An Introduction to a Practical UHPC Manufacturing in South ... · South Africa, a Fellow of both the South African Academy of Engineering (SAAE) and the South African institution

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In the Department of Civil Engineering at the University of Pretoria UHPC is manufactured with a compressivestrength of 180 MPa retaining the superior properties ofUHPC while reducing the costs. In order to achieve this,several modifications to the typical mix design for UHPCwere taken into consideration. The high compressivestrength can be attributed to both optimum packing den-sity as well as the low water to cement ratio (w/c).

Nowadays, interest is drawn toward manufacturing very high-strength concrete and using pre-cast elements in large-scaleprojects. Ultra-high performance concrete (UHPC) is a type ofconcrete which possess very high compressive strength(more than 150 MPa) and high ductility which makes it a suit-able material for manufacturing pre-cast structural members.Manufacturing this type of concrete in large-scale is howevernot appealing for industry and the main reason can be attrib-uted to the manufacturing costs of UHPC. The costs includethe material cost, and preparation cost as well as the cost ofequipment required.

Impact of Material on Costs

The basic principles of ultra-high strength concrete with duc-tile behaviour, known as Powder Reactive Concrete (PRC),were suggested by Richard and Cheyrezy [1]. Eliminating thecoarse aggregate to increase the homogeneity of mix wasone of their suggestions. In their proposed mix design, theratio of sand to cement (by weight) was 1.1 and 600 μm wasconsidered as the maximum size of aggregate. There are nu-merous studies [2-7] carried out to investigate the behaviourof UHPC following Richard and Cheyrezy’s proposed princi-ples. Some studies can be found which considered coarseraggregate with maximum particle sizes to 0.8 mm [8-9] and 1mm [10], but, this still does not make this concrete practical.Typical materials and a mix composition for UHPC is shown inTab. 1. UHPC contains a high amount of cement (800-1,000kg/m3) resulting in the higher cost of concrete. The high ce-ment content also causes higher shrinkage as well as highemission of CO2. Silica fume is used to not only increase the

packing density of concrete, but it also works as a cementi-tious material. This material is one of the most expensive ma-terials in the mix design and it is not widely available. The ratioof aggregate to cement (Agg/c) is mostly kept at 1.1. Usingvery fine aggregate (less than 600 μm) is another characteris-tics of UHPC. The relatively low aggregate content leads tohigher costs as aggregate is a cheaper material in comparisonto the other materials used in manufacturing concrete. More-over, providing aggregate in different gradings require siev-ing, which makes preparation expensive. All these material re-quirement make UHPC unpractical and too expensive for usein large-scale projects. Further costs are incurred when it isnot possible to obtain the required materials locally and ma-terials have to be imported.

Curing

UHPC often requires highly specific types of curing making itunpractical and expensive, as well. Typically, the specimen aresubmerged in 90°C water for several days. In order to achieveUHPC with ultra-high strength (more than 200 MPa) an auto-clave is used in the manufacturing process [12].

Manufacturing Economical and Practical UHPC

The main focus of the Department of Civil Engineering at theUniversity of Pretoria was to manufacture UHPC with a com-

An Introduction to a Practical UHPC Manufacturing in South Africa

Increasing the aggregate size and content while using locally available materials

� S. Vatannia, E. Kearsley and D. Mostert, University of Pretoria, South Africa

Tab. 1: Materials and mix design in typical UHPFRC [11]

Material Amount (kg/m3)Portland cement 712Fine sand 1,020Silica fume 231Ground quartz 211Superplasticizer 30.7Accelerator 30.0Steel fiber 156Water 109

03a-technology_040-061_us_Layout 1 09.11.17 15:43 Seite 56

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pressive strength of 180 MPa retaining the superior propertiesof UHPC while reducing the costs. In order to achieve this,several modifications were made to typical UHPC mix designsincluding:• The aggregate content has been increased to reduce

the consumption of cementitious materials. An aggre-gate to cement ratio (by mass) of 2.5 was used, thus reducing the cement content to 593 kg/m3.

• The aggregate size was increased. Andesite with a maxi-mum particle size of 4.75 mm and 6.7 mm was chosenas a fine aggregate and coarse aggregate respectively.No sieving was done for aggregates and they were usedin the mix as they were supplied from the quarry.

• Local straight steel fiber with 13 mm length and 0.26 diameter was utilized to reinforce UHPC.

• All the materials used to manufacture UHPC at Universityof Pretoria were locally available. In the process of manu-facturing, no milling or special mixer was used. Onlyequipment which is available in a standard concrete laboratory was used.

Materials

Cement, Undensified Silica Fume (USF) and Ground Granu-lated Blast Furnace Slag (GGBS) were used as the cementi-tious materials. A CEM I 52.5N with a relative density of 3.14

� Sharifeh Vatannia is a PhD student in Civil Engineering De-partment at the University of Pretoria under the supervision ofProfessor Elsabe Kearsley. She holds BSc (Eng) & MSc (Eng) de-grees in civil engineering at University of Zanjan (Iran). Her inter-ests are focused on structural engineering and ultra-high per-formance concrete. [email protected]

� Professor Elsabe Kearsley graduated with a degree in Civil En-gineering from the University of Pretoria in 1984 where after sheworked as structural engineer in in both the UK and South Africabefore becoming a staff member at the University of Pretoria in1990. She holds a PhD from the University of Leeds, UK. She is aregistered professional engineer with the Engineering Council of

South Africa, a Fellow of both the South African Academy of Engineering (SAAE)and the South African institution of Civil Engineering (SAICE) as well as a mem-ber of RILEM and the Concrete Society of Southern Africa. Elsabe was the 2009President of SAICE and the Head of Department of Civil Engineering at the Uni-versity of Pretoria from 2007 to 2015. She is still conducting research at the Uni-versity of Pretoria mainly aimed at reducing the carbon footprint of the cementand concrete industry. [email protected]

� Derek Mostert received his National Higher Diploma in CivilEngineering from the Pretoria Technicon in 1980. He studied afurther 4 years in concrete technology and received his Advancedconcrete Technology Diploma from City in Guilds London in1992. He holds Master of Science degree in Advanced ConcreteTechnology from the University of Queens, Belfast Ireland in

2015. Since 1995 he is employed at the University of Pretoria at the departmentof Civil Engineering as a Concrete Technologist and his research is focused onthe properties of Lightweight concrete and high-strength concrete.

[email protected]


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was chosen as cement. USF and GGBS had relative densitiesof 2.2 and 2.93, respectively. The chemical composition of ce-ment, USF and GGBS is provided in Tab. 2. The superplasti-cizer (SP) used is polycarboxylate-based with a relative densityof 1.06. A retarder was used to keep the mix workable for alonger period of time. Andesite with maximum particle sizeof 4.75 mm and 6.7 mm was chosen as a fine aggregate andcoarse aggregate respectively. The particle size distributionof the materials used in the study is shown in Fig. 1.

Mix composition

The main idea of the mix composition was to reduce the costswhile making it practical and providing optimal properties atthe same time. The water to cement ratio was kept constantat 0.25. The content of SP and retarder was 2.5 % of the ce-mentitious materials (by weight). The ratio of total aggregateto cement (by weight) was taken as 2.5. The mix design is pre-sented in Tab. 3. Procedures were carried out to maximize thepacking density of the aggregates (sand and stone). Enhanc-ing the packing density of the aggregate helps to reduce thevoids and gaps between the aggregate that has to be filledby the paste. In order to enhance the packing density of ag-gregate, loose bulk densities were obtained on blends con-taining different ratios of sand and stone. The optimum ratioof stone to sand was found at the ratio resulting in the highestloose bulk density.

Mix preparation

All the dry materials except USF were mixed together for 1minute. Water and admixtures were added to the mix beforeUSF was added. The reason for adding USF later was that it isvery light and thus disappears as dust during dry mixing.When the concrete was completely mixed and uniform in tex-ture (after 5 minutes), the fibers were evenly dispersed byhand and mixed in for 4 minutes. The casting was done on avibrating table and the moulds were vibrated for 1 minute.The specimens were covered with plastic sheets while theywere kept in the laboratory at 24°C and 98% relative humidityfor 24 hours.

Curing

The effect of different curing regimes and procedures on thecompressive strength of UHPC were studied by the authors[13]. An effective and practical curing regime was chosen inorder to achieve desirable compressive strength while consid-ering the cost. After demoulding, the specimens were sub-merged in water with a temperature of 80-85°C. In the curingprocess changes in the water temperature should be con-trolled to avoid thermal shocking. After placing the samplesin the water, the temperature of the water bath should begradually increased from room temperature to 80-85°C. At theend of heat treatment, the temperature of the water is allowed

Tab. 2: Chemical compositions of cement, USF and GGBS

SiO2 MgO Al2O3 SO3 K2O CaO Fe2O3 TiO2 Na2O LOICement (%) 31.6 1.39 3.75 3.4 0.18 54.75 4.09 0.27 <0.01 1.77USF (%) 84 1.08 0.75 0.09 3.34 2.22 1.96 0.03 0.17 5.56GGBS (%) 34.87 8.03 14.38 1.96 0.72 37.05 0.89 0.72 <0.01 0.16

0

10

20

30

40

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60

70

80

90

100

0,1 1 10 100 1000 10000

Cu

mu

lative

vo

lum

e,

%

Size, μm

Cement

Undensified Silica Fume

GGBS

4.75mm sand

6.7mm stone

Fig. 1: Particle size distribution of the materials

Tab. 3: Mix design of UHPFRC at the University of Pretoria

Material Mix design (kg/m3)Cement 593USF 178GGBS 119Andesite sand (Max: 4.75 mm) 446Andesite stone (Max: 6.7 mm) 1,036Water 148Hook-ended fiber 152Admixture 22.2


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to fall gradually to room temperature before samples are re-moved. The cooled specimen can then be kept in 24°C wateror wrapped in plastic sheets and kept in ambient conditions.

Results

Workability, compressive strength and splitting tensilestrength results of UHPC reinforced with steel fibers are pre-

sented here. The compressive strength was determined using100 mm cubes. To measure the splitting tensile strength, 200mm high cylinderical specimen with a diameter of 100 mmwas used.

• The slump ranged between 100 mm and 120 mm depending on the types of fiber. The flow table test diameter for the mix containing no fiber was 435 mm.

0

2

4

6

8

10

12

14

16

0 0,1 0,2 0,3 0,4 0,5 0,6

Sp

littin

g t

en

sile

str

en

gth

, M

Pa

Transversal deformation, mm

Fig. 2: Splitting tensile strength of UHPFRC


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175,1

186,3

175,8

185,3

171,8 173,1

0

20

40

60

80

100

120

140

160

180

200

220

Water cured at 24°C Heat-treated at 85°C

Co

mp

ressiv

e s

tre

ng

h,

MP

a

High strength hooked-ended fiber Micro fibers No fiber

14,7 15,3

19,1

15,5

8,7 8,3

0

5

10

15

20

25


Sp

littin

g t

en

sile

str

en

gth

, M

Pa


21,0

28,6

26,3 26,4

18,6 18,1

0

5

10

15

20

25

30

35


Fle

xu

ral str

en

gth

, M

Pa


55,8

51,9 51,9 51,4

62,9

54,8

0

10

20

30

40

50

60

70


Mo

du

lus o

f e

lasticity,

Gp

a


• An average compressive strength of 185 MPa isachieved for UHPC containing 2 % by volume fraction ofsteel fiber.

• A splitting tensile strength of 6 MPa was measured forthe UHPC. In the presence of 2 % steel fibers (by vol-ume) the splitting tensile strength improved to 14.6 MPa.As presented in fig. 2, ductile behaviour is observed.

The ductility of the splitting tensile behaviour is depend-ent on the type of fibers used to reinforce the UHPC.

The effect of different types of fibers on the mechanical prop-erties of UHPFRC was investigated by the authors [14] and thetrend observed can be seen in fig. 3. The specimens werecured either at 85°C water for 3 days or at 24°C water up tothe day of testing. The results showed that fibers have a sig-nificant effect on the post-cracking behaviour and the ultimatesplitting tensile strength of UHPFRC. For instance, the in-crease in the strength of UHPFRC after the first cracking canbe noted as 42 % and 38 % for high strength hooked-endedfibers (3,000 MPa tensile strength) and micro fibers (2,500MPa tensile strength), respectively. In order to measure theflexural strength, four-point bending tests were carried out on50x50x300 mm beams. The beams containing high strengthhooked-ended fibers and micro fibers gave a ductile respondunder flexural loads and 26 MPa and 28 MPa were achievedas the flexural strength, respectively. Beams had about 35 %and 50 % strength increase after the first cracking, respec-tively. Fig. 4 displays an UHPFRC prestressed beam testingunder flexural load to measure the flexural capacity and theeffect of fiber on the behaviour of the beam.

Fig. 3: Mechanical properties of UHPC reinforced by 2 % volume of different types of fibers [13]

Fig. 4: Testing UHPFRC prestressed beam under flexural load


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Conclusion

Economical and practical UHPC can be manufactured to besuitable in large-scale applications. This accomplishment ispossible as a results of several modifications to the typicalUHPC mix design. These modifications include: increasing theaggregate size and content while using locally available ma-terials. No milling or special mixers are required and onlystandard concrete laboratory equipment was employed in themanufacturing process. �

References

[1] Richard, P. and Cheyrezy, M., 1995. Composition of reactive powderconcretes. Cement and concrete research, 25(7), pp.1501-1511.

[2] Habel, K. and Gauvreau, P., 2008. Response of ultra-high perform-ance fiber reinforced concrete (UHPFRC) to impact and static load-ing. Cement and Concrete Composites, 30(10), pp.938-946.

[3] Yoo, D.Y., Lee, J.H. and Yoon, Y.S., 2013. Effect of fiber content onmechanical and fracture properties of ultra high performance fiberreinforced cementitious composites. Composite Structures, 106,pp.742-753.

[4] Kang, S.T. and Kim, J.K., 2011. The relation between fiber orientationand tensile behavior in an Ultra High Performance Fiber ReinforcedCementitious Composites (UHPFRCC). Cement and Concrete Re-search, 41(10), pp.1001-1014.

[5] Yang, I.H., Joh, C. and Kim, B.S., 2012. Flexural response predictionsfor ultra-high-performance fibre-reinforced concrete beams. Maga-zine of Concrete Research, 64(2), pp.113-127.

[6] Voo, Y.L., Foster, S.J. and Gilbert, R.I., 2006. Shear strength of fiberreinforced reactive powder concrete prestressed girders withoutstirrups. Journal of Advanced Concrete Technology, 4(1), pp.123-132.

[7] Park, J.J., Kang, S.T., Koh, K.T. and Kim, S.W., 2008. Influence of theingredients on the compressive strength of UHPC as a fundamentalstudy to optimize the mixing proportion. In Proceedings of the inter-national symposium on ultra-high performance concrete, structuralmaterials and engineering series (No. 10, pp. 105-12).

[8] Wille, K., El-Tawil, S. and Naaman, A.E., 2014. Properties of strainhardening ultra high performance fiber reinforced concrete (UHP-FRC) under direct tensile loading. Cement and Concrete Compos-ites, 48, pp.53-66.

[9] Wille, K., Kim, D.J. and Naaman, A.E., 2011. Strain-hardening UHP-FRC with low fiber contents. Materials and Structures, 44(3), pp.583-598.

[10] Voo, Y.L., Poon, W.K. and Foster, S.J., 2010. Shear strength of steelfiber-reinforced ultrahigh-performance concrete beams without stir-rups. Journal of structural engineering, 136(11), pp.1393-1400.

[11] Graybeal, B. A. (2006). Material property characterization of ultra-high performance concrete (No. FHWA-HRT-06-103).

[12] Yang, S.L., Millard, S.G., Soutsos, M.N., Barnett, S.J. and Le, T.T.,2009. Influence of aggregate and curing regime on the mechanicalproperties of ultra-high performance fibre reinforced concrete (UH-PFRC). Construction and Building Materials, 23(6), pp.2291-2298.

[13] Vatannia, S., Kearsley, E., Mostert, D., 2016. The effect of curingregime on the compressive strength of ultra high performance con-crete. In Proceedings of the 9th international concrete conferenceon environment, efficiency and economic challenges for concrete,Dundee, Scotland.

[14] Vatannia, S., Kearsley, E., Mostert, D., 2016. The mechanical proper-ties of UHPC reinforced by different types of fiber. fib Symposium:Performance-based Approaches for Concrete Structures, CapeTown, South Africa.


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An Introduction to a Practical UHPC Manufacturing in South ... · South Africa, a Fellow of both the South African Academy of Engineering (SAAE) and the South African institution