Issue 2 -  2001/02

 ISSN 1311-8978

Tensile Strength of Fine-grained Slag Concrete

Violeta J. Petkova-Russeva, Central Laboratory of Physico-Chemical Mechanics,

Bulgarian Academy of Sciences, “Acad. G. Bonchev” St. 1, Bl. 1, 1113 Sofia,

e-mail: malahit@clphchm.bas.bg

Received 10.10.2002; Cited 22.11.2002

 Abstract

A new formulation of fine-grained slag concrete has been developed, characterised by effective utilisation of secondary waste products from different industrial processes. The obtained concrete possesses high compressive strength (cubic specimens) and good adhesion. The relationship between the high compressive strength and adhesion and the increased axial tensile strength of the slag concrete has been proved. The empirical equation of Ferret has been applied for determining the tensile strength dependence on the strength value. The differences between the calculated tensile strength and the experimentally established one for the compressive strength (cubic specimens) exceeding 50 MPa have been established. They are the prerequisite for finding a new equation for empirical determination of the tensile strength of the fine-grained slag concrete. This equation combines the tensile and the cubic specimens. The new formula can be successfully applied for predicting the tensile strength of different slag concrete composites. It is shown that the axial tensile strength of fine-grained slag concrete with cubic specimens exceeding 60 MPa is approximately 40 % higher than the tensile strength of ordinary concrete with coarser aggregates.

Key words: fine-grained slag concrete, compressive strength (cubic specimens), tensile strength, a new formula for the tensile strength empirical determination.

The aggregates are the main component parts of concrete, occupying 80 % of its volume. The construction-technical characteristics of concrete, its durability and safe performance depend on the type and properties of aggregates. The combined utilisation of waste products from different industrial branches as active additives and/or modified artificial aggregate materials of low activity represent one of the effective methods for obtaining high strength and deformation parameters of concrete.

A formulation of a high-strength composite material (CM) has been developed on the basis of this concept. Its silicate matrix contains a new combined active additive (AA), artificial aggregate materials and preliminary treated water with pH= 11. The formulation can be successfully applied in building practice as fine-grained slag concrete (SC). The advantages of this composition are the high strength [1] and increased adhesion [2] between the surface layer of the cement stone and the aggregates due to the good physico-mechanical properties of the ingredients. The latter provide the possibility of increasing the axial tensile strength of slag concrete, which is one of the important mechanical characteristics used as the basic criterion for determining the crack resistance of concrete structures. The design compressive strength of construction equipment of national importance as runaways, road covers, hydrotechnical structures, etc., can be determined on the basis of the tensile strength. The testing of standard specimens with complicated eight-like shape, as well as the use of additional devices and procedures are the difficulties in the tensile strength determination. In the practice it has been accepted to calculate the tensile strength as a function of the compressive one – mainly the cubic strength, by means of well-known empirical relationships [3].

The basic aim of the present report is to develop a new mathematical expression for the cubic compressive strength and the tensile strength, which will be valid for the fine-grained slag concrete. At the same time the results should conform to those obtained from standard tests or calculated by means of the empirical formulas. The experimental data for the changes of the density, the cubic and axial tensile strength have been discussed. Comparative assessment of the three parameters has been performed by juxtaposing the analogous parameters of ordinary concrete control composition with natural aggregates. The effect of the granulated slag used as fine-grained gravel on the higher cubic strength of slag concrete has been proved.

Initial materials and methods of investigation

Two test series of concrete with composition 1:3 were investigated – new composite slag concrete (SC) and ordinary concrete with natural aggregates (OC). The methods of mixing, placing and compacting of the concrete mixes were in accordance with the Bulgarian State Standard BDS EN 206-1. The used cement from the “Devnya” Cement Plant meets the requirements of BDS EN 196-1. Finely dispersed sterile material and fine-grained granulated slag – both waste products from non-ferrous metallurgy, were used instead of sand and gravel aggregates for SC. They entirely replaced the natural aggregates. The choice of artificial aggregates with suitable activity is especially important for obtaining higher strength properties of mortars and concrete [4]. Different chemical methods were applied for determining the activity of the sterile material and the acidic granulated slag [5].

The formulation of the fine-grained slag concrete contains a combined active additive consisting of two components – alkaline and pozzolanic ones. These are cheap mineral waste products from the building and ceramic industry. The additive meets the requirements of BDS EN 196-5 and BDS EN 196-6 with respect to its dispersity, chemical composition and activity [6]. It can be used both in neat clinker cements and in cements of lower quality. The composition of the new additive was optimised using the methods of planned experiment and mathematical statistics. The possibilities of the Excel software and the Linest function for multi-linear regression were used for this purpose. The regression equations describing the changes in the compressive strength as a function of the alkaline and pozzolanic components were calculated [7]. The concentration of Ca(ОН)₂ was also investigated when varying the quantity of cement and active additive in a broad range [8]. Polynomial relationships have been determined for the different ages of curing, which can be used for the approximate determination of the safe calcium hydroxide content.

The obtained experimental results confirmed the effectiveness of the application of the new active additive. It contributes to the intensification of the hydration processes and to the structure formation of the slag-cement stone. Its plastifying effect leads to diminution of the water-cement ratio and to decreased water requirement. In this way the additive has a significant contribution to the recorded high strength parameters of the fine-grained slag concrete both at a very early age and at more advanced age of hardening.

Standard test specimens – cubes with sizes 15/15/15 cm and eight-shaped samples with defined sizes, were used to determine the cubic strength and the axial tensile strength. Hinge devices are preliminary fixed at the front sides of the specimens by means of epoxy adhesive for the creation of centric tensile stresses. The test specimens were cured under standard conditions till the corresponding age of treatment - 1, 28 and 365 days. The strength is calculated as the arithmetic mean of the single results of three test specimens for each of the above mentioned ages.

Results and discussion

The changes of the cubic strength of the two series – SC and OC, are shown in Fig. 1. Higher values for the ages of 1 and 365 days – 15.8 MPa and 56.3 MPa respectively, are reached for the new fine-grained slag concrete composition with the defined optimal content of the additive and the granulated slag [1, 7]. On the one hand, these results are due to the dense and strong granulated slag, which participates in the solid framework formation. At the same time it possesses good adhesion to the mortar ingredient. On the other hand, the new additive improves the hydrolysis and hydration processes of the melilite glass at an early age and contributes to the diminution of Ca(ОН)₂ concentration due to its components, which exist in a chemically active state. The recorded by X-ray phase analysis newly formed alkaline hydration products – mainly tobermorite gel and sub-disperse low alkaline calcium hydrosilicates of the CSH(В) type guarantee the dense structure and high strength properties of the fine-grained slag concrete.

Fig. 1. Changes in the cubic strength of SC and OC for different ages of curing.

The experimental data for the bulk density of three formulation compositions (1:2, 1:3 and 1:4) of ordinary concrete (OC) and fine-grained slag concrete (SC) are shown graphically in Fig. 2. It is known that this parameter depends mainly on the type of concrete and on the density of aggregates. When using granulated slag, the bulk density of concrete increases and it varies from 2.75 to 2.84 t/m³ for slag concrete. Hence it is classified as heavy concrete according to this parameter. The density values for ordinary concrete are considerably lower – 2.23 – 2.31 t/m³.

The effectiveness of the granulated slag application as fine-grained gravel for increasing the cubic strength of the fine-grained slag concrete has been proved (Fig. 3). It is seen that the strength is the highest when introducing 65 % slag from the total mass of the artificial aggregates and this refers to all ages of curing. The cubic strength at the age of 365 days is respectively 15.2 % and 17. 4 % higher than the strength of slag concrete with 55 % or 75 % content of granulated slag in the artificial aggregate composition.

The functional relationship between the tensile strength and the cubic strength is shown in Fig. 4. The investigation range of the latter (from 10 МРа to 80 МРа) is in accordance with the recorded maximum strength of slag concrete – 71 МРа. The tensile strength has been determined experimentally for the fine-grained slag concrete (curve 1), the ordinary concrete with natural aggregates (curve 2), or has been calculated according to the well-known formula of Ferret (curve 3):

                                                                                                     (1)

Fig. 2. Changes in the bulk density depending on the formulation of OC and SC.

Fig. 3. Influence of the granulated slag content on the cubic strength of SC.

The fine-grained slag concrete yields lower experimental values for the tensile strength compared to the calculated ones according to formula (1) for cubic strength lower than 50 МРа (Fig. 4). This was the main reason for the development of a new empirical expression, uniting both strengths in one relationship and having maximum reliability of the obtained results:

,                                                                                                        (2)

where g_о is the bulk density of the fine-grained slag concrete, t/m³.

a - experimentally determined empirical coefficient.

2.80 – the bulk density of slag concrete with cubic strength of 80 МРа.

It is clear from formula (2) that the cubic strength grows with the bulk density. The tensile strength increases too and can be calculated from equation (1) for a density of 2.80 t/m³. The difficulties in determining the bulk density at the construction site brought the necessity of transforming formula (2), replacing the density by the cubic strength according to equation (3):

,                                                                                                                                                      (3)

where b is an experimentally determined empirical coefficient.

After substitution of (3) in (2) and calculation of the mentioned coefficients by means of the least square method, the following equation is obtained for the tensile strength of slag concrete:

,                                                                                                           (4)

the correlation coefficient being 0.97.

Fig. 4. Relationship between the tensile strength and the cubic strength of slag concrete.

curve 1 – experimental data for SC, curve 2 – experimental data for OC, curve 3 – formula of Ferret for SC, curve 4 – new formula for SC.

It could be stated that for changes in the compressive strength from 60 МРа to 80 МРа, the axial tensile strength of slag concrete is on the average 30 % to 35 % higher than the tensile strength of ordinary concrete with natural aggregates (Fig. 4).

Conclusions:

1.   A new composition of high strength fine-grained slag concrete has been developed, which comprises a new active additive and artificial aggregates of low activity and defined grain-size distribution. Both components represent secondary waste products from various industrial branches.

2.   A new empirical equation has been developed, which combines the functional relationship between the cubic strength and the axial tensile strength of fine-grained slag concrete. This expression can be widely applied in building practice for the approximate determination of the tensile strength without testing of standard specimens. In this way significant economy of time and resources can be achieved.

3.   The fine-grained slag concrete has good adhesion and high mechanical parameters – compressive and axial tensile strength. When the cubic strength is higher than 60 МРа, the tensile strength is on the average 30 % to 35 % higher than the tensile strength of ordinary concrete with natural sand and fine-grained gravel. According to BDS ЕN 206-1 the slag concrete corresponds to class B_o 3.0 and the ordinary concrete – to class B_o 2.0.

References

1. Petkova-Russeva V., Three-parametric Model for Assessment of the 1-day Strength of a Silicate Composite, Journal of International Research Publications (serial online). Bulgaria, Science Invest Ltd. - Branch Bourgas, 2000Bo1, Issue 1, no. 9_2001 - [cited 9.10.2001], Annual. Available from Internet: <http://ejournal.topcities.com/Contents/Issue 1/9/9_ 2001.htm>. ISSN 1311-8978.

2. Panayotov P., V. Petkova. Adhesion of Polymer Coatings for Fine-grained Slag Concrete. Proc. IV-th International Symposium “Technomat’2002” - “Materials, Methods and technologies”, 12-14.09.2002, Bourgas (in press).

3.   NIIZhB, Strength, Structural Changes and deformations of Concrete, Stroiizdat, Moscow, 1978 (in Russian).

4.   Vinogradov B. Influence of the Aggregates on the Properties of Concrete, Stroiizdat, Moscow, 1979 (in Russian).

5.   Petkova V. Composition Grouts Formed by Combined Application of Waste Products, Stroitelstvo, No 3-4, 1992, 16-18 (in Bulgarian).

6. Petkova-Russeva V. Pozzolanic Activity of Combined Additive for a New Silicate composite, X-th Jubillee Symposium “Ecology’2001”, Bourgas, 07-09.06.2001, 353-358.

7. Petkova-Russeva V., R. Krastev. On the Variation of the Compressive Strength of a New Class of Silicate Composites, Journal of International Research Publications (serial online). Bulgaria, Science Invest Ltd. - Branch Bourgas, 2000/01, Issue 1, no. 8_2001 - [cited 9.10.2001], Annual. Available from Internet: <http://ejournal.topcities.com/Contents/Issue 1/8/8_ 2001.htm>.

8. Petkova-Russeva V. A New Composite Formulation with Combined Utilisation of Secondary Raw Materials, ХI-th International Symposium “Ecology’2002”, 06-08.06.2002, Bourgas, (in press).