Petrographic Methods of Examining Hardened Concrete: A Petrographic Manual
APPENDIX E. EXAMPLES OF REASONS PETROGRAPHIC SERVICES ARE REQUESTED AND CORRESPONDING PLANS FOR ANALYSIS
Situation: Concrete submitted failed test for required compressive strength.
- Client’s question: What makes this concrete so much weaker than it was designed to be?
- Preliminary plan of analysis: (1) Perform analysis of air-void system to determine if excess air is a major cause of low strength and (2) perform general examination with stereomicroscope with an emphasis on paste quality and aggregate-paste bond.
Situation: Mixture was harsh and workability was very poor.
- Client’s question: Why? Is the coarse aggregate too large, of a difficult shape, or both? Does the aggregate have abundant reentrant angles and angular particle shapes? Was the void content of the fine aggregate within specifications?
- Preliminary plan of analysis: (1) Perform general examination with stereomicroscope with an emphasis on size, shape, and lithology of aggregates and the quantity of paste and (2) consult with client and any involved testing laboratories to acquire test results on aggregates or obtain samples of aggregates for testing.
Situation: Inspector reported that contractor added water because workability was poor.
- Client’s question: Did the contractor add too much water? Did he or she add it too late? Was it mixed in properly?
- Preliminary plan of analysis: (1) Perform analysis of air-void system and (2) perform general examination with stereomicroscope with an emphasis on variations in paste color and quality throughout mass, condition of aggregate-paste bond, and distribution and size of air voids.
Situation: Inspector reported that tines used for surface texturing seemed to tear concrete surface rather than produce usual continuous lands and grooves.
- Client’s question: Why? What is the depth of the damage?
- Preliminary plan of analysis: (1) Perform general examination with stereomicroscope with an emphasis on size and shape of aggregates and condition of paste and (2) prepare several lapped surfaces that are at right angles to both wearing surface and grooves for examination of depth of damage. Consider using special preparation methods (see section 5.3.3).
Situation: Inspector reported that vibrating screed failed to operate and the concrete was handscreeded.
- Client’s question: Was the quality of the concrete adversely affected? Was the consolidation sufficient?
- Preliminary plan of analysis: (1) Perform analysis of air-void system, including sorting voids encountered into entrained, entrapped, and irregular and (2) perform general examination with stereomicroscope with an emphasis on paste quality.
Situation: Air-meter test showed that mixture had low air content; however, contractor placed mixture anyway.
- Client’s question: Is the air-void system sufficient to provide durable concrete that is able to resist damage from cycles of freezing and thawing?
- Preliminary plan of analysis: (1) Perform analysis of air-void system and (2) perform general examination with stereomicroscope.
Situation: There was a driving rainstorm during placement of concrete.
- Client’s question: Is the concrete overwatered? Depth of damage? Durability of the surface texture? Ability of the surface to resist abrasion?
- Preliminary plan of analysis: Perform general examination with stereomicroscope of several lapped surfaces that are at right angles to both wearing surface and grooves to examine depth of damage and quality of paste and check air-void distribution, size, and stability. Consider using special preparation methods (see section 5.3.3).
Situation: Specimens are from a test mixture employing a new material or construction practice.
- Client’s request: General petrographic examination and a report on anything unusual.
- Preliminary plan of analysis: (1) Perform analysis of air-void system and (2) perform general examination with stereomicroscope.
Situation: A crack system is developing in a pavement (or bridge deck) soon after placement.
- Client’ request: Is this plastic shrinkage cracking? Why did it happen?
- Preliminary plan of analysis: Perform careful examination of specimens according to recommendations and suggestions in chapter 4. A visit to the site and additional specimens may be required. Note: The data necessary to answer the “'why” part of this question cannot be found in the petrographic laboratory, but must be sought in the inspector’s notes, an analysis of the weather conditions, and the recorded observations of any people who passed by the site.
Situation: Placement developed a severe crack system over many portions of the surface. Deterioration is becoming worse at an ever-increasing rate, and concrete is requiring constant work from maintenance crews. (Obviously, the younger the placement, the more mystifying and crucial the problem. However, even when a pavement’s life has surpassed its planned life cycle, the need for extensive repairs can seem to be a disaster to a transportation department&s budget.)
- Client’s request: What is causing this problem? How bad is it going to get? What can be done to stop the deterioration?
- Preliminary plan of analysis: (1) Identify aggregate lithologies (thin sections may be required), (2) perform careful examination of specimens according to recommendations and suggestions in chapter 10, (3) inspect placement to determine extent of use of materials present in deteriorated areas and advise client on number and location of specimens required, and (4) obtain copy of chemical analysis of cement batch used if alkali-aggregate reaction is the likely cause. Note: If the deterioration is caused by an alkali-aggregate reaction, the client should be advised that the deterioration may proceed to complete destruction and that there is no known way to stop it. In the case of an alkali-silica reaction, the same materials can be used in the future if sufficient pozzolanic material or ground granulated blast-furnace slag is included in the mixture.
Situation: Shortage of a material from one source necessitated use of an unapproved material.
- Client’ request: Is there anything wrong with the concrete? (Make sure that the concrete fabricated with the approved material is available for comparison with the concrete fabricated with the substitute material.)
- Preliminary plan of analysis: (1) Perform analysis of air-void systems of the two concretes and (2) perform general examination with stereomicroscope of the two specimens.
Situation: Concrete telephone conduit (or drainage ditch lining) in contact with soil was distressed by some mechanism that seems to be dissolving it.
- Client’s request: What is happening?
- Preliminary plan of analysis: (1) Perform general examination with stereomicroscope with an emphasis on aggregate type and presence of minerals that may produce acid during weathering and (2) if no acid-producing agents are found in the concrete, perform petrographic examination of soil and nearby rock outcrops and chemical analysis for contaminants of soil and groundwater.
Client’s question: What is this slick-looking coating (or this dark coating or this white coating) that we find near the cracks in the placement? Does it indicate serious problems?
- Preliminary plan of analysis: (1) Determine composition of exudation (often this may be accomplished by examination of grain-mount preparations using petrographic microscope), (2) request specimens that will include a full specimen of concrete (a depth of 127 or 152 mm is usually sufficient), and (3) perform general examination of specimen of full concrete with stereomicroscope with an emphasis on aggregate types and any possible reaction products.
Client’s question: Why is this concrete such a funny color? Does it mean the concrete was made from the wrong materials? How durable is this sort of concrete?
- Preliminary plan of analysis: (1) Obtain copy of design of mixture and (2) explain to client how particular materials produce different colors.
Client’s question: Why is there a peculiar dark blue-green color so unevenly distributed in this concrete? Does it mean that the mixing was not sufficient? Are there weaker and stronger places in this concrete? (Suspected: Ground granulated blast-furnace slag (GGBFS) was used in the mixture.)
- Preliminary plan of analysis: (1) Inquire if GGBFS was included in mixture or examine thin sections to check paste for presence of GGBFS and (2) explain to client that dark color exists in portions of concrete that retain original moisture and that it fades unevenly as concrete becomes drier. Reassure client that mottled color does not indicate variation in the distribution of cementitious material. Note: The color cannot be restored by rewetting.
Client’s question: Why didn’t this concrete that was placed 3 weeks ago ever set? The concrete can be shoveled like gravel and is almost dry.
- Preliminary plan of analysis: (1) Rewet finer material and test for setting properties, (2) conduct chemical analysis to determine amount of retarder, and (3) recommend an investigation of admixture dispensing system at concrete plant to determine the likelihood of the addition of too much retarder.
Client’s question: What material is in these stalactites found hanging under this concrete bridge? Or, What is this white (gray, beige, etc.) material occurring as a coating on this concrete? What do they mean concerning the durability and strength of the bridge?
- Preliminary plan of analysis: (1) Collect some of material and determine its composition (often this may be accomplished by examination of grain-mount preparations using the petrographic microscope), (2) determine presence or absence of alkali-silica gel (see section 10.2.3), and (3) visit placement and evaluate condition of concrete.
Situation: This concrete set much faster than expected.
- Client’s request: is wrong with it? Did it get properly consolidated?
- Preliminary plan of analysis: (1) Perform analysis of air-void system, including sorting voids encountered into entrained, entrapped, and irregular; (2) perform general examination with stereomicroscope with an emphasis on paste quality; and (3) perform chemical analysis to determine if admixtures (accelerators, water reducers, etc.) are present in specified amounts.
Situation: This concrete had a very high slump and a runny consistency.
- Client’s request: What is the water-cement ratio of this concrete?
- Preliminary plan of analysis: (1) Perform examination of specimens for aggregate segregation; (2) perform analysis of air-void system, including sorting voids encountered into entrained, entrapped, and irregular; (3) perform general examination with stereomicroscope with an emphasis on paste quality and an estimate of water-cement ratio; and (4) if necessary, perform chemical determination of cement content.
Situation: Pavement is pushing out and causing humps in adjacent bituminous concrete.
- Client’s question: Can concrete grow? How? Why?
- Reply: Yes. Particular concretes are made using expansive cement and are designed to expand and thus minimize cracking. This type of concrete may have been used as a repair material. If an original placement appears to be expanding, the concrete may be affected by one of the alkali-aggregate reactions that cause expansion of the concrete (see chapter 10).
- Recommendation: Investigation of problem site by petrographer or engineer experienced in techniques in section 10.2.2, probably including a complete sampling program.
Client’s question: How deep are these cracks? Will they cause corrosion of the reinforcing rods?
- Preliminary plan of analysis: Measure depth of cracks and distance from end of crack (as visible with the stereomicroscope) to reinforcement. If distance to reinforcement is less than 50 mm, report that cracks will probably cause corrosion.
Concrete researcher’s question: Can you (the petrographer) actually see a difference between concrete with a low chloride ion permeability and that with a high permeability (see ASTM C 1202)?
- Reply: In extreme cases, yes. We can sometimes see the differences with the stereomicroscope. In other cases, we can see the differences in fluorescent-impregnated thin sections by the use of the P/EF microscope.
Concrete researcher’s request: Measure these cracks (depth and width at 25-mm intervals) and give me the data so that we can calculate the volume of the crack.
- Plan: (1) Prepare lapped surfaces at right angles to surface; (2) gently mark lapped surfaces with graphite as close to wearing surface as possible and then at 25-mm intervals down the depth of the cracks; and (3) using a stereomicroscope and various stage micrometers, measure the width of the cracks at marks; report results.
Concrete researcher’s request: Compare these two beams subjected to testing for resistance to freezing and thawing and report any internal differences.
- Plan: (1) Prepare lapped internal surfaces and mark microcracks as described in section 8.4 and (2) either count cracks along traverse lines in specimens or visually compare markings.
Concrete researcher’s request: Compare the bond in these different types of overlay (or different methods of achieving overlay bond) and report the location of any cracking or debonding.
- Preliminary plan of analysis: (1) Prepare lapped surfaces that cut across bond at right angles; (2) using stereomicroscope and a pen as described in section 8.4, mark all microcracks; and (3) study relationship of microcracks and any obvious debonding to line of bond between overlay and substrate. Hint: In many cases, especially when removal of damaged material has been vigorous, the break between the overlay and the substrate will not occur on the bond between the two materials, but rather in the substrate 2 or 3 mm below the bond.
Client’s question: “Why is this concrete so good?” (Only once in the more than 35 years of our experience was any HCC submitted with this request.)
- Plan: Examine a number of lapped internal surfaces.
- In this case, the answer is: The concrete in question had a low water-cement ratio and had been fabricated with a particular sort of expanded aggregate that provided a pasteaggregate bond that was much better than that usually found in concrete fabricated with natural crushed stone or gravel.
