Adaptation of the German “Guideline for Preventive Measures Against Harmful Alkali Reactions in Concrete” on Dolomite Rock Testing for Highway Projects in Bosnia and Herzegovina

Abstract

The subject of application of dolomite and dolomitic limestone in structural concrete has been a debate in Bosnia and Herzegovina for more than four decades. In many regions engineers usually tried to avoid the use of dolomite and dolomitic limestone in concrete mixtures. It was a kind of engineering know-how without a specific code or guideline covering this matter. “Motorways of Federation of Bosnia and Herzegovina” in cooperation with the “MFPA – Materialforschungs- und -prüfanstalt” Institute of the Bauhaus-Universität in Weimar, Germany developed a code adapting the German “Guideline for preventive measures against harmful alkali reactions in concrete” for testing of dolomite and dolomitic limestone aggregates. Such aggregates from several quarries on the route from Sarajevo to Zenica were tested for potential risk of alkali reactivity in the laboratory of the “MFPA” in Weimar and the results of the testing will be presented in this work.

1 Introduction

1.1 The Motorway Project

The Public Company of “Motorways of Federation of B&H” on behalf of the “Government of Federation of B&H” was in charge to implement a range of motorway construction projects during the period from 2013 to 2015 in value of approximately 500 million €.

Fig. 1.

Sarajevo Bypass Motorway – LOT 3

This infrastructural challenge included numerous concrete structures. Almost 2 km of different types of bridges, 10 km of tunnels over 20 underpass/overpass cross passage structures, kilometers of different types of retaining structures as well as dozens of kilometers of internal and external drainage systems and concrete piles had to be constructed.

Fig. 2.

Sarajevo Bypass Motorway – LOT 1 and 2

The specific motorway projects were on the Sarajevo Bypass (Figs. 1 and 2) including a section going further to south in the direction to the city of Mostar and the section towards the city of Zenica (Fig. 3).

The structural concrete volume was over 1 million cubic meters. Aside from the structural and constructional site organization complexities an important role was to provide sufficient concrete volume and to assure its quality. One of the difficulties in respect of the structural concrete was to provide an adequate volume of aggregates fulfilling the requested quality standards.

Fig. 3.

The motorway towards the city of Zenica

2 The Quarries

In the Sarajevo region at that time six (6) active quarries have been found that are in the range that could be considered for supply of aggregates for the construction sites of the motorway project. Three of the quarries where basically dolomite quarries, one was a dolomitic limestone quarry and two where limestone quarries.

Basically the dolomite and dolomitic limestone in Bosnia and Herzegovina were mainly used in the production of asphalt mixtures whereas the “pure” Limestone was used for concrete mixtures. After the war in the 1990’s a massive reconstruction process has been carried out in Bosnia and Herzegovina, so to find the so called “pure” limestone quarries of sufficient capacity for extensive and high demand projects became a difficult issue.

So it was obvious that it had to be analyzed whether dolomites and dolomitic limestone quarries might be used for concrete mixtures for the structures on the motorways. The other reason why the “Motorways of Federation of B&H Ltd.” had to provide proper regulations is that most quarries had gone through the privatization process. In former Yugoslavia all the quarries where public-owned (governmental properties), so there was no real competition among them, they were all used depending on the requirements. Because nowadays all the quarries are privately owned, was expected that this issues had to be regulated. It was essential to develop an appropriate code or a guideline that defines the dolomite and dolomitic limestone testing of aggregates in respect of the potential alkali reactivity.

2.1 Mechanism of the Alkali-Carbonate Rock Reaction

The use of limestone and dolomite as aggregates in concrete in terms of service performance has in most cases been recorded as excellent. Nevertheless there are some dolomites as well as few argillaceous dolomitic limestones that produce excessive expansion and breakdown of concrete. Excessive expansion occurs when these dolomites or argillaceous dolomitic limestones – present as coarse aggregates – and Portland cement with relatively high percentage of alkalis react in such extent that it causes destruction of concrete. This phenomenon is known as the alkali-carbonate rock reaction. Extensive studies have been conducted since its discovery. The research on this phenomenon has shown that deleteriously reactive rock aggregates influenced by the external factors (moisture and temperature) react with alkali cements. Petrographic and other methods of distinguishing expansive from non-expansive carbonate rock components have been established but – despite extensive research having been carried out on this matter – there is still disagreement on the mechanisms causing the expansion. Rate and degree of expansion of reactive rock exposed to alkalis increases, if the alkali is derived from the cement or is added as sodium or potassium hydroxide. The expansive reaction is also intensified by the increase of temperature indicating some chemical reactions. Moisture, necessary for the expansive reaction, is taken up during the progress of the reaction. The dolomite constituent of carbonate rock is attacked by the alkali with the formation of brucite, calcite and alkali carbonate. This reaction may be written in its simple formula:

$$ {\text{CaMg}}\left( {{\text{CO}}_{ 3} } \right)_{ 2} + 2 {\text{NaOH}}\quad\longrightarrow\quad{\text{CaCO}}_{ 3} + {\text{Na}}_{ 2} {\text{CO}}_{ 3} + {\text{Mg}}\left( {\text{OH}} \right)_{ 2} $$

This transformation is known as the dedolomitization reaction, the rate of it is directly affected by the size of the dolomite crystals, the temperature, and apparently by the particle size of calcite. No quantitative relations have so far been identified between expansivity and such variables as the state of crystalline order in the dolomite, porosity and permeability of the rock as well as type, amount and distribution of clay minerals. Alkali concentration affects the conversion rate and probably also the composition of the reaction products.

2.2 The Guideline for Aggregate Testing

The “Guideline for aggregate testing” was developed to define the minimum requirements for aggregate manufacturers. The Instruction is defining the testing program and aggregate certification in terms of alkali reactivity in case of dolomites and dolomitic limestone.

The testing program shall be as defined under:

• ASTM C295 2012

• DIN EN 12620

• German Alkali Guideline (DAfStb- Richtlinie Vorbeugende Maßnahmen gegen schädigende Alkalireaktion im Beton 2007)

Testing procedure in terms of alkali reactivity in case of dolomites and dolomitic limestone:

Steps	Explanation	Reference standards
1	Sampling the dolomite rocks in the quarries of Bosnia and Herzegovina, Further processing and preparation of the samples at the MFPA Weimar	DIN EN 932-1 and DIN EN 932-2
2	Preparation of thin sections with dimensions of 50 × 80 cm	–
3	Determination of the HCl-insoluble residue	Based on DIN EN 196-2
4	Microscopic examination on the thin sections and classification into harmless or harmful	ASTM C295/C295 M
5	In case that in step 3 the samples are considered to be potentially harmful the Rapid test method of the Alkali-Guildline (DafStb), Appendix B applies (Reference method), afterwards the samples are classified as potentially reactive or non-reactive	Alkali-Guildline (DafStb) Appendix B
6	In case that in step 4 the samples are considered to be potentially harmful the Concrete test at 60 °C of the Alkali-Guildline (DafStb), Appendix C applies, afterwards the samples are classified as potentially reactive or non-reactive	Alkali-Guildline (DafStb) Appendix C
7	Evaluation of results and reporting	–

2.3 Petrographic examination according to ASTM C295

Because there is a relationship between the textural composition of dolomitic limestones and ACR expansion, their texture should be explored under the microscope. Basically there are two identifying features – first dispersedly distributed rhombic dolomite crystals with small size in a micritic matrix (Gillot 1969; Hadley 1964; see also Sommer & Katayama 2006) and second a significant content of ooids (rock type oosparite) or fossiliferous and pseudo-oolitic allochems (Qian et al. 2002). A reliable microscopic determination of those components requires firm knowledge about petrography of carbonate rock. The cited literature should be studied carefully and improvements of the international state of knowledge to that subject have to be considered. At first the available rock samples respectively agglomerate fractions have to be investigated macroscopically with the help of a hand lens to determine major dolomite types as well as structural and textural differences. The calcite content should be estimated with diluted hydrochloric acid. Then petrographic thin sections should be made of all major rock types in a laboratory specialized on this kind of work. Staining of the pore space is strongly recommended; selective staining of the carbonate minerals would be an option.

The thin sections have to be studied using at least a polarization binocular microscope to get an overview of the minerals as well as their microstructure characteristics. Since the dolomites and dolomitic limestones are relatively homogeneous, the constituent minerals can be detected under the microscope alone – they consist almost entirely of the minerals dolomite and calcite, therefore a precise quantification of different main minerals is not necessary here. However it is recommended to use SEM (scanning electron microscopy) to determine either the quartz content of selected samples – which in some cases may reach critical percentages – and if the quartz is mono- or cryptocrystalline. Detecting the cryptocrystalline quartz and also the clay minerals in the matrix is impossible by examining thin sections under the optical microscope due to the much lesser resolution compared to SEM. According to Beyene et al. (2013) SEM observation is indispensable for the investigation, as it detects the cryptocrystalline quartz in the matrix, which is responsible for the ASR gel.

2.4 German alkali guideline

In case of the dolomites and dolomitic limestone of B&H the German Alkali Guideline can be adapted as follows: As for alkali reactivity, approvals for specific aggregates shall be granted for materials that are classified as follows, according to the aforementioned Alkali Guideline (DAfStb-Richtlinie, Vorbeugende Maßnahmen gegen schädigende Alkalireaktion im Beton - Alkali Richtlinie):

“X” – according content of HCl-insoluble substances and results of mineralogical and petrographic microscopy, rapid test method and 60 °C concrete test.

2.5 Rapid test method

Aggregate fractions with grain sizes of ≥2 mm can be tested with the rapid test method. In the case of crushed rock material it is usually sufficient to test the grain fraction 8/16 mm for the assessment and classification of all aggregate fractions.

• The aggregate fractions 2/8 mm and 8/16 mm are broken up and tested together as a grain mixture in the volume ratio 57:43.

• The aggregate fractions 2/8 mm, 8/16 mm and 16/22.4 mm (screened out from 16/32 mm) are broken up and tested together as a grain mixture in a volumetric ratio of 28:29:43.

Three mortar prisms with measuring pins in the dimensions of 40 × 40 × 160 mm have to be fabricated for the strain measurement with alkali-silica reaction test cement CEM I 32,5 R and a given grain composition. The Na₂O equivalent of 1.30 M.-% is set by the addition of potassium sulphate to the mixing water. The prisms are stored for 13 days in a (1.00 ± 0.01) molar, (80 ± 2) °C hot NaOH solution in a drying cabinet, and their change in length is measured 4 times during the storage period. Limits for the strains in the rapid test methods are specified in the alkali guideline (Figs. 4 and 5).

Fig. 4.

Stainless steel container for testing of three prisms.

Fig. 5.

Test equipment with an air temperature (60 ± 2) °C and at least 98% rel. humidity

2.6 Concrete test at 60 °C

To get an authoritative classification at least three prisms with a predetermined composition and the dimensions of (75 ± 5) mm × (75 ± 5) mm × (280 ± 10) mm have to be fabricated for the elongation measurement according to DIN EN 12390-2.

After the reference measurement three specimens each are placed in a sealed stainless steel container. These stainless steel containers are placed in a climatic exposure test cabinet with an air temperature of (60 ± 2) °C and at least 98% of relative humidity.

The zero measurement of the length (l₀) and mass (m₀) takes place at (20 ± 2) °C immediately after striking the specimen from their molds. Further measurements (l_t and m_t) take place at the age of 28, 56, 84, 112 and 140 days. Limits on the expansion of concrete prisms in the concrete test under 60 °C are specified in the alkali guideline.

3 Results and Discussion

Four quarries were tested according to the testing procedure for the dolomite rocks in respect to potential alkali-dolomite reaction. After sampling on site, the HCL-insoluble residue was determined and microscopic examination was carried out on the thin sections. In order to substantiate the findings both the Rapid test method and Concrete test at 60 °C were carried out. The results are shown in the Figs. 6 and 7. In the Rapid test method all specimens from the quarries remain well below the limit of 1.00 mm/m. In the Concrete test at 60 °C, more severe strains of the specimens are determined. However, after 140 days of storage period the tested specimens did not exceed the limit value.

Fig. 6.

Results of the rapid test method

Fig. 7.

Results of the concrete test at 60 °C

4 Conclusions

Based on the observations in the quarries, the determination of the insoluble HCl residue and the microscopic investigations on thin sections as well as from the results of the Rapid test method and Concrete test at 60 °C it can be certified that the Dolomite rocks of all four quarries in terms of alkali reactivity are harmless.

The higher strain results of the specimens in the Concrete test at 60 °C showed a much higher sensitivity of this test method in respect of the reactivity. Thus, these results are of particular importance for the assessment of the alkali - dolomite reactivity testing.