Successful ASR prevention in Germany – Influencing factors and adequate measures
Robin Przondziono, Research Assistant, Institute for Building Materials, Ruhr University Bochum, Germany
Rolf Breitenbücher, Full Professor, Institute for Building Materials, Ruhr University Bochum, Germany
Corresponding author: [email protected]
KEYWORDS: Alkali-Silica Reaction, Concrete Pavements, Performance Test, Regulation
Conflict of Interest: None
1. ABSTRACT
Alkali-Silica Reaction (ASR) in concrete pavements has become a real problem in Germany in the end of the 1990s / beginning of the 2000s. In an extensive research project, the background for such ASR-damaging has been examined intensively at the Ruhr University Bochum. ASR in concrete pavements is not only influenced by the reactivity of the aggregate, but rather by a superposition of different influencing factors. For concrete pavements for example, there are specific conditions that increase an ASR significantly. On the one hand, concrete pavements are microstructurally damaged by the superposition of cyclic stresses induced by traffic and climate changes, and on the other hand they are exposed to alkaline de-icing agents during the wintertime. Thereby, an ASR-promoting external alkali supply is given.
Three absolute preconditions are necessary for an ASR to occur: potentially reactive aggregates, sufficient supply of alkalis and an adequate degree of moisture. In Germany, there have been numerous measures taken in the last 10 to 15 years in order to prevent ASR-damages in concrete pavements. Already in 2005 the alkali-content (Na2O-Equivalent) allowed in cements for concrete pavements has been limited to 0.8 % by mass. Additionally, in each case the aggregates intended to be used must be assessed beforehand in a special procedure.
Since these requirements were established by the highway-authorities in 2005 (with modifications in 2013) there have been no new damages related to ASR observed on concrete pavements, which have been constructed in compliance with these guidelines.
2. INTRODUCTION
Cracking in concrete pavements is one of the main problems of this pavement type as their durability can be significantly impaired by cracks. In many cases, cracks are caused by restraint stresses e.g. due to the restraint of thermal or hygric concrete deformations. Beside this, in various German regions cracking in concrete pavements has been often associated with an Alkali-Silica-Reaction (ASR) in the last decades.
In Germany about 1/3 of the 12,000 km highways are constructed with concrete (4,000 km). In about 11 to 15 % of these concrete lanes (i.e. about 450 to 600 km) extensive cracking has been observed within about 5 to 10 years after construction. In most cases ASR-products could be proved in these concretes. These damages are mainly concentrated in Eastern regions, where especially greywacke and rhyolite have been used as coarse aggregates.
However, even if relevant reaction products of an ASR are detected in samples from affected areas, cracks result only in the rarest cases solely from an ASR. Rather it has to be assumed, that cracks in concrete pavements are caused by a superposition of several stress impacts. In order (to be able) to define adequate technological measures for avoiding critical cracking in pavements, it is necessary to identify the most significant sources first.
In order to prevent a damaging ASR in concrete pavements the three prerequisites, alkali-reactive aggregates, alkaline environment and moisture, have to be addressed. The alkali-reactivity of the aggregates can be assessed via suitable performance tests prior to the application. Additionally, the alkaline environment can be limited by use of appropriate constituents with low alkaline-content. Lastly, also the prevailing moisture conditions on concrete pavements are considered within the regulation.
Research Significance
This paper presents recent research results regarding the superposition of influencing factors, i.e. degradation and fluid transport, as the knowledge of these interactions contributes the adjustment of regulations with regard to the use of reactive aggregates which are exempt from use in concrete pavements under current regulation. However, the paper further illustrates the status quo of German regulation regarding ASR prevention, which has proven to be successful in recent years.
3. ALKALI-SILICA REACTION
In the well-known Alkali-Silica-Reaction (ASR), amorphous silica (SiO2) reacts with alkali hydroxide ions (NaOH, KOH) to a viscous alkali-silica-gel, if simultaneously an appropriate amount of humidity is available (Equation 1).
2NaOH + SiO2 + nH2O Na2SiO3 + nH2O (1)
Normally the amorphous silica is brought into the concrete by coarse aggregates like opal, greywacke and rhyolithe due to their SiO2 content, while the alkalis derive mainly from the cement. In some aggregates the reaction starts in the contact zone between mortars and aggregates, in other types mostly in the interior of the grain itself, where the alkalis penetrate into microcracks of these particles (Fig. 1).
Fig. 1. Alkali-silica-gel formation and cracking around the aggregate (left) and within the aggregate (right) according [1]
The main problem of this reaction is the volume expansion of the reaction product due to the absorption of water. Normally this expansion is restrained to a large extent by the encircling concrete. The resulting internal pressure can mount up to 20 MPa in the surroundings of coarse aggregates [2]. Such stress exceeds the tensile strength concrete so that internal cracking starts (Fig. 1). Cracks due to an ASR are usually map-distributed and netlike in all three dimensions. So they are not limited only to the visible surface, but extend all over the concrete structure, so that finally the structure will be destroyed as long as the mentioned prerequisites are on hand.
4. INFLUENCING FACTORS ON CONRETE PAVEMENTS
Degradation
Concrete pavements are subject to static, cyclic and dynamic influences. In order to assess these loads, different stress frequencies should be considered. Due to changing climate conditions, thermally and hygrally-induced deformations are inevitable. These corresponding restraint stresses are changing when exposed to varying influences. In this context, it has to be distinguished between daily temperature changes (day – night) and seasonal temperature and moisture changes (summer – winter). These effects are characterized as low-frequent. Apart from these low-frequent influences, high-frequent load-dependent stresses due to traffic also act on a
concrete structure. Especially concrete pavements are exposed to a high number of load cycles day by day. These effects should be considered as high-frequent.
Such cyclic influences only rarely cause macrocracking, which would lead to a fatigue failure of the whole structure. However, due to these loads, fine cracks might form in the concrete's microstructure, which may then lead to the gradual degradation of the concrete properties, while, at the same time, the material's stiffness decreases [3,4].
The extent of degradation is dependent on two factors: the number of load cycles and the magnitude of stresses. This relation is usually described as the ratio between the stress σo and the adequate strength f. In pavements, in which degradation is caused by compressive as well as tensile stresses, the tensile properties are more relevant (σo/fct,fl). At the same time, it is essential to superpose the actual stresses with those that are already active in the concrete itself, i.e. residual stresses caused by former hydration heat release [5].
As suggested by Holmen [6], at a stress-ratio of σo/fct,fl, below approx. 40 %, microcracks will not turn into macrocracks, even after millions of load cycles. At a stress-ratio above 65 %, fatigue failure of the concrete may occur considerably earlier (already after a few 10,000 load cycles).
Microstructural degradation as a result of the concrete's cyclic loading can be assessed on the basis of ultra-sonic runtime measurements of the longitudinal acoustic wave close to the concretes surface. Based on the results of the ultrasonic runtime measurements, the relative dynamic E-Modulus can be calculated. [7]
Thereby, Comparative ultrasonic runtime measurements are an appropriate non-destructive method to assess these effects. Sievering [7] measured the surface-near ultrasonic runtime on concrete pavements of German highways. Herein the service lane was compared to the driving lanes, as it was exposed significantly less cyclic traffic loads. Correspondingly, the ultrasonic runtime was higher in the driving lanes and thus the calculated relative dynamic E-Modulus significantly lower in those lanes (Fig. 2). This indicates a higher microstructural degradation within the driving lanes, which is caused by the additional traffic loads compared to the service lane.
Fig. 2. Relative Dynamic E-Modulus of the service lane (SS) and the driving lanes (1.FS and 2. FS) [7]
In experimental studies, large concrete beams measuring 180 x 50 x 27 cm³ were exposed to cyclic stresses. All beams were produced using a typical pavement concrete mix according to the German guideline “TL Beton-StB 07” [8]. As for a cement, CEM I 42 N with a maximum aggregate size of 22 mm was chosen. Finally, the concrete surface was broom finished.
The concrete beams were exposed to cyclic stresses at a minimum age of 56 days. The tests were conducted in a four-point-flexural setup, in which the chosen upper and lower stresses represent different stress conditions of superposing loads acting on a concrete pavement. Slowly recurring thermal restraint stresses were to be superposed with high-frequent cyclic traffic stresses. Thus, an upper stress level and a corresponding lower stress level (σmax and σmin) could be derived from these boundary conditions [7]. The study yielded the following variations:• Maximum stress to bending tensile strength (σo/fct,fl = 0.35; 0.50; 0.60)• Number of load cycles (N = 0; 1.0; 2.0; 5.0 million)
Figure 3 shows the development of the relative dynamic E-Modulus for the different stress-ratios as well as over the course of different load cycles. As expected, the rel. dynamic E-Modulus decreased stronger with a higher stress-ratio. After five million load cycles, the remaining relative dynamic E-Modulus amounted to about 91 % for a stress ratio of σo/fct,fl = 0.35 and to about 87 % and 75 % for σo/fct,fl = 0.50 and σo/fct,fl = 0.60, respectively.
Fig. 3. Decrease of the rel. dyn. E-Modulus of beams after 5 million load cycles and at different stress-ratios
After completion of the cyclic loading a microscopic investigation of the crack characteristics on thin polished sections has been conducted. Table 1 lists the corresponding crack characteristics. Compared to the undamaged reference specimens (rel. Edyn = 100 %), with decreasing relative dynamic E-Modulus, the number of cracks increased significantly. At a relative dynamic E-Modulus of about 70.2 % the number of cracks increased by more than factor 30. Nevertheless, the average crack width and length stayed rather constant at about 5 µm and 1.5 mm, respectively. It can thus be concluded that the observed increased degradation rather resulted from the formation of new cracks than from the expansion of already existing cracks.
Table 1. Crack characteristics
rel. dynamic E-Modulus [%]100 92.7 91.5 86.5 80.2 70.2
Number of cracks [-] 12 74 104 128 203 403
Average crack width [µm] 6.6 5.2 5.3 4.8 5.7 4.6
Average crack length [µm] 1,400 1,600 1,000 1,400 1,600 1,500
Average crack cross section [µm²] 13,400 8,100 6,100 6,700 9,200 8,500
Total cross section [µm²] 160,000 600,000 630,000 860,000 1,800,000 3,450,000
External alkali supply
In common concrete structures an ASR can be prevented by the limitation of the alkali content in the cement as this is normally the single alkali source. In concrete pavements, however, the external alkali supply by de-icing salts plays in particular an important role on the ASR-gel-formation as so, the total amount of reactive alkalis is increased significantly.
In this context, also a modification in the application of the de-icing salt is relevant. In former times salt was dispersed, only when ice or snow were already present on the pavement. For some years these agents have been already applied, when such conditions were forecast. That way, the alkalis are in touch with the concrete surface in a stage at which the concrete is well absorptive. The absorption in the concrete is particularly enhanced by the above mentioned microcracks. Further the de-icing agents are pressed quite intensely into the concrete microstructure by the over rolling traffic. Hence, in combination with the prophylactic application of NaCl the external support of alkalis into concrete pavements currently is significantly intensified in comparison with the past.
The described microstructural degradation also influenced the penetration behavior of fluids into the concrete. An investigation of the capillary water uptake in concrete was carried out on specimens previously subjected to cyclic loads with varying degrees of damage. Therefore, Karsten-Tubes were applied on the surface exposed to tensile stresses. The water uptake was assessed over time. It showed, that the water uptake (more or less merely through capillary suction) occurred faster and higher with increasing damage (Fig. 4). In a pre-damaged concrete with a relative dynamic E-Modulus of approximately 92.7 % the water uptake already increased by about 60 %. At a damage of rel. Edyn = 70.2 % the water uptake increased by about 250 %.
Fig. 4. Water uptake in concrete with different relative dynamic E-Modulus
Alkali solutions not only penetrate in in-situ concrete pavements through capillary suction, their ingression is in fact considerably increased by the load of the surface traffic (Fig. 5).
Fig. 5. Increasing alkali ingress with microstructural damage and overrolling traffic
In a test setup that was developed at the Ruhr University Bochum to specifically explore these effects, the aforementioned concrete beams were arranged in a circular array, to ensure that they could be loaded with six tires consecutively. The top surface of the beams had previously been exposed to flexural tension in the cyclic four-point-flexural setup. While the beams were loaded, their top surface was treated with a sodium-chloride solution of 5%-NaCl. The tires that were used to apply the load were additionally equipped with a dead weight of up to one ton in order to achieve near to real-life loading conditions for each beam. Since the solution was only pressed into the material within the track width of the tires, a merely capillary solution uptake was to be expected for the adjacent areas. This setup thus allowed the comparative assessment of the two types of liquid penetration into the concrete. [9]
After the overrolling of the large-format concrete beams up to two million times, small specimens were cut from the different areas of liquid penetration. The specimens were then split perpendicular to the overrolled surface. The freshly broken surfaces were sprayed with silver nitrate to visualize the chloride penetration depth. The penetration depths are visualized in Figure 6.
Fig. 6. Average chloride penetration depth dependent on the number of tire passes and microstructural damage
The average penetration depth on the specimens with the most damages (rel. Edyn = 80.2 %) without tire passes amounted to about 23.8 mm already above the penetration depth on the undamaged specimens (rel. Edyn = 100 %). This effect was also observable for the less damaged specimens but less distinctive. It can further be noticed, that with increasing tire passes, the penetration increases in damaged as well as undamaged specimens. The average penetration depth of an undamaged specimen after 2 million tire passes already amounted to 27.8 mm, whereas for a strongly damaged specimen (rel. Edyn = 80.2 %) the average penetration depth amounted to 35.2 mm. The influence of the overrolling traffic could not be observed for little damage (rel. Edyn = 93.3 %). Concludingly, it can be stated, that the influence of the locally induced external pressure through the tire passes is increasing fluid penetration. However, this effect is limited to a depth of about 2 to 4 cm and becomes more significant with increasing microstructural damage.
5. ASR-PREVENTION MEASURES
In Germany, the "alkali guidelines" [10] issued by the German Committee for Structural Concrete (DAfStb) specify measures designed to prevent the occurrence of any damaging ASR. To determine suitable preventative measures, it is necessary to characterize the ambient conditions of the concrete by means of the moisture class and the alkali-reactivity potential of the aggregate. For this purpose, every aggregate according to DIN EN 12620 [11] to be used in Germany for concrete in accordance with EN 206/DIN 1045-2 [12] needs to be categorized by
an alkali reactivity-class. Preventative measures have to be taken in the case of certain combinations of moisture class, alkali reactivity-class and the cement content (if any) of the concrete. The alkali guidelines include test methods as well as criteria for the categorization of aggregates into one of the alkali reactivity-classes, in addition to preventative measures based on concrete technology.
A damaging ASR can basically be prevented in two ways:
- replacement of the alkali-reactive aggregate- reduction of the alkali content in the pore solution of the concrete.
The General Road Construction Circulars [13], introduced in 2005 and 2006, excludes the use of definitely alkali-reactive aggregates, such as opal, siliceous chalk and flint. Potentially ASR-sensitive coarse aggregates, such as greywacke (crushed), rhyolite (crushed), crushed quartzite gravel (Upper Rhine), recycled aggregates and all aggregates with unknown sensitivity have to be evaluated by an expert report. With an updated Road Construction Circular in April 2013 basically all coarse aggregates have to be evaluated regarding their suitability.
The approval of coarse aggregates for the use in road construction can happen in three different ways. One option is to have the coarse aggregates approved by an expert, who conducts a performance test for the specific concrete composition. There is also the possibility of approval through the so called “WS-basic test”. This test is used for coarse aggregates from a specific quarry. The testing is carried out under pessimal conditions, with a high cement content and a high effective alkali content.
If coarse aggregates are already approved through the expert solution or the WS-basic test they may be listed in the “BASt-list” by the Federal Highway Research Institute (BASt). There are previously approved aggregates and the corresponding concrete properties listed in the “BASt-list”. An exemplary excerpt from the “BASt-list” is displayed in Figure 9. Aggregates on this list may be used in road construction without further testing.
Fig. 9. Excerpt from the BASt-list [BASt]
The second measure to prevent an ASR can be achieved by using special cements with low effective alkali content. In Germany, these cements, which are also referred to as “NA cements”,
are standardized in DIN 1164-10 [14]. They include Portland cements, Portland-slag cements and blast furnace cements. The use of pozzolana as a main constituent of cement or an additive to concrete is in principle also suitable for this purpose. No regulations in this regard are yet applicable in Germany.
The limitation of the effective alkali content is regulated in the General Road Construction Circular from 2005 / 2006 [13] (Tab. 2). The limit value is dependent on the type of cement and if applicable the amount of granulated blast furnace slag.
Table 2. Limitation of the alkali content according [13]
Cement Granulated Blast Furnace Slag (GBFS)
[M.-%]
Alkali content of the cement
Na2O-equivalent[M.-%]
Alkali content of the cement without GBFSNa2O-equivalent
[M.-%]
CEM I + CEM II/A
0.80 ---
CEM II/B-T --- 0.90
CEM II/B-S 21 to 29 --- 0.90
CEM II/B-S 30 to 35 --- 1.00
CEM III/A 36 to 50 --- 1.05
In many cases alkali-reactive aggregates in concrete can also be used for structures in the building and structural engineering sector without any need for special measures. This, however, depends on the composition of the concrete (cement content) and the environment (moisture class).
6. PERFORMANCE TESTS
The Bauhaus University (FIB) and the Research Institute in Düsseldorf (FIZ) each developed a method for investigating the alkali reactivity of concretes for structural elements, namely the “Climate Simulation Concrete Prism Test” and the “60 °C concrete test with external supply of alkalis”. The two methods lead to comparable evaluation of identical concretes and are recognized by the BMVBS (Federal Ministry of Transport, Building and Urban Development)”.
Regardless of the test methods used, the establishment of the evaluation criteria must satisfy two important criteria:
- Avoidance of ASR damage- Retention of the competitiveness of concrete construction
Thus, the criteria are chosen carefully to eliminate aggregates and compositions that have demonstrably led to ASR damage, but also to allow to continue to use mixtures and aggregates that have proven to be successful in practice.
The BMVBS published a new General Road Construction Circular in 2013 [15] introducing a WS aggregate test, which is designated to assess the suitability of coarse aggregates for concrete pavements with concrete of the WS moisture class, referring to the special conditions present on concrete roads regarding ASR. Therefore, the aggregate is to be investigated in an unfavorable condition with one of the aforementioned test methods.
Climate Simulation Concrete Prism Test (CS-CPT)
The CS-CPT was developed at the Bauhaus University Weimar to test specific concrete compositions (job mixtures) for their ASR potential, with the option to take external alkalis into consideration. Concrete prisms (100 × 100 × 400 mm3) with an applied test solution (water, de-icer solutions, etc.) are subjected to cycles of alternating temperature and moisture conditions. One cycle runs for 21 days and consists essentially of 4 days of drying at 60 °C and ≤ 10 % relative humidity (RH); 14 days of wetting at 45 °C and 100 % RH; and finally 3 days of freeze-thaw-cycling between +20 and −20 °C (Fig. 7). Besides three initial subcycles between 5 and 65 °C within 12 hrs, three more subcycles between +10 and −10 °C within 12 hrs are done prior to the six freeze-thaw cycles in order to simulate rapid temperature changes that cause stresses in the prisms. The cycling is automated within a walk-in climate simulation chamber for a period of 9 months or 12 cycles. After every cycle the expansion and mass of the prisms are recorded.
A set of three prisms is prepared for every test variation. In every prism two stainless steel studs are embedded for expansion measurements. About 24 hr after casting, the prisms are demolded, wrapped airtight in polyethylene foil, and stored for 5 days at 20 °C. Subsequently, the foil is removed completely and a flexible foam rubber tape is glued around the edges of the flattened prism side to form a guard that will retain the applied test solution later on. Finally, the CS-CPT is initiated 7 days after casting of the prisms.
At the end of the first drying phase, the initial length and mass of the prisms are measured at 20 °C and 400 g of the test solution is applied to each prism for the first time. The test solution remains on the prisms until the end of the cycle. After the cycle, the test solution is removed to measure the length change and mass of the prisms at 20 °C and is replaced once the readings are obtained. During the second drying phase, the test solution evaporates, leaving behind dissolved substances as NaCl from the applied NaCl de-icer solution. At the end of the second drying phase, 400 g of new test solution is applied again on each prism. This routine is repeated until 12 cycles are completed.
Fig. 7. Alternating temperature and moisture conditions in the CS-CPT [16]
Concrete Prism Test
In contrast to the 60 °C concrete test according to the alkali guideline, the Research Institute in Düsseldorf (FIZ) developed a modification of the 60 °C concrete test with external alkali supply. This test was designed for in situ extracted drilling cores, but can also be applied for specimens created in the laboratory (75 x 75 x 280 mm³). The extracted drilling cores or manufactured specimens have to undergo a homogenizing pretreatment before they are subjected to the alternating storage cycles. They are being stored at 60 °C and 100 % RH in a climate chamber for six days. After a one day storage at 20 °C and 100 % RH the initial length of the specimens is recorded. Subsequently the specimens are subjected to 10 storage cycles. Each storage cycle begins with five days storage at 60 °C and 65 % RH, followed by two days submerged in NaCl-solution (3 % or 10 %) at 20 °C. After the external alkali supply the specimens are stored at 60 °C and 100 % RH. On the last day of each storage cycle, the specimens are stored at 20 °C and 100 % RH (Fig. 8). After completion of each storage cycle the length of the specimens is measured and in relation to the initial measurement the elongation is documented. Depending on the concentration of the NaCl-solution the limit strain value amounts to 0.3 mm/m and 0.5 mm/m respectively.
Fig. 8. Alternating temperature and moisture conditions in the 60 °C concrete prism test [FIZ]
7. DISCUSSION/ CONCLUSION
In the last few years cracking has been observed in various concrete pavements. Besides restraint stresses due to restrained hygric and thermal deformations alkali-silica-reactions (ASR) can be relevant for this. Due to the multiplicity of potential causes for pavement cracking all possible influences have to be considered in order to reveal the relevant causes. In the minority of the cases only one single reason could be found as responsible for the crack formation. In the same manner also only very few cases cracks are solely caused by ASR, although reaction products can be verified in the concrete structure. In most cases a superposition and interactions of various influences e.g. thermal/ hygral restraint stresses, traffic loads and/ or ASR are responsible for cracking.
During the service time, the concrete pavements are stressed continuously by cyclic loads due to the overrunning traffic in combination with seasonably changing restraint stresses. It has to be taken into account that in most cases concrete pavements show some microcracking due to this continuous loading. In combination with an external alkali supply by de-icing-agents in winter, the risk for a damaging ASR is enhanced significantly in pavements in comparison to common concrete structures. This could be proved in the described investigations.
To reduce the risk of ASR-damaging, on the one hand – beside cements with low alkali content – especially aggregates without amorphous silica (SiO2) can be used. With the introduction of the General Road Construction Circulars in 2005, 2006 and 2013 [13, 15] in Germany, a successful regulation was established in order to prevent ASR-damage in concrete pavements. Since 2006, no new ASR-related damages have occurred thus far.
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