Prof. J. Szwabowski

Dr. J. Golaszewski

Departement of Building Processes

Silesian University of Technology

ul. Akademicka 5, Gliwice, Poland

tel. +48 32 237 22 94; fax. +48 32 237 27 37

E-mail
http://www.polsl.gliwice.pl

WORKABILITY

OF FRESH HIGH STRENGTH CONCRETE

The key elements in HSC production ARE:

• maximum reduction of water content in concrete mix; Demanded workability is obtaining by ADDING high dosage of superplasticizer to the mix

• improvement of concrete structure by Condensed Silica Fume addition.

Workability and workability loss of HSC ARE result of:

• low water cement ratio,

• effectiveness of superplasticizer (depending on: chemical origin, content, and dosage procedure of superplasticizer; W/C ratio, cement type, CONCREting conditions)

CSF content and type.

Presented in report results of workability of fresh HSC are effect of several research programs performed in:

• Building Materials Laboratory of Department of Building Processes; Silesian University of Technology

• Building Materials Laboratory of Norwegian Institute of Technology; University of Trondheim.

Poor workability and workability loss of fresh HSC are often referred as main problems in proper utilisation of HSC. Despite that, only little information about workability of such concrete is available in the literature. Moreover, workability of HSC is often tested and expressed in terms of slump test, which do not properly express HSC properties. Production of HSC requires more appropriate specifications of fresh concrete workability and properties of fresh HSC have to be identified in basic rheological terms by two parameters: yield value and plastic viscosity, using special concrete rheometers. General lack of information in literature about rheological properties of fresh HSC makes shaping of HSC workability very difficult.

As commonly known the key elements in HSC production are:

• maximum reduction of water content in concrete mix; demanded workability is obtaining by adding SP,

• improvement of concrete structure by microfiller (usually CSF) addition.

The properties of fresh HSC are thus mainly results of:

• low water cement ratio,

• effectiveness of SP (depending on water cement ratio, cement type, SP chemical origin, content, and dosage procedure),

• CSF content and type.

In present paper the results of influence of these factors on workability and workability loss (which is more rapid than that of normal strength concrete) of fresh HSC are presented and a procedure of workability formation is proposed. This results were obtained in several research programs performed in:

• Building Materials Laboratory of Department of Building Processes; Silesian University of Technology,

• Building Materials Laboratory of Norwegian Institute of Technology; University of Trondheim.

Concrete mixes were designed according to typical HSC mix proportions usually used in practice. Rheometers ROD-1E (constructed in Silesian Technical University), Viskomat PC, and BML Viscometer (constructed in Building Materials Laboratory NTH, Trondheim) were used in tests. All test results are presented in conventional units: g is a measure of yield value and h is measure of plastic viscosity.

EFFECT OF SP CONTENT ON HSC WORKABILITY (1)

Effect of SP addition on fresh HSC properties – constant W/C.

Addition of SP to concrete mix with constant W/C always produce improvement of workability. For mixes with high W/C SP addition decreases yield value, while plastic viscosity remains roughly on the same level. For mixes with low W/C (as typical HSC mix) changes of rheological properties proceed in two phases: low addition of SP cause decrease of yield value, when plastic viscosity is nearly the same, further increasing SP dosages cause decrease of plastic viscosity, corresponding changes in yield value are small. In presented tests SP of SNF type was used, for other types of SP changes of rheological parameters, especially plastic viscosity, can have different range and nature.

EFFECT OF SP CONTENT ON HSC WORKABILITY (2)

Effect of SP addition of different chemical origin on fresh mortars – constant W/C=0,53.

Effect of SP type on concrete mix rheological properties were also tested using mortars of W/C =0,53. SP of AP type decreases yield value in highest level, effect of SMF SP on yield value is weakest. Generally differences in SP action on yield value disappear with increasing SP dosage. Only SP of SNF type shows no effect on mortars plastic viscosity (this effect was also observed in previous presented result for high W/C mixes). Addition of other type SP (AP and SMF) increase plastic viscosity, especially AP SP.

EFFECT OF SP CONTENT ON HSC WORKABILITY (3)

Effect of SP addition on fresh HSC properties – constant slump.

Constant slump of tested mixes was obtained by adequate to SP and CSF content changes of water content in concrete mix. Increasing SP content significantly increased plastic viscosity, while the yield value was appro-ximately the same (but generally higher for mixes contain SP then for mixes without SP addition). It very well proves inadequacy of slump test to testing of fresh HSC properties: the same slump suggest the same workability while mixes with low W/C ratio and high SP dosage have risen plastic viscosity which cause fall of HSC workability.

EFFECT OF SP DALEYED ADDITION ON WORKABILITY (4)

Effect of SP moment of adding and chemical type of SP on fresh HSC properties.

Moment of adding of SP and its chemical origin significantly affects rheological properties of fresh HSC. Highest yield value and plastic viscosity were obtained when SP was added with the mixing water. Delayed addition of SP cause decrease of yield value and plastic viscosity and thus improvement of workability - higher with increasing time of delay of SP adding. Effectiveness of SNF type SP is clearly higher then SMF type SP – more information gives investigation performed on mortar.

EFFECT OF SP DALEYED ADDITION ON WORKABILITY (5)

Effect of SP moment of adding of different chemical origin on fresh mortars – constant W/C=0,47.

Delayed addition of SP of all tested types decreases yield value – the same effect was observed for HSC concrete’s. On the other hand delayed addition cause only a little changes of plastic viscosity of mortars – tendency to increase of plastic viscosity can be observed.

EFFECT OF CSF CONTENT ON HSC WORKABILITY (1)

Effect of CSF content on rheological properties of mortars.

Addition of CSF to mortars without and with SP dramatically increases yield value and decreases plastic viscosity. Increasing yield value makes workability of such mixes poor. Different effect of addition of CSF we can observe for HSC mixes with constant slump.

EFFECT OF CSF CONTENT ON HSC WORKABILITY (2)

Effect of CSF content on rheological properties of fresh HSC - constant slump.

Addition of CSF only a little influencing yield value of concrete mixes with constant slump. On the other hand increasing CSF addition cause strong decrease of plastic viscosity, especially for mixes with low W/C and high SP dosage. This indicates that the CSF addition at presence of SP produces the higher workable mix. Type of CSF (dry or slurry) do not influence workability of HSC.

WORKABILITY LOSS OF HSC (1)

Workability loss – effect of SP content and W/C ratio on rheological properties of fresh HSC– constant initial slump.

Workability loss of HSC is primarily effect of increase in time of yield value, but also a increase of plastic viscosity can play a meaningful role. In fresh HSC increase both yield value and plastic viscosity, and this increase take place in two phases. In phase I, directly after end of mixing, both yield value and plastic viscosity increase. In phase II only yield value increases. Duration of phases and range of rheological parameters changes depends first of all on SP content and W/C ratio. Figures shows that strong increase of plastic viscosity apply especially to mixes with low W/C and high SP content. Increase of W/C ratio and decrease of SP dosage produce mixes with still high yield value increase, but low changes of plastic viscosity in time. For concrete with high W/C ratio and low dosage of SP increase of plastic viscosity in time was not observed and workability loss proceed due yield value increase.

WORKABILITY LOSS OF HSC (2)

Workability loss – effect of SP content and W/C ratio on rheological properties of mortars. – constant W/C.

Tests performed for mortars with high W/C and relatively low SP dosage are supplement to previous presented results. Workability loss of these mortars is only effect of yield value increase, while plastic viscosity slightly decreasing in time. It is worth to notice that for high W/C ratio workability loss (yield value increase) is higher for control then SP mortars.

WORKABILITY LOSS OF HSC (3)

Workability loss – effect of moment of SP dosage on rheological properties of fresh HSC– constant initial slump.

Delayed addition of SP cause slower increase yield value in time, which produce lower workability loss. On the other side delayed SP addition for mixes with low W/C ratio and high SP dosage can produce substantial increase of plastic viscosity in time.

WORKABILITY LOSS OF HSC (4)

Workability loss – effect of moment of SP dosage and SP type on rheological properties of mortars– constant W/C.

Effect of moment of adding of different type SP was investigated using mortars of W/C = 0,47. Generally delayed addition of SP produce mix with lower yield value increase in time (as fresh HSC presented before) and with higher plastic viscosity (especially for SMF type). Especially sensitive for moment of adding is SP SMF type. SP of SNF type is less sensitive, AP SP action is practically independent on moment of adding.

WORKABILITY LOSS OF HSC (5)

Workability loss – effect of CSF content and type on rheological properties of fresh HSC – constant initial slump.

Increasing CSF amount cause substantial increase of yield value in but in the same time reduce increase of plastic viscosity. CSF type has no effect on workability and workability loss of HSC.

PROCEDURE OF WORKABILITY SHAPING.

Procedure of workability shaping.

Presented procedure relate only to workability shaping ,basing on assumption that basic mix proportion of HSC is defined – it means that cement type and content, W/C ratio, CSF dosage, aggregate type and content are determined. Procedure of workability shaping depend on successive experimental optimisation of type of SP and its procedure of addition and SP dosage for point of view of required workability - that is rheological parameters g & h. First steps of SP optimisation tests – selection of SP type and procedure of addition - should be performed on mortars using rheometrical workability test. SP dosage by reason of real W/C and cement content in concrete must be optimised using concrete specimens also using rheometrical workability test. If measurement of rheological parameters of such mix is in accordance with required rheological parameters process of workability shaping can be terminate. If not, correction of workability is necessary, depending on obtained rheological properties of fresh HSC. Procedure and methods of correction of rheological parameters is presented in diagram.

CONCLUSIONS

• Workability of fresh HSC depends on type, content and moment of adding of SP and content of CSF. Increasing content and delaying moment of incorporation of SP and addition of CSF (but only at presence of SP) improving workability of HSC.

• Workability of HSC depressed with time (it is so called workability loss). Rheological parameters g & h increasing with time, speed and range of changes depends on w/c ratio, content, moment of introducing and chemical origin of SP and content of CSF.

• Rheological test can be successfully used to design, forming and quality control of workability of fresh HSC.

Papers in English to deal with workability of fresh concrete problems:

• Szwabowski J.: Influence of Three-phase Stucture on the Yield Stress of Fresh Concrete. "Rheology of Fresh Cement and Concrete". Proc. Intern. Conf. of the British Society of Rheology, E&FN Spon, London, 1991

• Punkki J., Golaszewski j., Gjorv O.E.: Workability loss of High-Strength Concrete. ACI Materials Journal, V.93, No. 5, September-October 1996

• Szwabowski J., Golaszewski J.: The effect of superplasticizer and silica fume on workability of high strength concrete. Conference Concrete and Concrete Structures, Technical University of Kosice, Slovac Republic, 1998

• Szwabowski J., Golaszewski J., SuchoD S.: About testing of plasticizer and superplasticizer performance. Medzinarodna Vedecka Konferencia. 60. vyrocie Stavebnej fakulty STU v Bratyslave, Bratislava, Slovac Republic, 1998

• Szwabowski J., Golaszewski J.: Effect of ground brick particle size on mortars and concrete workability. Medzinarodna vedecka konferencia. 60. vyrocie Stavebnej fakulty STU v Bratyslave, Bratislava, Slovac Republic, 1998

• Golaszewski J., O’Farrell M., Sabir B.B., SuchoD S., Szwabowski J., Wild S.: The influence of ground brick on the physical properties of mortar and concrete. International congress “Creating with Concrete” Dundee, Scotland, 1999

• Szwabowski J., Golaszewski J.: Workability of light weight aggregate high strength concrete. XI Mezinarodni Vedecka Konference Brno, Czech Republic 1999

• Szwabowski J., Golaszewski J.: The effect of superplasticizers on rheological properties of mortars. Building materials and testing - Brno, Czech Republic, 1999

• Faroug F, Szwabowski J., Wild S.: Influence of Superplasticizers on Workability of Concrete. Journal of Materials in Civil Engineering, 1999

© Markus Greim, Schleibinger Geräte GmbH, greim@schleibinger.com, 31.03.2000, 12:16,

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