Application of steel slag concrete in the foundation slab and basement wall of

the Labein-Tecnalia Kubik building

Idoia Arribas, José T. San-José, Iñigo Vegas, Jose A. Hurtado and Jose A. Chica

Abstract: The study described in the present article is motivated by a desire for the

consolidation, on the market, of sustainability concepts in the construction hyper-

sector. Within the framework of the Kubik initiative, developed by Labein -Tecnalia

over the period 2008-2010, a concrete structure which incorporated black steel slag

was constructed, as the foundation for the Kubik building laboratory. The results set

out in this study cover the dosage phases of the steel slag aggregate concrete, with

volumes of over 75% black slag. It is a pioneering structural application involving

slightly over 140m3 of reinforced concrete (basement walls and foundation slab),

which was manufactured in a concrete factory (Hormigones y Minas SA) and poured

on site without interruption by means of a concrete pump.

Introduction Basic cement materials are the most widely used materials in the world in terms of

investment and production volume. As an example, global cement production in 2007

was over 2,500 million tons. An estimated dosage of between 250 and 300 kg of

cement per cubic metre of concrete means that 8,000 to 10,000 million cubic metres

could be produced which is equivalent to 1.5 cubic metres of concrete per person. No

other construction material has been used in such quantities, and it appears unlikely

that another construction material could compete with concrete in terms of volume, in

the future. This is principally due to it being a relatively low-cost, easily manufactured

material, which has excellent qualities, both in terms of durability and mechanical

strength.

Over the last decade, the European construction sector has undergone periods of

stagnation with regard to its production levels. A downturn in Europe of -4.3% was

envisaged for 2009, in relation to the preceding year. There is widespread agreement

that the sector will be incapable of recovering earlier levels of growth before a

complete return to normality in the economy is confirmed. Accordingly, even if we

accept that in 2010 the economies of the Euro zone may start to free themselves

from sluggish growth levels, the construction sector will have to live through a further

full year of stagnation, before returning to the path of growth.

In the present situation, two tendencies may be observed among firms in the sector:

cost reduction maximization in response to falling demand or, to risk pursuing

innovation as a strategy that will result in greater strength when activity picks up

again. Some of the opportunities of the sector (civil building works or infrastructure)

lie in the development of systems (Glass et al 2008) with greater levels of

commitment towards global sustainability (better safety, low energy consumption,

reduced emissions, environmental compatibility, improved functionality, etc.).

In this first decade of the 20 c., greater environmental lobbying, on the part of public

authorities and social agents, has heightened interest in the assessment of steel slag

aimed at the production of steel slag aggregate. Ever since the 1990s, LABEIN-

Tecnalia, currently integrated in Tecnalia-construction, has been working in the field

of recycled industrial by-products (black slags, foundry sands, paper mill sludge,

etc.). Its technological focus has been on their incorporation in construction materials

(concrete aggregates, cements, bituminous asphalts, mixed asphalts, etc.),

generating a range of satisfactory products and solutions that perform to a required

standard (Vegas, 2009).

The industrial by-products examined in this study are none other than slags from the

first fusion of Electric Arc Furnaces (black slags), produced in steelworks. Towards

the end of the 1990s, a (nationwide) initiative was taken in the Autonomous

Community of the Basque Country [Comunidad Autónoma del País Vasco (CAPV)],

that led to the preparation of a White Paper on slags (Ihobe, 1999) applicable to the

Basque Country (CAPV). The regulatory standard became a reference in Spain that

made it possible to use black slags in resurfacing layers, road bases and sub-bases.

Given the high production of black slag in Spain (some 2 million tons/year), it is

considered necessary to diversify the commercial uses of steel slag aggregate, with

a view to ensuring greater penetration into potential markets. On this point, one of the

priority lines of research over the last five years has centred on the use of steel slag

aggregate to prepare hydraulic concretes in bulk, with low structural capacity.

Scientific progress has also led to the emergence of business interests around steel

slag aggregate. The next challenge associated with this line of research is to study

the performance of structural concretes made with steel slag aggregate: designing

new dosages, looking into technological improvements for on-site use, compatibility

between materials (use of cements with additions), widen knowledge on physico-

mechanical behaviour and aspects related to the durability of reinforced concrete,

development of structural/functional prototypes on a real scale and analysis of

different architectonic performances (acoustic, energetic or aesthetic efficiency).

Benefiting from the work of other Spanish research groups, located in the universities

of Catalonia (Vázquez-Ramonich et al. 2004 and Berridi, 2008), Burgos (Manso,

2001 and Rodríguez, 2008) and the Basque Country (Losáñez, 2005), the approach

of this research is at more of an industrial level, on a real scale. The purpose of the

work consists in validating, both technologically and industrially, the use of black slag

in basement elements which certainly have complex execution conditions,

summarized as: a need to use concrete pumps for on-site execution, a large

continuous concrete slab, thin elements (basement wall), high geometric quantities of

steel in its reinforcements and large distances between plant and work site.

The manufacture of concrete with black slag as a steel slag aggregate The manufacture of steel in electric steelworks entails two very different processes at

present: load fusion and dephosphoration in the electric arc furnace (EAF) and

desulpheration and refining in the ladle furnace (LF).

The manufacturing process in an EAF begins with a pre-heated charge that is placed

in the furnace hearth. This charge is made up of pre-reduced steel scrap (main

component, in proportions of 50 -90%), and slag-forming materials (calcium, silicon,

magnesium, alumina) in suitable proportions so that they form slag and protect the

furnace lining. Subsequently, the electric arc generated between three graphite

electrodes is started up, in order to melt the load very rapidly. A pool or core of liquid

steel is left in the lower bowl, and the protective slag floats above it in the upper part,

also in a liquid state, given that the fusion point of the compounds so formed

(silicates and calcium and magnesium aluminates) is lower than that of the steel. The

presence of iron oxides and calcium silicates, together with other minority

compounds, form the EAF black slag, the black colour of which at atmospheric

temperature is due to the iron oxides.

The generation of black slag in the CAPV amounts to almost 50% of national

production in Spain (937,000 tons, according to data in 2007). The first studies in the

Basque country [CAPV] (San-José et al 2000), on the possibilities of making use of

EAF black slag, which date back almost 12 years, mainly covered physico-chemical

aspects for its use as granular material in the manufacture of road surfaces (Rubio,

1991). In the field of concrete, and in conjunction with other national initiatives

(Manso et al, 2005), important steps have been made over the past 10 years in the

incorporation of this by-product, which we shall call Steel Slag Aggregate (SSA), as a

material for possible future use in concrete: up until now in non-structural concrete,

and as shown in this present study, in structural concrete, with load bearing capacity.

The main concern for over a decade was to guarantee that steel slag aggregate

would be stable from the dimensional point of view (Frías et al 2004). The presence

of excessive free calcium and magnesia leads to the expansion of the aggregate

over time. The consequence of the hydration of free calcium that the slag might

contain is an increase in volume (it can even double in size) that generates internal

tensions, causing small “wedge effects”, which fracture the periphery of the calcium

nodules.

MgO that is not chemically bounded is known as free magnesia, which in its

crystalline form is called periclase, as well as magnesium wüstites the general

formula of which is (Fe2Mg)O with more than 70 % MgO in weight. Unlike free

calcium, the humidity reaction takes place in a considerably slower manner. The

negative influence due to free MgO has solely been observed to date in the case of

slags with over 4 % MgO in total weight.

At present, the producers of steel slag aggregate produced from EAF black slag

guarantee expansion values of almost 0%. Among other similar initiatives in the

Basque country, one example of a black slag processing plant is the Guipúzcoa

plant, which is managed by Corrugados Azpeitia, S.A and designed to produce

around 90,000 t-AS/year.

Design and execution of a foundation slab and basement walls Following a laboratory study of dosages, as a preliminary step in the manufacture of

the definitive product, a study of the mechanical behaviour of the product was

undertaken at the Hormigones y Minas concrete plant in Mañaria – Durango

(Vizcaya), in order to evaluate difficulties associated with scaling up for the market.

The steel slag aggregate in use came from Corrugados Azpeitia (Grupo Alfonso

Gallardo) and its titration and storage for one month took place at the Arroa Bea-

Zestoa instalations of HORMOR, until the date of its delivery to the Mañaria plant of

Hormigones y Minas in September 2008.

A cement concrete mix of 375 kg/m3 was used, and the cement (MP-CEM II/B-M(V-

LS) 42,5R) originated from at the Rezola plant (Arrigorriaga-Vizcaya).

The evolution of the compressive strength values (28days: 50 - 58MPa), performed

at the concrete plant shows that the dosage is appropriate for placing on-site using a

concrete pump.

The following table sets out the adjustments to concrete dosages manufactured with

over 75% steel slag aggregate in the execution of the foundation slab and basement

walls of the KUBIK building.

Table 3. Dosage of the foundation slab and basement walls. Dosage HA-30/F/20/IIa+Qa

Product Foundation slab Basement walls

Cement: II/B-M(V-L-S)

42,5R* 375+/-15 kg 375+/-15 kg

Fine aggregate 46% 40%

Thick aggregate 54% 60%

Cement/aggregate 0.46+/-0.02 0.46+/-0.02

Additives 1+/-1.4spc 1+/-1.4spc

* FYM (Arrigorriaga factory)

(Source: Hormigones y Minas - FyM - Italcementi Group)

0

10

20

30

40

50

60

70

80

90

100

0,010 0,100 1,000 10,000 100,000

FULLE R OBTE NIDA

0.010 0.100 1.000 10.000 100.000

FULLER OBTAINED

0.010 0.100 1.000 10.000 100.000

FULLER OBTAINED

Figure 1 Dosage adjustment curve (Source: Hormigones y Minas - FyM - Italcementi Group)

First of all, uninterrupted concreting of around 140 m3 (25 lorries) was carried out

using SSAC type HA-30/F/20/IIa+Qa.

Figure 2 Concreting and vibrating of SSAC in the foundation slab.

Subsequently, the concreting of the basement walls was executed in two parts, the

north half-wall and the south half-wall.

Unlike the foundation slab, these elements require greater vibration intensities and

more fluid consistencies to reach all of the crevices around the rebars. This structural

element has a thickness of 30cm and a height of 3m. Hence, concreting is carried out

in batches of approximately 60 cm. with continuous and sufficiently intense vibrating

(2 simultaneous vibrators) to avoid spalling. Nevertheless, some spalling appeared,

probably due to vibrating that was not sufficiently intense (and not always possible),

coupled with an excessively long delay of the two in-transit mixers, due to last minute

inaccuracies in the assembly of the shoring-reinforcement bars of the wall.

All these issues are not always unrelated to it being a large-scale execution (never

undertaken before in an international context, as mentioned beforehand), and

perfectly excusable in view of the complications associated with its uninterrupted

execution, in keeping with the construction schedule, and such thin elements.

Hormigones y Minas supplied a total of 26 m3 in 5 in-transit mixers from its plant at

Mañaria-Durango (Vizcaya) for the execution of the south half-wall. Likewise, 5 days

later, a further 5 in-transit mixers supplied a total of 28.5 m3 for the second concreting

phase of the north half-wall; excess production being returned to the plant for

recycling.

Figure 3 View of the foundation slab and basement walls constructed by means

of pumped SSAC.

Evolution of strength

Over the following months, Labein-Tecnalia took three concrete specimen test

samples of different sizes and volumes, with a view to implementing exhaustive

quality control and follow up of the properties of the SSAC.

Thus, breakage of 3 ø15 x 30cm specimens was performed at different curing ages:

3 days, 7 days, 28 days, 90 days and 180 days. Moreover, six ø15 x 30cm

specimens were used to characterize the modulus of longitudinal deformation,

Poisson’s ratio, the modulus of transverse deformation and the load-deformation

curve under compression. The remaining 9 specimens, at 180 days were exhibited at

the KUBIK experimental laboratory.

Prismatic specimens of different dimensions were also taken which were later

subjected to accelerated ageing tests. These specimens will be subjected to

systematic study in accordance with the testing-inspection-assessment cycle for

degradation evaluation, so as to draw conclusions on the envisaged behaviour of this

material in its lifecycle.

Strength control performed by Labein-Tecnalia on the concrete foundation slab gave

the figures that are shown below in Table 4.

Table 4. Compressive strength of concrete foundation slab. Compressive strength (MPa)

1st Sample (20.9ºC Tª amb.

60%HR)

1st Sample (20.9ºC Tª amb.

60%HR)

1st Sample (20.9ºC Tª amb.

60%HR) Age

Cone: 20 cm Cone: 17 cm Cone: 18 cm 28.1 23.6 23.6

26.3 24.4 24.4 3d

29.4 27.9 25.7 24.5 25.7 24.0

39.2 37.0 37.0

36.0 40.3 40.3 7

38.0 37.7 39.3 38.8 39.3 40.4

51.7 51.7 51.7

51.9 51.3 51.3 28d

51.1 51.5 51.4 51.5 51.4 57.1

55.7 54.6 54.6

53.0 57.7 57.7 90d

56.6 55.1 52.0 54.7 52.0 62.3

56.5 56.9 56.9

57.3 58.0 58.0 180d

58.3 57.4 57.1

57.3

57.1

64.3

Based on these values, the graphs were drawn up to show the evolution of the

compressive strength of S1, S2 or S3, which refer to the specimens taken during the

concreting of the slab.

Evolution of compressive strength SLAB

0

10

20

30

40

50

60

70

0 28 56 84 112 140 168 196

AGE (days)

Com

pres

sive

str

engt

h (M

Pa)

SLAB 1 SLAB 2 SLAB 3 SLAB

Figure 4 Evolution of the compressive strength of the SSAC foundation slab

over 180 days.

This highlights the notable improvement in the strength of the SSAC that was applied

to the foundation slab at 180 days, which had increased by 10% with respect to its

value at 28 days.

The results of the strength control performed on the concrete applied to the

basement walls are presented in table 5.

Table 5. Compressive strength of concrete foundation slab. Compressive strength (MPa)

South Wall (20.9ºC Tª amb. 51%RH)

North Wall (20.9ºC Tª amb. 60%RH) Age

Cone: 16 cm Cone: 17 cm 15.7 23.2 15.3 24.2

3d

16.4 15.8 22.3 23.2 33.5 36.2 34.5 36.1

7d

34.3 34.1 35.1 35.8 45.0 47.3 47.8 44.0

28d

48.5 47.1 49.1 46.8 51.7 54.6 50.4 53.7

90d

55.5 52.5 53.1 53.8 57.9 56.3 55.5 56.8

180d

57.3 56.9

57.8 57.0

On the basis of these values, the graphs showing the evolution of the compressive strengths of the South Wall and the North Wall were drawn up:

Evolution of compressive strength BASEMENT WALL

10

20

30

40

50

60

70

0 28 56 84 112 140 168 196

AGE (days)

Com

pres

sive

stre

ngth

(MPa

)

SOUTH WALL NORTH WALL WALL

Figure 5 Evolution of the compressive strength of the SSAC basement walls

over 180 days.

In figure 6, it may be seen that the SSAC applied to the basement walls at a curing

age of 180 days had undergone an increase of around 20% with respect to its

strength at 28 days.

Elasticity modulus

The moduluses of longitudinal and of transverse deformation were both calculated,

as well as the Poisson’s Ratio of the concrete foundation slab and basement walls,

as shown in table 6. In all three cases, the modulus of elasticity and Poisson’s ratio of

the concrete in both the foundation slab and the basement walls reaches very similar

average values of around 33 GPa and 0.26 respectively.

Future lines of research Given that no two natural aggregates are exactly alike each other (morphology,

texture, granulometry, etc.), neither are there two exactly similar batches of black

slag. Some more immediate lines of work could be defined over coming years,

according to the following breakdown:

• Examine the technology of placing SSAC (pumping and spraying) in greater

depth.

• Explore other cementitious matrices.

• Define more practical and new methodological durability tests.

• Value the improvements/variations introduced by the new aggregate-paste

reactions in the new SSA environments, etc.

For all these reasons, the research team committed to this work believes it will be of

great interest in the future to look at questions such as pre-cast concrete structures,

hybrid structures, etc.

Conclusions The construction of the foundation slab and basement wall has demonstrated the

viability of the structures in every sense. However, a new road is now open to explore

other improvements with regard to the technology of execution, variation of dosages

and components, the study of other aggressive environments, aspects of accelerated

durability, etc.

From a purely economic point of view, apart from the agreements reached between

different parties (producers/consumers), it should certainly be taken into account that,

according to the conclusions of a recent study by Labein, if the same volumes of both

black slag and natural aggregate had to be transported (same volume of works), then

the greater density of SSA would require a 21% increase in lorry transport (10-15%

denser aggregate), in comparison with the same volume of natural aggregate that

would have to be transported, which could, for example, be compensated by air

entrainment additives that would reduce its density. Furthermore, whilst maintaining a

global vision of the problem (lifecycle) at all times, this data should also be contrasted

with reductions in energy consumption and greenhouse gas emissions, due to:

1. Reductions in large amounts of energy and emissions needed for extraction

from the aggregate quarries.

2. Reductions in large amounts of energy and emissions required for crushing,

screening and cleaning the natural aggregate that is replaced by SSA.

3. Reductions in large amounts of energy and emissions needed for part of the

transport of the natural aggregate to concrete plants.

Thus, the solution to apply black slag from steelworks for mass and/or structural

concrete should undergo control processes identical to those for natural aggregate,

in addition to physico-chemical characterization of its inert properties.

Acknowledgements The authors would like to express special thanks to Hormigones y Morteros Agote

S.L., represented by Modesto Etxeberria, for technical collaboration and the supply of

steel slag, and likewise for the funds made available through various research

programmes of Science and Innovation Ministry.

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