Report on the Diagnosis, Prognosis, and Mitigation of Alkali-Silica Reaction (ASR) in Transportation Structures
5.0 ASR Investigation Program Level 3: Detailed Studies for the Diagnosis/Prognosis of ASR
The selection of appropriate remedial measures for ASR-affected structures requires a good knowledge of the current condition of the concrete and its potential for future deterioration. For instance, in the case of reinforced concrete members, determinations of the expansion reached to date, the current rate of expansion and the potential for future expansion of the concrete are particularly critical in order to determine whether or not the reinforcing steel in the structural element has reached or will at some point reach its plastic limit, thus creating risk of structural failure. This is particularly important in mildly reinforced structural members affected by ASR. Similarly, the above information will be crucial for highway concrete pavements where extensive cracking and spalling at joints would create serious serviceability issues. Also, as cracking develops due to the expansion in any ASR-affected unreinforced or reinforced transportation structure, durability issues related to freezing and thawing (where present), external sulfate attack, steel corrosion, etc. will likely develop and affect the service life of the affected member.
A framework for the selection of the time/frequency and magnitude of the remedial action(s) is proposed below, as a function of estimate/ measurements of the following parameters:
- The expansion reached to date.
- The current expansion rate.
- The potential for further expansion due to ASR (e.g., number of years before the steel reinforcement yield could occur (in the case of RCC members) or joints in concrete pavement will close).
- The stability (or structural integrity) of the structure.
- The safety of persons/vehicles.
- The potential for further deterioration due to other mechanisms (extended service life).
The third parameter above is a function of the first two; in other words, remedial actions will be required more rapidly in structures where steel reinforcement or joints in pavements may shortly become "at risk of failure/serviceability issues," no matter what age it is.
In order to generate the information required for the selection of remedial actions, as mentioned above, a combination of field (in-situ testing) and laboratory activities/investigations is proposed in Figure 1. Table 8 summarizes the information required and the various approaches/ activities proposed to obtain it. The selection of the activities to be carried out will depend on the criticality of the structure, the amount of time available to generate the data, and the degree of precision expected. For instance, a more precise indication of the expansion rates will be obtained from the in-situ monitoring of expansion in affected structural members. However, considering the seasonal and thermal effects, it will generally take a minimum of 2 and preferably 3 years to obtain reliable data from that approach. Expansion tests on cores submitted to high temperature (38°C)(100°F) and humidity (> 95 percent RH) conditions will generate data in 6 to 12 months. However, how representative of the behavior of the overall structure will that data be, is still under debate as this kind of approach/investigation program is still very much under development worldwide.
|Information||In-situ testing||Laboratory investigation|
|Expansion reached to date||
|Current condition of the concrete||
|Current expansion rate||
|Potential for future expansion||
5.2. In-situ Investigations
Detailed field investigations are carried out through site inspection and expansion, deformation and displacement measurements on the structure.
5.2.1. Detailed site inspection and selection of sites for further in-situ investigations
Detailed investigations are proposed that complement the observations made as part of the condition survey (Section 3). The necessity to carry out such investigations will depend on the criticality of the structure, the extent of damage and the extent of information required; special attention will be given to generate a quantitative assessment of the extent (severity) of deterioration between the different members of the structure. This will provide an overall condition rating of the structure (and of its members); the Cracking Index method can be used for that purpose (see Section 4.2 and Appendix B). The detailed inspection will also focus on locating the signs of expansion and relative movement in the structure (and measure the extent of the relative movement to date), the sources and availability of moisture and the effect of the restraint on expansion for the various members of the structure. It will also help in selecting locations for:
- Additional coring.
- Installation of probes for temperature and humidity measurements.
- Installation of demec points3 and other devices for expansion and/or relative movement measurements.
- Stress measurements (if required) in the steel reinforcement (see Section 5.2.4) and/or in the concrete.
- Possibly where some remedial actions could be implemented (e.g., modification in the drainage system, application of surface treatments, etc.).
5.2.2. Surface cracking
Surface cracking measurements will be carried out using the Cracking Index method described in section 4.2 and Appendix B. This crack mapping process will however be carried out on members of the structures showing a range of deterioration, but always exposed to moisture. As mentioned before, this will provide a comparative, quantitative rating of the "surface" deterioration affecting the structure as a whole. The measurements of the Cracking Index will then be used to estimate the expansion reached to date by the concrete (see Section 5.5.2), as well as the current rate of expansion (see Section 5.5.3). The CI measuring grids will be well identified so that crack mapping could be repeated over a 2- to 3-year period to allow the monitoring of changes in the extent of surface cracking affecting the structural members selected.
5.2.3. Expansion and deformation measurements
In-situ deformation measurements will be performed by installing demec points and/or metallic references/devices at the surface of selected concrete members; periodic length-change measurements are then taken using extensometers of various shapes and ranges, invar wires/rods or optical systems (leveling). Fiber-optic and vibrating wire systems can also be used, with deformation measurements being performed and the data transmitted automatically to central servers for further treatment. Further details on the setting-up for expansion measurements are given in Appendix D.
5.2.4. Stress measurements
Evidence of stress build-up in reinforcing steel and the surrounding concrete resulting from restrained ASR expansion can be obtained from the measurements of stress in reinforcing bars (which are exposed, strain gauged, then cut to release the tensile stress) (see Appendix D) and the overcoring4technique (Danay et al. 1993).
5.2.5. Temperature and humidity measurements
It is commonly accepted that ASR develops or sustains in concrete elements with internal relative humidity >80-85 percent [relative to 23°C (76°F)]. The relative humidity in a concrete structure can be measured over time with depth or laterally in different concrete elements using various techniques such as wooden stick, or portable or permanent probes (Stark and Depuy 1987, Stark 1990, Siemes and Gulikers 2000, Jensen 2000) (see Appendix D). Humidity and temperature readings can provide useful information in the treatment (e.g., applying any corrections required for length-change measurements) and interpretation of expansion and crack measurements as those are influenced by the above conditions.
5.2.6. Non-destructive testing
Periodic pulse velocity measurements can be made on specific members or parts of the affected structure (at the surface or in the bottom of drilling holes), and might permit assessment of the evolution and the extent of internal cracking or deterioration. Impact echo measurements have been used to monitor the performance of concrete pavements affected by ASR after topical treatments with a lithium nitrate solution (Johnston et al. 2000, Stokes et al. 2003).
5.2.7. Structural evaluation
Especially in the case of reinforced concrete bridges where the visual survey and the in-situ measurements indicate a severe level of deterioration (e.g., C.I. > 0.5 mm/m (0.018 in/yd), crack width > 0.15mm (0.006 in), expansion > 0.05 percent), it will likely be necessary to determine whether or not the stability of the structure is at risk. This should be carried out through a full-scale investigation performed by a competent structural engineer. Full-scale loading tests in the field will ultimately permit assessment of the real structural loss in performance (or serviceability) of the affected structure (see Appendix D). The criteria for load tests are usually based on some limiting deflection criteria and recovery of the deflection with time (CSA 2000).
The assessment of the structural stability of an ASR-affected structure could also be made using a macro-scale numerical ASR model provided this model is sophisticated/reliable enough and fed with the required/pertinent information gathered from all in-situ and laboratory investigations (see Section 5.4).
5.3. Detailed Laboratory Investigations
A series of tests are described hereafter that can be performed on samples cored from various members of the structure and that will provide useful data in the determination of the current condition of the concrete, the expansion reached to date, the current rate of expansion, and the potential for future expansion of the concrete.
5.3.1. Sampling and treatment of samples
Requirements for the sampling process and the treatment of the specimens at site are similar to those described in Section 4.3 (Preliminary sampling). However, a much larger number of cores will be required, because of: 1) the various laboratory tests to be performed; and 2) the interest of sampling various components of the structure to assess the effect of various parameters (such as exposure conditions and restraint) on the results of the tests. Such requirements will be discussed in more detail as part of the various sections hereafter.
5.3.2. Detailed petrographic examination
Special attention is given at quantifying the degree of ASR-related damage between the different members showing various visual degrees (severity) of deterioration and/or various degrees of exposure conditions. This will provide a better picture of the overall condition of the structures and of its members. In order to do so, a quantitative petrographic technique is proposed, the Damage Rating Index (DRI). The method is described in Appendix C; it evaluates the condition of concrete by counting the number of typical petrographic features of ASR on polished concrete sections (16x magnification) (Grattan-Bellew 1992, Dunbar and Grattan-Bellew 1995). The DRI represents the normalized value (to 100 mm2) of the presence of these features after the count of their abundance over the surface examined has been multiplied by weighing factors representing their relative importance in the overall deterioration process (Appendix C).
The DRI method is a useful tool for the quantitative assessment, based on petrography, of internal damage in concrete due to ASR or other mechanisms. However, as the results are very much related to the experience of the petrographer and since there is currently no standard test procedure available, the method is fairly subjective and the results can be quite variable from one operator to another. Consequently, it is highly recommended that the method be carried out by the same petrographer, at least on the sets of cores from a same structure.
5.3.3. Mechanical testing
Compressive and tensile strengths and elasticity modulus - All mechanical properties of concrete are negatively affected by ASR, however not at the same extent or at the same expansion levels. Reductions by up to 60 percent for the compressive and splitting strengths, 80 percent for the direct tensile strength, and 60 percent for the elastic modulus have been reported. In most cases, the properties the most rapidly affected are the modulus of elasticity and the direct tensile strength, even before significant levels of expansion are attained. The compressive and splitting strengths generally behave similarly, being significantly affected only at relatively high expansion levels. The following tests/parameters are thus proposed for the early detection of ASR:
- The compressive strength (which is normally not significantly affected), required for determining the direct-tensile strength-to-compressive strength ratio.
- The direct-tensile strength, which may be significantly affected.
- The direct-tensile strength-to-compressive strength ratio; this ratio lies normally between 0.07 and 0.11 for sound concrete while values below 0.06 would indicate internal damage due to AAR (Nixon and Bollinghaus 1985).
- The elastic modulus, which is rapidly affected even at relatively low expansion levels (even less than 0.08-0.10 percent), then appearing relatively low with respect to the theoretical value calculated from the compressive strength; the elastic modulus, however, tends to stabilize at high expansion levels.
The diagnostic value of the above tests, however, lies in the capacity to establish the reduction in mechanical properties due to ASR, which may require some assumptions/tests to establish the properties of the original/unaffected concrete5. Cores can be extracted from various members showing mild to severe surface cracking (and other symptoms suggestive of ASR) to determine if any changes in mechanical properties of the concrete are observed and, if so, the extent of such a variation.
Modified Stiffness Damage Test (SDT) - The Stiffness Damage Test was originally proposed by Chrisp et al. (1989) and adopted by the Institution of Structural Engineers (ISE 1992). Recently, the method was slightly modified and used for estimating the expansion attained to date by ASR-affected concrete (Smaoui et al. 2004a, 2004b, 2004c, Bérubé et al. 2005). In the modified test procedure described in Appendix E, the test specimens, which consist in 100 mm (4 in) cores extracted from the concrete member to be evaluated, are subjected to five cycles of uniaxial loading/unloading up to a maximum of 10 MPa (1450 psi). The following parameters are then used for assessing the ASR expansion attained to date: (1) the energy dissipated during the first cycle (hysteresis loop), and (2) the accumulated plastic strain after the five load/unload cycles. Both parameters progressively increase with increasing internal microcracking and expansion in concrete affected by ASR (see Figure E1 in Appendix E); however, the energy dissipated parameter was found to best correlate with AAR expansion, and is consequently recommended for assessing the expansion attained to date in concrete affected by ASR. However, the results were found to be aggregate-dependent, which requires the use or the establishment of a calibration curve suitable to the particular reactive aggregate involved or a similar type (see Section E.2.9 in Appendix E).
Cores for stiffness damage testing can be extracted from various members showing mild to severe surface cracking (or other symptoms suggestive of ASR) to determine if any changes in properties are observed and, if so, the extent of such a variation. This testing is recommended because of the inherent variability in the internal damage of deteriorating concrete members, which a set of SDT specimens consists of a minimum of three cores. Moreover, when the number of cores available for mechanical testing is limited, the modulus of elasticity can be determined in the modified SDT and the compressive strength can be obtained by reloading the cores up to failure after the five load/unload cycles (see Appendix E).
5.3.4. Expansion tests on cores
The potential for further expansion due to ASR is a critical parameter to consider when selecting the most appropriate remedial action(s) for concrete affected by ASR. Current rates of expansion are best established from periodic or continuous in-situ monitoring of deformations, which can then be extrapolated for estimating the potential for future expansion. However, in-situ monitoring will generally take a minimum of 2 and preferably 3 years to yield useful information, i.e., where permanent and cumulative deformation due to ASR could "reliably" be differentiated from reversible and cyclic movements related to mechanical (loading, traffic, operation conditions, etc.), thermal and climatic (daily and seasonal) variations.
On the other hand, expansion tests on cores can yield results in a relatively short period of time, e.g., 6 months to 1 year, which makes them fairly popular techniques for assessing the potential for further expansion of ASR-affected concrete (prognosis), and for diagnosis as well. It is, however, important to mention that expansion tests on concrete cores extracted from deteriorating concrete structures will often lead to more pessimistic results than that obtained from in-situ monitoring, as the cores are not tested under the same environmental (temperature, humidity, wetting-drying, freezing-thawing, etc.) and stress conditions.
The procedures used for testing cores from ASR-affected structures vary greatly from one study to another (Bérubé et al. 2004). Based on several studies carried out over the past decade, two following test procedures are recommended, which are described in Appendix F:
- Expansion test in air at > 95 percent R.H. and 38°C (100°F).
- Expansion test in 1N NaOH solution at 38°C (100°F).
The first test is the most realistic for evaluating the potential for further expansion of ASR-affected concrete. The concrete is tested with its own alkali content and the test conditions used are similar to those used in the Concrete Prism Test (ASTM C 1293). The second test in alkaline solution is recommended for determining the "residual reactivity" of the aggregates present in the concrete under study.
Because of the inherent variability in the extent of ASR in deteriorating concrete members, it is recommended that a set of specimens for expansion testing should consist of a minimum of three cores for cores stored in humid air and two cores for testing in the NaOH solution), minimum 100 mm (4 in) in diameter and 200 mm (8 in) long. One of the main issues with the expansion test on cores at 38°C (100°C) and R.H. > 95 percent is that cores suffer from significant alkali leaching under those test conditions; consequently, the expansion tends to level off after a certain time, which is not really due to consumption of reactive mineral phases or alkalis. The use of larger cores, e.g., 150 mm (6 in) in diameter, could contribute to reducing that effect, as they would be less susceptible to alkali leaching during the test and would likely lead to more reliable results (see Appendix F).
5.3.5. Expansion tests on concrete made with coarse aggregates extracted from cores
A method of determining the "residual aggregate reactivity" when the reactive phases belong to the coarse aggregates is to extract those aggregates from cores and test them through the standard Concrete Prism Test ASTM C 1293. The procedure for aggregate extraction has been recently revisited and is described in Appendix G.
5.3.6. Water-soluble alkali content of concrete
The higher the alkali concentration in the pore solution, the higher the OH-concentration (thus the pH), and the higher the risk for ASR. Therefore, the determination of the above concentrations could be useful for assessing whether or not it is sufficiently high for ASR to be maintained, and may thus provide valuable insights in the process of selecting remedial actions.
As it is difficult to extract pore solution from old concrete, an indirect way of measuring the chemical composition of this solution consists in determining the water-soluble alkali content of the concrete by subjecting a representative sample of concrete to hot-water extraction. In the method described by Bérubé et al. (2002a) and presented in Appendix H, two 10g subsamples of ground concrete (< # 100-sieve) are placed in boiling water for 10 minutes and the solution left to stand overnight. The solution is then filtered, the volume of solution completed to 100 ml with distilled water and the Na and K concentrations measured through chemical analysis. The alkali and OH-concentrations in the original pore solution could be calculated from the above values (see Appendix H), and expressed on a kg/m3Na2Oeq basis. It is important to mention that about 40 percent of the cement alkalis are incorporated in the cement hydrates, thus not extractable through the hot water extraction method. For example, a concrete with a total alkali content of 3 kg/m3(5 lb/yd3) Na2Oeq(e.g., concrete incorporating 300 kg (660 lb) of a cement with 1 percent Na2Oeq) would yield about 1.8 kg/m3(3 lb/yd3) of water-soluble Na2Oeq (i.e., 60 percent x 3 kg/m3) (i.e., 60 percent x 5 lb/yd3).
The measurements of the water-soluble-alkali content in concrete can yield interesting information in assessing whether or not the concrete tested contains sufficient alkalis for ASR (diagnosis) or to sustain this reaction (prognosis). However, the results should be used with care as the experimental variability of the method is relatively high, the estimated coefficient of variation being about 20 percent (between 10 and 15 percent when a control concrete is tested in parallel, as recommended).
5.4. Numerical AAR Simulation Modeling
A large number of more or less sophisticated numerical models have been developed for the diagnosis of ASR, the assessment and structural integrity, forecasting of further expansion and long-term stability (prognosis), and prediction of structural responses to various mitigation measures (Léger et al. 1995, CSA 2000). Due to the complexity of the ASR expansion process, any simulation model, as sophisticated as it might be, must be calibrated based on the monitoring data and pertinent information obtained from in-situ and laboratory investigations; for instance:
- Geometry of the concrete member or structure under study (drawings).
- Aggregate reactivity and concrete expansivity (in-situ monitoring, petrography, expansion tests on cores, water-soluble alkali content, etc.).
- Environmental conditions (humidity, temperature) (which can be monitored or assessed).
- Mechanical, physical, and thermal properties of concrete (laboratory testing).
- Internal stress conditions (reinforcement, prestressing, postensioning, loading, confinement, etc.; stresses in concrete and reinforcing steel) (which can be measured in-situ or assessed).
- Location of discontinuities (important cracks) (site inspection).
- Results from in-situ structural investigations (loading capacity, concrete/steel bond, etc.).
- Current rates of dimensional changes and buildup of internal stresses (movements, deformations, crack width, stresses in concrete and reinforcing steel, etc.) (from in-situ monitoring or using indirect procedures).
5.5. Collective Assessment of In-situ and Laboratory Investigations
The collective assessment of all in-situ and laboratory investigations, and from modeling as well, addresses the following subjects:
- ASR expansion attained to date.
- Current expansion rate.
- Potential for further expansion due to ASR.
- Risks relative to structural integrity.
- Risks relative to security of persons and properties.
- Potential for further deterioration due to other mechanisms.
- Decision criteria for application of mitigation/remediation measures.
In the case of reinforced concrete members (e.g., bridges), the potential for further expansion due to ASR will be expressed by the number of years before the reinforcing steel yields (in the direction of lower or lack of restraint) could occur, which requires data on the ASR expansion attained to date and the current expansion rate. In the case of concrete pavements, the potential for further expansion due to ASR will be expressed by the number of years before the joints could close, which requires information on the current expansion rate and widths of joints.
Decision criteria for application of mitigation measures are proposed in the following sections, which are based on the above time limits and on other critical considerations, such as the risks relative to structural stability and security of persons and properties, and the potential for further deterioration due to other mechanisms. The selection of the particular measures to apply in the particular case under study is discussed in Section 6.0.
5.5.2. ASR expansion attained to date
Three methods can be used for estimating the expansion attained to date in ASR-affected concrete members; those are listed hereafter from the most to the least recommended method:
- In-situ monitoring of deformations and movements since construction.
- Modified Stiffness Damage Test (SDT) (see Section 5.3.3 and Appendix E).
- Surface Cracking Index (see Section 4.2 and Appendix B).
Monitoring since construction - This is the best method for determining the expansion attained to date in ASR-affected concrete members. This is often possible in the case of large concrete dams or civil structures that have been instrumented during, or shortly after construction; however, this was not a common practice and is consequently rarely possible in the case of transportation structures.
Modified Stiffness Damage Test (SDT) - The modified SDT can be used for determining the expansion attained to date by the concrete under study. The test procedure and the determination of the expansion attained to date are described in Appendix E.
The results obtained appear more reliable and relatively less variable than those obtained from surface cracking measurements, while not being affected by the outdoor exposure conditions. However, as mentioned in Section 5.3.3, the results were found to be aggregate-dependent, which requires the establishment of a calibration curve suitable to the particular reactive aggregate involved or a similar type. A number of such curves are presented in Appendix E for different types of reactive aggregates, and could be used in the absence of more pertinent data. Appendix E also describes a methodology for the establishment of calibration curves for the SDT.
It must be mentioned that the information gathered from the modified SDT strictly applies along the axis of the cores tested, which direction, however, is generally the most critical (i.e., the direction along which the expansion is the most important since cores are usually taken perpendicular to the direction of the main reinforcement or restraint). However, the test seems to underestimate expansions in directions perpendicular to prestressing cables or rods (Bérubé et al. 2005).
Cracking Index - As mentioned in Section 4.2, the cracking at the surface of a concrete member reflects differential deformations (expansion or contraction) between the surface and the inner concrete due to various mechanisms such as ASR, sulphate attack, freezing/thawing, and shrinkage.
In the absence of any information from monitoring since construction or from modified SDT's, the expansion attained to date in concrete members where ASR is the main contributing factor to cracking, can be roughly estimated from the measurement of the Cracking Index. In the method presented in Section 4.2 and described in Appendix B, measurements along the three or two accessible directions are recommended in order to separately assess the expansion attained to date along each direction. For each direction of measurement, the expansion due to ASR (in the absence of any other mechanism causing surface cracking) is considered equal to the Cracking Index discussed in Section 4.2 and Appendix B, i.e., to the sum of the openings of all cracks intersected along the measured lines divided by the total length of these lines.
ASR expansion to date (%) =100 ∑ crack widths / total length of measurement lines
It must be emphasized that the measurements must be made on the most exposed concrete sections, i.e., where the conditions (variations in humidity conditions, alkali leaching) are largely contributing to limit ASR-expansion in the near-surface concrete layer. It is thus considered that the expansion obtained from the measurements of surface crack widths, resulting from that differential expansion (inner expansion - surface expansion) is then closer to the actual expansion encountered within the whole mass of the concrete member under investigation.
5.5.3. Current rate of expansion due to AAR
Three methods can be used for estimating the current rate of ASR expansion in the concrete member under study; those are listed hereafter from the most to the least recommended method:
- In-situ monitoring of deformation for at least 2 and preferably 3 years (see Section 5.2.3 and Appendix D).
- Expansion tests on cores (see Sections 5.3.4 & 5.3.5 and Appendix F) combined with determination of the water-soluble alkali content (see Section 5.3.6 and Appendix H) and the humidity, temperature, and stress conditions prevailing in the field (Figure 2 and Appendix I).
- In-situ monitoring of surface cracking for at least 2 and preferably 3 years (see Section 4.2 and Appendix B).
In-situ monitoring - As discussed previously, monitoring the movements and deformations within an AAR-affected concrete member is the best method of determining the current rate of expansion in this member, which can be then extrapolated to the next 5 to 10 years. Because of the variability on the temperature and moisture conditions in the field, in-situ monitoring of expansion and deformation should generally be carried out for a minimum of 2 years, preferably 3 years, in States where large annual temperature variations are observed.
Expansion tests on cores combined or not with the water-soluble alkali content (and considering the humidity, temperature, and stress conditions in the field) - In the absence of reliable monitoring data, the approach proposed by Bérubé et al. (2002b), illustrated in Figure 2 and described in Appendix I, can be used for estimating the current rate of expansion in ASR-affected concrete in the field. The method uses the following parameters:
Figure 2. Laboratory assessment of the current rate of AAR expansion in concrete members in-service either already affected by AAR or not. (After Bérubé et al. 2002b).
- The "residual concrete expansivity," which is determined by testing core samples in air at > 95 percent R.H. and 38°C (100°F) (see Section 5.3.4 and Appendix F).
- The "residual aggregate reactivity," which is determined by testing core samples in 1N NaOH solution at 38°C (100°F) (see Section 5.3.4 and Appendix F) or, when the reactive aggregates belong to the coarse fraction, by testing coarse aggregates extracted from cores through the Concrete Prism Test ASTM C 1293 (see Section 5.3.5 and Appendix G).
- The amount of (water-soluble) "active" alkalis in the concrete, which is estimated through the hot-water extraction method described in Section 5.3.6 and Appendix H.
- Estimates of the humidity, temperature, and compressive stress conditions (confinement, reinforcement, prestressing, postensioning, loading, etc.) to which the concrete member under investigation is subjected in the field.
The individual risk indices corresponding to each of the above parameters are combined to estimate the current rate of ASR expansion in this member in the field, either already or not yet affected by ASR.
Cracking Index - In the absence of any information from in situ monitoring or from expansion testing on cores, the current rate of expansion in ASR-affected concrete members can be roughly estimated from the periodic measurement of the Cracking Index (see Section 4.2 and Appendix B).
5.5.4. Potential for further expansion due to AAR
Ultimate ASR expansion in the field - According to the Institution of Structural Engineers (ISE 1992), the maximum ASR expansion that a concrete member may suffer in the field can be obtained from the determination of the maximal expansion of concrete cores tested in humid air (> 95 percent R.H.) at 38°C (100°F). However, such an assumption does not take into account the alkali leaching during the test and the consequent flattening-off of the expansion curve (see Section 5.3.4).
Reinforced concrete members of bridges (delay before steel yielding) - According to the Institution of Structural Engineers (ISE 1992), the most important criterion behind the decision of applying some remedial measures for an ASR-affected reinforced concrete member is its expansion attained to date. Indeed, if the steel/concrete bond remains good, which would suggest that concrete and steel reinforcement expansion is similar, one can estimate the condition of the steel with respect to its elastic limit. This limit is considered to be about 0.20 percent expansion in absence of any more relevant information.
Moreover, based on the current rate of expansion, one can estimate the number of years before the reinforcing steel could reach/exceed its elastic limit, thus before the structural integrity could be at risk:
Delay before steel yielding (yrs) =[0.20 -Expansion to date (%)]/Current expansion rate(%/yr)
The expansion to date and the current expansion rate can be determined using one of the methods proposed in Sections 5.5.2 and 5.5.3, respectively. However, results from cracking measurements on concrete surfaces where the steel/concrete bond may be defective could lead to misleading estimates of expansion to date and /or current expansion rate.
In the case of reinforced concrete members (bridges), the urgency of applying remedial actions will be partly based on criteria related to the delay before steel yielding. More rapid intervention is required in the case of a reinforced member whose reinforcing steel is near yielding conditions, whatever its age is. In any case, it is recommended that further action be taken when the delay before steel yielding is estimated to be less than 5 years, for example by starting an in-situ monitoring program of expansion, with measurements at least on a yearly basis, and/or by performing a structural assessment of the member/ structure in question; it would be appropriate to confirm an assessment that has been based essentially on expansion tests on cores rather than on in situ monitoring.
Concrete pavements (delay before closure of joints) - In the case of concrete pavements, the current rate of ASR expansion is a most critical parameter that can be used for estimating the number of years before the expansion joints could close, thus causing spalling at joints :
The current expansion rate can be determined using one of the methods proposed in Section 5.5.3.
In the case of concrete pavements, the urgency of applying some mitigation measures will partly be based on criteria related to the delay before the closure of expansion joints occur. In any case, it is recommended that further action be taken when the delay before joint closure is estimated to be less than 5 years, for example by starting an in-situ monitoring program of expansion, if not done, with measurements at least on a yearly basis; it would be appropriate to confirm an assessment that has been based essentially on expansion tests on cores rather than on in situ monitoring.
5.5.5. Risks relative to structural stability (reinforced members of bridges)
The structural assessment of an ASR-affected concrete member/structure requires the knowledge of the current stress conditions and strength of the member/structure. The stress conditions may be determined from in-situ measurements (e.g., compressive stresses in concrete using overcoring techniques and tensile stress determination in reinforcing steel; see section 5.2.4), manual calculations, or numerical modeling. The strength of materials of the structure may be determined based on results from mechanical tests on cores that are adjusted to field conditions, usually using a numerical model. Full-scale load tests can also be used to assess the load carrying capacity of the structure (see Section 5.2.7). The evolution with time of the strength of materials of ASR-affected concrete structures can also be predicted using a numerical ASR model which is populated with the data resulting from all relevant in-situ and laboratory investigations.
ASR-affected concrete may present significant losses in strength and elastic modulus (see Section 5.3.3). However, for the whole reinforced concrete member from which the cores were taken, the structural strength (flexural, compressive, and shear strengths) may remain quite satisfactory if well-anchored three-dimensional reinforcement is present in sufficient amount. Such reinforcement reduces the expansion of the surrounding concrete which can then become "chemically" prestressed. However, the concrete expansion can also result in steel yielding, loss of concrete/steel bond, concrete delamination, with potential weakening of the structural integrity of the concrete member or structure.
The structural assessment of an AAR-affected member or structure must focus on the following aspects:
- Presence or potential for formation of major cracks that could affect the stability of the structure.
- Potential for sliding along horizontal construction joints.
- Potential for bearing or crushing failure of concrete where sharp changes in structural geometry occurs.
- Potential for steel yielding, concrete/steel bond reduction, and concrete delamination.
The decision should be made concerning the application of appropriate remedial measures based on the presence of one or more of the above features, which all constitute a risk for the integrity/stability of the structure and for the security of peoples and vehicles as well.
5.5.6. Additional risks relative to security of persons and vehicles
The stability/integrity of an ASR-affected concrete member or structure may be totally safe; however, concrete fragments of various sizes can be detached from portions of the deteriorating pavement or bridge structure, due to various deterioration mechanisms causing spalling, and overall degradation (ASR, corrosion, freezing/thawing, sulfate attack, excessive loading, etc.). The detachment of such fragments would represent a threat to the public and should thus be avoided by taking immediate remedial actions.
5.5.7. Potential for further deterioration due to other mechanisms
Concrete bridges and pavements are most often exposed to aggressive conditions which will eventually impair their durability or serviceability. In particular, exposure to chloride (deicing and/or anti-icing chemical, sea water, etc.) may cause concrete degradation, corrosion of reinforcing or prestressing steel and/or concrete spalling. Wetting/drying and freezing/thawing cycles may cause/enhance surface cracking, and deterioration. Freezing/thawing may significantly increase the expansion and the deterioration in ASR-affected concrete due to the presence of preexisting ASR microcracking, despite the presence of an appropriate air void system and even when ASR is almost completed.
When exposed to such aggressive conditions, the potential for further deterioration of ASR-affected concrete members shall be investigated, even if they do not present any significant potential for further expansion due to ASR. A decision shall then be made about the necessity or not to immediately apply some mitigation measures (e.g., penetrating sealer, etc.). According to CSA A23.36("Design of Concrete Structures"), the crack widths in reinforced concrete for durability should not exceed 0.1 to 0.4 mm. The ISE (1992) considers that cracks in excess of 0.15 mm (0.006 in) in width should justify further investigations; ACI committee 224 also suggests a 0.15 mm (0.006 in) limit criterion for structures exposed to wetting and drying conditions. The decision criterion proposed in this document corresponds to a maximum of 0.15 mm (0.006 in) for the crack width.
5.5.8. Summary of decision criteria for application of mitigation/remediation measures
Based on the above discussion, the decision to immediately apply some appropriate mitigation/remediation measures could be taken on the basis of the criteria given in Table 9.
|Decision criterion||Based on following investigations||Immediate action(s) recommended|
|Risk of steel yielding (expansion > 0.2 percent) in 5 years or less (reinforced members of bridges).||
|Risk of closure at joints by 5 years or less (unreinforced members of bridges, pavements).||
|Risk relative to structural stability, excluding imminent steel yielding, considered above (reinforced members of bridges).||
|Additional risk relative to security of persons and vehicles due to detached concrete fragments (bridges and pavements).||
|Low potential of further expansion due to AAR but risk for rapid further deterioration due to other mechanisms (corrosion, sulphate attack spalling, scaling, degradation,...) (bridges and pavements).||
3Strain measurements are taken between different points on a structure using a single instrument, the demountable mechanical strain gauge (DEMEC), which consists of a standard or a digital dial gauge attached to an Invar bar. The points between which those measurements are taken are pre-drilled stainless steel discs or bolts which are attached to the structure with adhesive or drilled and "cemented" into the concrete (see Appendix D).
4In the overcoring technique, a hole is drilled in a concrete element to a selected depth (as required for the investigations). The end-face of the hole is ground flat to allow fixing of a strain gage. Drilling is then performed at a smaller diameter around the gage. Unexpected/excessive increase in strain could be measured as a consequence of relaxation in compressive stress (due to confinement release) in concrete experiencing expansive pressures due to deleterious mechanisms such as ASR.
5One should remember that any reduction in performance may be due to a combination of mechanisms, and that the test results were obtained on specimens that have been extracted from their restrained conditions; consequently, they may not represent the exact condition (or structural capacity) of in-situ concrete.
6Canadian Standards Association (CSA), "Design of Concrete Structures," CSA A23.3-94, Canadian Standards Association, Mississauga, Ontario, Canada, 2000.