CONCRETE TESTING

ASTM C 856 "Standard Practice for Petrographic Examination of Hardened Concrete"

Before entering into the discussion of various sample collection tips, a few things are important to note:

​

• Locations to collect samples for petrographic examinations must be decided carefully, not randomly, with a prior discussion with the petrographer. Field photos of the distress to investigate should be provided to the petrographer to review before initiation of any sample retrieval step. Petrographer should have an opinion with the project engineer to collectively make a proper planing for sample retrieval.

• Along with overall field photos of the distress under investigation, close-up photos of condition of concrete at the locations selected for sample retrieval must be documented before any retrieval process starts. 

• Number of samples to collect depends on various factors, e.g., variations in the extent of distress, variations in the types of distress, area affected, any variation in concrete mix and its role on the distress, location of distress (if it it preferential), testing budget, etc. A reference sample from a sound location, if any, may be taken for comparison.

• A detailed background information of the project should be provided in a letter. For a progressional report, a petrographer requires to review all background information, product literature, mix design, and prior tests to understand the purpose of investigation, which is usually used in the report to set the stage for first providing the results of petrographic examinations and then interpret the results by incorporating the background information.    

​

• Concrete samples can be provided in saw-cut sections, as a chunk, or, most preferably as cores drilled from the chosen locations of structure. Cores should be 3 to 4 inches in diameter, where the diameter of core should be at least 3 times the nominal maximum size of coarse aggregate.  For concrete slab-on-grade, drill the core through to the slab down to the sub base. For a suspended slab, if full-depth drilling is not possible, drill at least 75% of total thickness of slab. Vertical structures (i.e., tilt-up or perimeter walls, etc.) drill through the wall thickness, if possible.

• Every sample must be properly marked, exposed surface end of the sample should be marked, and the sample should be adequately protected for shipment. 

• If you are investigating a problem or signs of a distress, the sample should contain said distress (i.e., take sample from over a crack, delaminated area, scaled area, etc. if those are your purpose of investigation). 

​

1. Floor-covering Failures - For collecting samples from floor covering failures such as blisters or de-bonding, coring should be done carefully, preferably from over the blisters or de-bonded areas, which are either visible during field survey or detected from subsequent field testing. Care must be taken to prevent damage of the surface beneath the blister or de-bonded coaver, especially if there is evidence of moisture accumulation, re-emulsification of adhesive, or alkali-silica ereaction gel present. The interfacial zone between floor covering and concrete is extremely important in investigation, which must not be contaminated by drilling mud or any other contaminants during core extraction. Many times, a hypodermic syringe is used to collect liquid from beneath the blister before coring, which is then analyzed in our laboratory for pH and water-soluble cation (sodium, potassium, calcium, magnesium), and anion (chloride, sulfate, nitrate, nitrite, phoisphate, bromide, and fluoride) analyses. 

2. Delamination - For investigation slab surface delamination, a prior field survey is mandatory to determine the hollow-sounded versus ring-sounded areas so that appropriate sampling can be done from both areas. For coring over a hollow-sounded area, the very top finished surface may or may not be adhered to the main body of core after retrieval, in which case the delaminated top surface should be fitted to the main body and wrap with a duct tape for shipment.  

3. Surface Scaling - For investigation of surface scaling, core should be collected from scaled locations encompassing all types of visible distress, e.g., from visible scaling or loss of finished surface to areas where near-surface coarse aggregate particles are exposed, to the areas of aggregate pop-outs where fractured remains of aggregates are exposed. For areas where original finished surface is still adhered but only loosely to the main body of slab, a few thin sheets of loose scales or loosely adhered sheets of scales should be collected along with a core drilled through the loosely adhered scales. 

4. Staining, Discoloration - For investigation of surface staining and discoloration, cores must be drilled from over such stained and discolored areas to investigate the compositions of stains and discolorations and materials responsible for those distress. For soft, white powdery efflorescence deposits, such samples should be brushed off with a metal brush and collect in a plastic ziplock bag for analysis.

5. Fire Damage - For investigation of fire damage, cores should be collected from multiple locations covering locations from the most damaged (often cracked, spalled, discolored, or covered with black smoke) to the least damaged (or undamaged) areas. Care must be taken to collected the core from most damaged ares, especially if damage is severe from fire, in which care intact core may not be possible to retrieve instead of rubbles at least near the exposed surface that was in direct contact with fire (from extensive cracking of concrete surface from fire).  

6. Freezing Damage - For investigation of potential freezing-related deterioration of an outdoor slab such as bridge deck, core should be collected from variably distressed areas through majority of the thickness of deck. For old bridge decks, often surface-parallel cracks are formed from cyclic freezing and thawing which makes retrieval of an intact core almost impossible. In such cases parallel pieces of concrete are often retrieved from coring, all of which must be aligned in order after core retrieval and carefully wrapped with tape and bubble wrap and send through plastic ziplock bags. 

7. Sulfate Attacks - For investigation of external sulfate attacks, depending on the severity of attack, exposed surface of concrete may be severely corroded often with white secondary deposits of gypsum, ettringite, or thaumasite. In those cases, surface deposits must be collected separately in ziplock bags before the cording operations. Cores should be through the full depth of the structural element of investigation where sulfate profiles will be determined from subsequent water-soluble sulfate analyses of concrete along with detailed petrographic examinations, SEM-EDS studies, and XRD-XRF studies. 

8. Iron Sulfide Distress - For investigation of cracking due to oxidation of pyrite or pyrrhotite, cores must be collected from over visible distress, e.g., from over parallel or map cracks in case of cracking or crumbling of walls or from cracked slab in case of pyrite oxidation related heaving. Any white deposits on the exposed surface must be collected for analysis. 

9. Alkali-Aggregate Reactions - For investigation of distress due to alkali-aggregate reactions, often cracks along with white deposits of reactions gels are seen in which case both the core drilled from over visible cracks as well as surface deposits should be collected. Cracks often appear as network of closed polygonal-shaped map cracking where cores should include some of those visible cracks on surface. 

10. Proprietary Grout Distress - For investigation of distress of anchoring grout or any proprietary rapid setting, shrinkage-compensating, high early strength products distress often originates as cracking and spalling at the product under investigation and often radiates out to the surrounding concrete. Core should be collected from the distressed locations of those products. Depending on the extent of damage, only powders or rubbles instead of intact core may be able to retrieve. 

11. Cracking - For investigation of any visible surface cracking, e.g.. from mechanisms related to volumetric instabilities from shrinkage expansions due to physical or chemical deteriorations in concrete to subbase settlement, irrespective of its origin, it is advisable to collect cores from over such cracks so that the depth of cracking can be measured along with petrographic examinations to investigate the reasons for cracking. 

12. Low Strength - For investigation of lower-than-design compressive strength of concrete, along with samples companion to the ones provided the low strength, a reference sample of good/anticipated strength should be provided to further examine the determined reasons for low strength in the good sample. Usually, for low strength investigation of laboratory-cured cylinders, an intact untested cylinder companion to the ones showed low strength from ASTM C 39 test should be provided assuming all cylinders have experienced the same curing condition until the tests. If an intact cylinder is not available, then a chunk of a tested cylinder can be provided. For low strength investigation of field samples, a prior investigation of strength variations of concrete either from in situ impact hammer testing or, more preferably, testing of a drilled core according to ASTM C 42 should be done to determine the cores to select for petrographic examinations for investigation the reasons for low strength. Mix design (not batch ticket) of the concrete should be provided to compare the design air content and water-cementitious materials ratios with the estimated values from petrography.   

13. Chemical Attacks - For investigation of various chemical attacks in water treatment plants, cores must be drilled from the corroded surface that was in contact with the chemicals through measurable depths, which should be determined from discussions with the project engineer. A plethora of testing will be needed from petrography to various chemical profiles through depth.

14. Pipe Distress - For investigation of mortar coatings on prestressed concrete cylinder pipe, bar-wrapped pipe, or reinforced concrete cylinder pipe, or sewer pipe usually broken/failed pieces of pipe are collected for investigation where the exposed (convex) end of the sample often weathered was in contact with the outer environment (e.g., soil) and inner end, often corroded was in contact with the effluent. In PCCP samples, often corroded prestressing wire or the wire cast surface is seen in the fractured inner surface end. Often in reinforced concrete pipe samples both the coating from outer end and the liner from inner end are collected with a prestressing steel in the middle.  These samples are carefully processed for detailed petrography, water absorption and volume of permeable voids, and chloride and sulfate profiles from the exposed end to the interior. Effects of chemical alterations of liner or the inner surface of pipe in contact with effluent is examined in thin sections with petrographic microscope and SEM-EDS analysis. 

15. Fill Materials - During investigation of fill materials for potentially expansive materials, e.g., in cases of heaving of asphalt pavement, a representative sample of the fill must be carefully collected from which after coning and quartering a smaller sub-sample is selected for analysis. 

16. Parking Garages - For assessment of existing condition of a parking garage, cores should be collected from over the protective coating, if any, through-depth for a variety of tests from petrography to SEM-EDS, various chemical profile analyses, air-void analysis, compressive strength, FTIR analysis of coating, etc. Number of cores to be collected depend on various factors, as mentioned before, which should be determined from prior discussion with the goal that number of cores to be collected must adequately cover and represent the overall condition or the distress, if any, under investigation.    

17. Bridges - For evaluation of an existing bridge deck and substructure, cores are collected not only vertically from the deck but also horizontally from the walls of substructure where latter cores could be quite long depending on the depth of drilling through the substructure. In such cases, a prior discussion is needed to determine which portions of the recovered core from the substructure should be tested for what purpose with the rule of thumb that the core for petrography must be collected from the exposed end, which was in contact with the environment, along with one from the interior end, if possible. Any portion decided for compressive strength testing should preferably be free of any visible cracks or other forms of distress (otherwise presence of any such must be mentioned in the strength report).    

18. Pipe Clogs - For investigation of any clog material from a pipe, the clog material itself along with the suspected concrete (the source), which is suspected to have caused clogging should be collected where a side-by-side comparison of components of both samples during petrographic examinations will be provided to assess the potential source for clogging of pipe. 

19. Admixture Issues - For investigation of the effect of a chemical admixture in concrete, which may have caused, e.g., delayed setting, accelerated setting, or any other abnormal behavior during construction, a sample of the admixture in question must be provided with the concrete since FTIR analysis, which is a common investigation for such projects often requires a reference run with the admixture in question before running a solvent-extracted residue of concrete to search functional groups of the admixture.

20. Miscellaneous Issues - For miscellaneous other investigations, not possible to include in this list a prior phone call with the petrographer is needed to decide the proper sampling protocol. For over 25 years of our investigation of various concrete deterioration testing more than 10,000 samples it is impossible to cover the ranges of investigation we have done where sample collection strategies varied from project to project. 

​