Bulletin of the Transilvania University of BraşovCIBv 2015 • Vol. 8 (57) Special Issue No. 1 - 2015

TECHNICAL-ECONOMIC INDICATORS OFSELF-COMPACTING CONCRETE

STRUCTURES

N. CAZACU1 A. BRADU1 N. FLOREA2

Abstract: Self compacting concrete (SCC) was presented by Japaneseprof. Okamura in 1986. It was developed to increase the quality anddurability of concrete structures. This new mixture achieves an adequatecompaction under its own weight due to its high fluidity that providesopportunities to create complicate structural and architectural shapes. Thehigher powder content is often supplemented by waste materials andindustrial by-products that make the concrete friendly with the environment.The material costs of SCC are higher than costs of vibrated concrete, butthanks to its workability, it can be placed at a faster rate with no mechanicalvibration in a shorter construction period, which leading to cost savings inplacement.

Key words: self compacting concrete, labor, construction time, cost.

1 Phd Technical University of Iasi, Faculty of Civil Engineering and Building Services.2 Prof. dr. Technical University of Iasi, Faculty of Civil Engineering and Building Services.

1. Introduction

Concrete has become one of the mostpopular construction materials inworldwide due to the possibility of use thelocals raw materials. This fact led to manyinventions in the field of concrete anddetermined many researches in order toenhance its quality, reducing the cost ofimplementation. Another test facing theengineering community is to performprojects using the concept of sustainabledevelopment. [2]

SCC is one of the most significantdiscoveries of contemporary concrete. Itwas developed to increase the quality anddurability of concrete’s structures with lesslabor that reduce the amount of skilledworkers. This innovative mixture is

characterized by:- flowability - fills thoroughly the

formwork, - passing ability -flows thoughtrebar, around obstructions

- stability - without segregation.SCC achieves an adequate compaction

under its own weight, without anyvibration. The higher powder content ofSCC is often completed by supplementarycementitious materials, most commonlyused are waste materials and industrial by-products that makes the concrete friendlywith the environment specifically when itis used as a exposed construction material.

The advantages of SCC in fresh andhardened states lead to an economicalefficiency: reduce of labor, increase ofsafety, improving product finish, decreaseform maintenance, the equipments costs,

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the electrical consumptions andconstruction time [3].

2. Composition of SCC

The constituent materials used for theproduction of SCC are the same as thosefor conventionally vibrated normalconcrete, except that SCC involves a lowwater/powder ratio, a limited quantity ofcoarse aggregate and an increased volumeof paste. The higher powder content isoften supplemented by mineral admixtures– waste materials and industrial by-products

To achieve an SCC of high fluidity andavoid the segregation it is normal to use ahigh dose of the polycarboxylateadmixture of new generation or viscosity-modifying agent (VMA) into the concretemix. The use of these admixtures increasessignificantly its cost compared to the costof conventional high quality concrete.

The constituent materials of SCC are:cement, mineral additions, aggregate,admixture and water.

The mineral additions improve andmaintain the cohesion and segregationresistance, reduce the heat of hydration andthermal shrinkage, according to theirreactive capacity with water they areclassified: inert, pozzolanic and hydraulic.

The aggregate properties should berigorously evaluated. The moisturecontent, water absorption, grading andvariations in fines content of all aggregatesshould be closely supervised in order toproduce SCC of constant quality. [4]

The shape and particle size distributionof the aggregate is very important andaffects the packing and voids content.

The maximum aggregate size shouldgenerally be limited to 12 – 20 mm toavoid the aggregate blocking through thereinforcement.

Superplasticizers of high range waterreducing admixtures are an essentialcomponent of SCC.

Choice of admixture for optimumperformance may be influenced by thephysical and chemical properties of thebinder/addition.

Japanese professor H. Okamura andOttawa presented the recommendations forthe mix proportions of SCC, Fig. 1:

-content of coarse aggregate is fixedat 50% of the total solid volume.

-volume of fine aggregate should be40% of the mortar volume.

-water / powder ratio can bebetween 0.9... 1, depending on thecharacteristics of the powder and thesuperplasticizer dosage.

The required water /powder ratio shouldensure the selfcompactibility.

Fig. 1 Composition of SCC versus vibrated concrete [1]

N. CAZACU et al.: Technical-economic indicators of self-compacting concrete … 29

3. Construction Labor

Plant’s industrialization is essential togain the benefit of modern technology andaugment the level of economic prosperity.Construction industry still remains as oneof the few homemade products puttogether piece by piece by workers, that’swhy it is one of the most labor-intensiveindustries in the world. The constructionlabor is a vital component of a buildingproject that justifies its high costs, Fig. 2.

Fig. 2. Necessity of SCC [7]

Labor production rates are defined as anumber of units of work produced in aspecified time

Pouring concrete is a very complexprocess with high consumption ofworkmanship and can be divided intomany more subprocess:

- cleaning the formwork;- handling, horizontal and vertical

transport of concrete to the desiredlocation;

- casting of concrete;- compacting and leveling concrete;- providing work platforms for

concrete’s transport at casting places.- supervision and required interventions

to the formwork during the pour ofconcrete;

- control the quality and patching asneeded.

The placing of workmanship is a majorcontributor to the quality of structures

made with conventional concrete.SCC was developed to reduce

significantly the need of skilled labor dueto elimination of mechanical compaction.It flows under its own weight and fills evenintricate forming with dense rebar, Fig. 3.

Fig. 3. Workers on pouring concrete

The quality of concrete structures madeof SCC nearly entirely depends on thequality of the concrete. Concretemanufacturers are not directly benefitingfrom the SCC technology, but theincreased of responsibility for the qualityof SCC demands tighter quality controlmeasures and assurance of the quality.[6]

4. Equipment and Placement ofConcrete

Modern construction is defined by theincreasing utilization of equipment toaccomplish numerous constructionactivities. Equipment refers to all theequipments, tools, and apparatus necessaryfor the proper construction and acceptablecompletion of a project. In a constructionproject, equipment costs are typicallydivided into portions. The first and biggerportion covers the cost of equipment and isoften referred to as equipment cost. Thesecond and smaller portion covers the costof hand tools.

Placing concrete consumes a lot of

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Fig.4. Energy demand for producing and placing concrete [9]

energy, includes increased utility rates,expensive and scarce skilled labor, risingmaterial and equipment costs. SCC is aperforming material of high fluidity thatdoesn’t need any mechanical compactionto be placed.

SCC pumps easily with reducedpumping pressures. Air content tends toremain more stable in an SCC mix duringpumping than in conventional concrete

The hydrostatic pressure of SCC ishigher than that of conventional concreteand the formwork could need alteration toavoid the leak paste. The casting speedshould be properly adjusted: pouring toorapidly may cause entrapped air and bugholes; pouring too slowly may lead to aninsufficient head pressure, decreasingSCC’s ability to fully compact in the form.To protect the surface from drying out isrecommended to use an evaporationretardant where needed similar toconventional concrete mix.

5. Construction Time

Construction time represents the lengthof the period between the beginning of

construction till its end and defines all theconstruction activities.

Casting concrete represents a complexprocess that requires skilled labor andaffects the construction time. Its mainactivities are: placing, compaction, curingand demolding.

The vibration is the most commonmethod of compacting of the concrete andrepresents a process of expelling theentrapped air. It has been found from theexperimental studies that 1% of air in theconcrete approximately reduces thestrength by 3- 5%. The air voids increaseconcrete's permeability, reduce itsdurability and decrease the ability towithstand aggressive environment.

Curing is the protection of freshconcrete from evaporation and temperatureextremes which might adversely affectcement hydration. The concrete must beprotected from the harmful influences ofwind, sun and variable weather to gaindesigned strength and durability.

Concrete elements can be demoulded asthey attempt sufficient strength to takeover all or part of load for which they weredesigned. The recommended values of

N. CAZACU et al.: Technical-economic indicators of self-compacting concrete … 31

Fig. 5 Compressive strength of SCC at early ages [8]

minimum compressive strength which theelement can be demoulded [4]:

- 2.5 N/mm2 to remove the sides offormwork

-70-85% of designed strength to removeshuttering plates with maintaining safetyprops

SCC gave high values as for earlystrength (before 28 days) as for the entirecuring duration compared to the normalconcrete as shown in Fig. 3. The strengthgained at 7 days was about of 90%percentage at 28-days strength for SCCand of 80% for normal concrete, while itwas 96% for SCC at the age of 14 days.The duration of an activity may beestimated:

Work duration=quantity of work/numberof crews x production rate

6. Cost of Construction

The total cost of a concrete structuredepends of the expenses on labor,materials and equipment.

Labor costs are calculated by twofactors: monetary and productivity.

The monetary factor itemizes the hourly

wage rates, premiums, insurance and taxes.The productivity represents the quantity ofwork produced in a given amount of timeby a worker or a special crew, that is thequantity of construction output unitsproduced in a given amount of time or aunit time.Labor cost is determinate bymultiplication the total labor hours bywage rate. It reduces with the increase ofconstruction productivity. In a constructionproject, equipment costs are typicallydivided into portions: the cost ofequipment and the cost of hand tools.

Equipment refers to all the equipment,tools, and apparatus necessary for theproper construction and acceptablecompletion of a project.

The cost per unit of time of eachequipment has to be determined.Estimating equipment costs involvesidentifying the ownership and operatingcosts. Ownership costs include: initial cost,financing (investment) costs, depreciationcosts and taxes and insurance costs.

The operating costs contain: maintenanceand repair costs, storage costs, fuel andlubrication costs. According to the studymade by Sue McCraven [9] in a plant, to

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determine the technical economicindicators of a product by cubic metermade with conventional mix and SCC. Theresults of the research are listed below:

The cost of labor of SCC was lowerwith 84;

material cost of SCC was higher with18% by cubic meter,

the total time of casting SCC wasshortened with 85%

The total cost for placing a product(labor + materials) by conventionalconcrete was higher than the cost by SCCwith 8%.

Material costs are offset by laboreconomy.

7. Conclusions

The main technical economic indicatorsof a building are:

- total cost – labor, equipment, materials- duration quantity/production per unit

of time x number of crew- unit cost - total cost/ quantity.Based on this study, the following

advantages of SCC used as a structuralmaterial may be described:

- depend less on skilled work force dueto the elimination of vibration concrete

- placement at a faster rate due to thehigh workability of concrete mix.

- shortened construction periods as resultof elimination of some technologicalprocesses as vibration, remedy pouringdefects.

- reduced total cost of building due todiminish of labor, duration etc.

increase the safety by eliminating theneed for consolidation.

References

1. BIBM, CEMBUREAU, EFCA,EFNARC,ERMCO, The EuropeanGuidelines for Self-Compacting ConcreteSpecification, Production and Use, May2005.

2. E. Elbeltagi, Cost Estimation, 2014.3. Indicator de norme de deviz pentru

lucrări de constructii administrative,social-culturale rezidentiale si industrial,volumul C, Bucuresti 2007

4. NE-012-2 Normativ pentruproducerea betonului si executarealucrărilor din beton, beton armat si betonprecomprimat. Partea 2: executarealucrărilor din beton

5. K.M. Mozgalev “Efficiency of self-compacting concrete use during theconstruction of monolithic buildings inwinter conditions”, Bulletin of the SouthUral State University Series “ConstructionEngineering and Architecture vol. 14, no.1,2014, pp.33–37

6. D. Kapooor, Self compactingconcrete,an economical approach, j.Masterbiulder, 2013

7. Okamura, M. Outchi Selfcompacting Concrete, Journal ofAdvanced Concrete Technology, vol 1,nr.1 pp5-15, april 2003

8. Al-Feel: Properties of SelfCompacting Concrete at Different CuringCondition and Properties of SelfCompacting Concrete at Different CuringCondition and their Comparison withproperties of Normal Concrete, journ. Al-Rafidain Engineering, Vol. 17, nr.3, 2009

9. www.precast.org