrete with a low carbon print and improved compressive strength and durability,as reported also by Corbu et al. (2010). Last but not least, for the same bearing capacity in flexure andshear, concrete members similar to steel members in terms of mass and dimensions weredeveloped.

2 Addressed requirementsFor this study the Portland cementwas supplied by Lafarge Romania, from the mill in Hoghiz.

A policarboxilate based chemical admixturecommercially available as ACE 440 BASF wassupplied by BASF Romania alongside the fine-grained silica fume commercially available asElkem Microsilica Grade 940 U Undensified. Two types of steel fiberswere used, furnished by


2/4



496

BAUM CAS FIBRES Romania: long with tapered ends defined by a ratio d/l of 0.4/25(diameter/length in mm) and straight defined by a ratio d/l of 0,175/6 (diameter/length in mm),respectively. The quartz sand powder was provided by Bega Minerale Industriale from the quarry

in Aghire.Following the aim of maximum compactity given by the Apollonian distribution(each sizegrading particles filling the holes in-between next size grading particles) and the basic principlesfor Ultra High Strength Concrete as stated by Lee and Chisholm (2005), the preliminary mixesdesigned in a 1st stage (no disperse reinforcement but various ratios of cement and chemicaladmixtures) underwent significant improvement in a 2

nd stage (by increasing the chemical

admixture ratio as well as by the addition of disperse reinforcement). As a result of the 2 ndstage,two mixes were considered suited: ME 22(no fibers) chosen to be the reference mix, and OE 23(long tapered end steel fibers) chosen for further study in the 3

rd stage by prolonging the

prescribed mixing time of constituents as well as by creating various types of dispersereinforcement: long tapered end steel fibers (the same as for the 2

nd stage), short straight steel

fibers and hydrid (equal mass mix of long and short steel fibers), respectively. For all stages

a forced mixing concrete mixer was used.

3 Comparison no fiber vs. long steel fiber vs. hybrid steel fiber reinforcedconcrete

Tab. 1lists the slump and flow properties for the above concrete mixes. It may be observed thatsimilar values occur for no fiber and hybrid mixes. For long fiber mixes, although smallervalues occur, the results are consistent with superior workability, as presented in Fig. 1. Also, theaddition of long steel fibers does improve workability even for small quantities as reported byMarkovic (2006).

Tab. 1

Fresh and hardened concrete propertie (71 mm cubes)

Properties

90C and RH 8090%

2nd

stage3

dstage

prolonged mixing period

ME 22, no

fibers

OE 23,

long steel

fibers

OE 23,

long steel

fibers

OE 23

short steel

fibers

OE 23

hybrid

steel fibers

W/C 0,22 0,23 0,23 0,23 0,23

W/B 0,18 0,18 0,18 0,18 0,18

Slump [mm] 265 247 270 270 260

Flow [mm] 10 min. 560 428 560 565 545

Flow [mm] 60 min. 508 515 500

Temperature [C] 26.7 29.5 32,5 32,28 35,3

Contained air [%] 4.60 4.56 4.50 4,35 4.40

Experimental density [kg/m3] 2260 2400 2395 2437 2413

Calculate density [kg/m3] 2289 2410 2410 2410 2410

fcm, 1 day [MPa] 65 78 87,8 80,2 84,2

fcm, 6 days [MPa] 131 105 171,3 169,3 172.8

fcm, 28 days [MPa] 124,5 129 157 172

Concrete temperature is under the influence of both the cement and admixture ratios and in somecases even under the influence of the mixing period.

Contained air values are consistent with a very high density concrete as air can be onlyeliminated by vibration. Smaller values for long steel fibers as compared with the reference mix


3/4



497

may be the result of further hydration reactions in the presence of increased temperature andhumidity caused by the fine-grained silica fume thus reducing porosity (Cheyrezy et al., 1995).

It may be observed that for fresh concrete properties hybrid (OE23) and no fiber mixes (ME

22) are very similar, yet superior to the long fiber mix (OE 23). Similar compressive strengthvalues are obtained only for fiber reinforced concrete. Also, there is a visible increase in theworkability of the mixes as blending time increases due to properties of the chemical admixture.Similarly, there is an increase in concrete temperatures, maybe due the increase in cement dosage.

Fig. 1 Plastic moulds texture transfer

4 ConclusionsUsing local available materials to create Ultra High Performance Concrete is possible as well asremarkable for compressive strength over 170 MPa may be achieved with addition of steel fibers.

Moreover, since the quartz fine powder was considered by the supplier a by-product with noeconomic value, its use as a prime constituent for Ultra High Performance Concrete makes thismaterial a valuable asset.

Prolonging the mixing periodhas proven to improve workability. This may be attributedto the properties of the superplaticizer which had the necessary time to fully develop and properlyinteract within the mix. Temperature of the mix reached a maximum of about 35

oC with no

negative impact on workability.The compressive strength is also dependent on the curing method. Standard conditions

(submerged in water at constant temperature of 20 2oC) lead to a maximum compressive strength

gain at about 28 days, just as for normal strength concrete. Specified conditions ( 90C and RH8090%) lead to a maximum compressive strength gain at the end of the curing cycle. Therefore,adopting an adequate curing method is mandatory to obtain design compressive strength.

Several ratios may be of interest (i.e. the increase of strength over time from one mix toanother). Tab. 4 and Tab. 5 lists an increase of about 1,542,74 at the age of 6 days for plainconcrete mixes and of about 1,401,73 for fiber reinforced mixes. Also, OE23L (long fiber mix) ascompared to ME 22 (reference mix) shows an increase of 1,31.

Choosing constituents may be sometimes a not so straight answer but prior laboratory trialsmay ease this procedure.

This outcome has been achieved with the financial support of research project granted by NationalGrant Agency CNCSIS PN II IDEI ID 1053/2007. Extensive material support was provided byLafarge Romania. Another party contributing to this study is BASF Romania. All support isgratefully acknowledged.


4/4

Study in Romania

Documents

Transcript of Study in Romania