Chapter 2. Primary Case Studies

Figure 3-19. Flowchart for assessing the condition of the concrete aggregates as applied to the Spearfish, South Dakota site.

Figure 3-20. Flowchart for identifying infilling materials in cracks and voids as applied to the Spearfish, South Dakota site.

Table 3-9. Identified diagnostic features along with their associated MRD type and significance as related to SD-090-019.

Recommended Prevention Strategies

For the distresses noted, the best preventative strategy is to use a different source of aggregate. Testing in accordance with the guidelines should show that the current source would be unacceptable without mitigation. Mitigation strategies for aggregate freeze-thaw deterioration that could be used if current aggregate source is all that is available include:

To address the potential for ASR, the following strategies can be employed to reduce the reactivity of the aggregate:

If aggregate benefaction is not feasible or cost effective, other strategies can also be employed including:

Regardless of the approach, the design PCC mixture must be tested to ensure that the aggregate freeze-thaw deterioration and ASR have been mitigated.

2.2 TH 65 in Mora, Minnesota (MN-065-064)

Project Description

The Minnesota DOT provided several candidate projects with durability problems. One of the projects-located on TH 65 in downtown Mora-was experiencing severe durability problems concentrated at the transverse joints. This project was selected as the primary case study site for the wet-freeze climatic region. This area receives approximately 660 mm of annual precipitation and has a freezing index of 1030 °C-days.

Table 3-10 presents a summary of the design information for this project. This project extends from milepost 64.2 to 65.0 and is located in both the northbound and southbound lanes. It is a four-lane divided roadway separated by a concrete median; some sections also include an additional lane for left-turn traffic. The pavement, which was constructed in 1989, consists of a 200-mm jointed plain concrete pavement (JPCP) with a 75-mm granular base and a 305-mm granular subbase. The transverse joints are skewed and have a variable joint spacing pattern of 4.0-4.6-5.2-4.6 m. Load transfer is provided by aggregate interlock only; no additional load transfer devices have been employed. The only variation in the two sections is the transverse joint sealant-Section 001 uses silicone sealant and Section 002 uses hot-pour sealant. The longitudinal joints are not sealed. A 2.4-m-wide AC shoulder is placed at the outer edge; there is no inside shoulder due to the concrete median.

Table 3-10. Summary of design features for MN-065-064.

¹ Climatic data are for Minneapolis, Minnesota.

Distress Survey Results

Although the design and construction details are the same, the initial investigation revealed that the southbound lanes were in better condition than the northbound lanes. Thus, two sections were selected for survey, one in each direction. Section 001 is located in the northbound outer lane beginning at milepost 64.6, and Section 002 is located in the southbound outer lane beginning at milepost 64.4. Both sections are constructed at grade. Tables 3-11 and 3-12 provide a summary of the distress survey results for Sections 001 and 002, respectively.

As previously mentioned, Section 001 is exhibiting the worst performance, with most of the deterioration limited to the transverse joints. Joint spalling and bituminous patching are predominant along the transverse joints. The spalling appears to be materials-related and progressed to medium severity in most cases. In some cases, the surface has scaled off and aggregate particles have been exposed. Every transverse joint has been patched over a portion of its length to help address the spalling problem. In fact, the transverse joints were so deteriorated that faulting could not be measured. In addition, maintenance forces on hand during the surveys indicated that removal of the material during the patching operation often extended through the entire depth of the slab. The only other distress noted was a longitudinal crack that extended the length of one slab.

Table 3-11. Summary of pavement condition surveys for MN-065-064-001.

¹ Faulting could not be measured due to deterioration at transverse joints.

Section 002 is in better overall condition but still exhibits some deterioration at the transverse joints. Medium-severity spalling is observed at 6 of the 33 joints (18 percent) and bituminous patches are observed at 12 of the 33 joints (36 percent). However, the deterioration at the affected joints is less severe than that observed on Section 001. Faulting could be measured on this section and averaged 2.0 and 1.7 mm at 0.30 and 0.75 m from the outer slab edge. The only other distresses include a single transverse crack and a single longitudinal crack.

MRD Field Characterization

A more detailed evaluation of the attributes of the MRDs was also conducted in the field. Table 3-13 provides the results of this characterization. Figure 3-21 shows some typical distress manifestations. Although a definitive diagnosis should not be drawn from this evaluation, it does provide information that can help diagnose the distress in conjunction with the laboratory results.

Table 3-12. Summary of pavement condition surveys for MN-065-064-002.

On both pavement sections, the MRD is confined to the transverse joints; it is not exhibited over the entire slab. The distress is exhibited as hairline cracks that typically run parallel to the transverse joint. The cracking appears to initiate at the transverse joint and progresses outward. As the deterioration progresses, scaling occurs at the surface and exposes the aggregate particles. For the most part, the cracking and deterioration is confined to 150 mm on either side of the joint. The cracks do not have any staining or exudate in or around the cracks.

Interestingly, adjacent to Section 001 was a left-turn lane that was part of the original road constructed in the 1950's. This turn-lane appeared to be in excellent condition and it was decided that the turn-lane should also be sampled, thereby providing an insight into a concrete pavement from the same environment, made from similar materials that performed well.

Table 3-13. Summary of MRD characterization for MN-065-064.

Figure 3-21. Typical distress manifestations observed at MN-065-064.

Laboratory Analysis

Core Selection/Visual Inspection

Based on the field survey and site inspection by researchers, the distress was determined to be concentrated at the transverse joints. In addition, the northbound turning lane was recognized as being in good condition even though it was approximately 40 years older than the badly distressed northbound Section 001. The guidelines for sampling were applied and four cores were retrieved from each section. Also, one mid-panel core (core D in standard pattern) was retrieved from the northbound (Section 001) left-turn lane. The cores selected for laboratory analysis were A from Section 002, cores B and D from Section 001, and core D from the left-turn lane. Pictures of these cores are shown in figure 3-22.

Inspecting the cores visually before and after slicing, mix proportions were only noted. If required to understand the distress, an estimate of the relative phase abundance is obtained simultaneously with determination of the hardened air content. The concrete was well consolidated with no apparent segregation or parallelism of the aggregates. Scaling was present in the deteriorated areas but not on the cores examined. Also, no evidence of sub-parallel cracking was apparent on these cores, no entrapped water voids were seen under aggregates, and no surface cracking was seen in the cores. In the mid-panel core D from Section 001, abundant infilling of the entrained air voids was seen and the paste appeared "soft." The mid-panel core from the left-turn lane also had abundant air void infilling but had a much harder paste. Also, the turn-lane concrete had a coarse aggregate that was not crushed and had a larger (25.4 mm) top size. This is in contrast to the concrete in Section 001, which used a crushed aggregate with a 19-mm top size. The concrete at the joint and in contact with the subbase had a depth of carbonation of 5-10 mm. This indicates some deterioration of the paste at those locations.

Stereo Optical Microscopy/Staining Tests

The rock type for the coarse aggregate was characterized as varying, but was dominantly mafic rock types typical of the Superior Lobe. The fine aggregate was the same, but with more quartz. No unusual alteration of the aggregate was observed. For this study, the most useful role for the stereo microscope was for observing stained specimens and determination of the hardened air content. The barium chloride/potassium permanganate stain described in Guideline II colors sulfate minerals a brilliant pink to purple hue as shown in figure 3-23.

Hardened Air-Void Analysis According to ASTM C 457

The hardened air-void content was determined in accordance with ASTM C 457. For the work performed in this study, the modified point count method was used for analysis and the required software was written in house using the free image analysis program from NIST Image 1.62. The results of the analysis are presented in table 3-14.

Table 3-14. Results of ASTM C 457 on concrete from MN-065-064-001.

As can be seen, the original air-void system and the existing air-void system (after infilling) are both inadequate in terms of the Power's spacing factor for each. This indicates a cement paste that is not adequately protected from the cyclic stresses of freezing and thawing. This usually results in cracking of the paste that is then susceptible to ingress of water and deicers.

Figure 3-22. Cores evaluated for MN-065-064.

Figure 3-23. Stereo optical micrographs showing sulfate minerals filling air voids.

Petrographic Optical Microscopy

Petrographic microscopy was used to further examine the infilling material present in the air voids. In addition to ettringite, which was common, hydrocalumite (Friedel's Salt) was also identified. The hydrocalumite was most common in the concrete in contact with the subbase. There appeared to be little difference between the cores from the joint and from the mid-panel of Section 001. In general, there was extensive infilling of voids and the cement paste appeared to be very porous and soft. Therefore, in addition to identifying infilling material within voids and cracks, epifluorescent techniques were used for examining the cement paste in the mid-panel cores from Section 001 and the left-turn lane of the northbound section. This analysis leads to an estimate of the w/c ratio for the concrete.

The technique used is commonly known as the UV dye method for determining w/c and uses a fluorescent dye epoxy impregnation preparation and microscopic observation using UV illumination. This method of w/c determination relies upon the relationship between measurements of cement paste fluorescence and the w/c values of known concrete standards (Mayfield 1990; Elsen et al. 1995; Jakobsen et al. 1997). To determine the w/c of a sample of concrete, fluorescence measurements are made from the cement paste, and related back to fluorescence measurements from the concrete standards. The intensity of the fluorescence measurements depends upon the amount of dyed epoxy absorbed by the cement paste. Cement pastes of higher w/c absorb more of the dyed epoxy because they possess a larger volume of capillary porosity than do cement pastes of lower w/c.

To measure the fluorescence of the cement paste, a concrete thin section is illuminated from above with blue light. The blue light causes the dyed epoxy to fluoresce yellow-green. A blocking filter is used remove the blue light reflected from the surface, allowing only the yellow-green fluorescence to reach the camera (Walker 1992). The camera generates a video signal, which is converted to an RGB digital image on a computer monitor. In the image, each pixel is assigned a 0-255 intensity, where 0 represents pure white (high intensity) and 255 represents pure black (low intensity). The G band of the image contains the most information about the fluorescence, and is used to make the cement paste fluorescence measurement.

To ensure that the illumination of the blue light and the performance of the camera are constant, a method of calibration is needed. Prior to collecting any measurements, a thin section composed of quartz sand in a matrix of dyed epoxy is used to calibrate the system. A digital image of the calibration slide is collected. In the image, the quartz sand appears dark, and the dyed epoxy matrix appears bright. If a histogram is plotted of the image, two distinct peaks are present, one for the quartz sand, and the other for the dyed epoxy matrix. It is important that the positions of the peaks on the x-axis do not shift in order to ensure consistent measurements. Figure 3-24 shows a summary of histograms collected from our calibration thin section. If the peak positions are out of alignment, then adjustments need to be made, either in the illumination, shutter speed of the camera, or gain and offset of the digital capture card. It has been reported that the fluorescence of the dyed epoxy decreases under constant illumination, but recovers to its initial fluorescence if allowed to sit in darkness for a period of 2 hours (Jakobsen et al.1995). Furthermore, the drop-off in fluorescence is most dramatic within the first 2 minutes, so it is important not to pause too long over any given area before collecting a fluorescence measurement.

Another set of parameters that can affect the fluorescence measurements is consistency in thickness of the thin sections, uniformity of impregnation by the dyed epoxy, and the consistency in dosage of dye. It is imperative that the thin sections used are of high quality (Elsen et al. 1995; Jakobsen et al. 1997).

Since concrete is a combination of cement paste, aggregate, and air bubbles, it is necessary to distinguish between the cement paste fluorescence and the fluorescence from aggregates and air bubbles. Generally, the aggregates are less porous than the cement paste, and therefore fluoresce at lower intensity levels, although this may not always be the case, especially when porous aggregates are used. At the other extreme, total porosity, the air bubbles fluoresce at a higher intensity level than the cement paste. At intermediate intensity values, most of the fluorescence can be attributed to the cement paste. Although this is a good approximation, intensity level alone is not enough to determine whether any given pixel represents cement paste, aggregate, or air bubble. Rigorous schemes have been proposed to ensure that the pixels used to make the cement paste fluorescence measurements do not include air bubbles or aggregate, but they were not employed here (Elsen et al. 1995; Gerold 2000). Instead, the distinction between cement paste, aggregate, or air bubble was based solely on fluorescence intensity.

Figures 3-25 through 3-31 summarize the fluorescence measurements from the different standards. Three thin sections were prepared from each w/c standard, and 10 fluorescence measurements were made from different locations on each thin section for a total of 30 measurements per w/c standard. Each fluorescence measurement represents a 2.493-mm x 1.870-mm region on the thin section; therefore, a total area of about 140 mm² was measured per w/c standard. Each measurement consists of a 640 x 480 pixel image, so a total of 9.216 million pixels were collected per w/c standard. Regions containing coarse aggregates were avoided during the sampling. The regions sampled were chosen by the operator, which may introduce bias.

Figure 3-24. Histogram of 16 fluorescence measurements from calibration thin section composed of quartz sand in a dyed epoxy matrix. The peak on the left represents the dyed epoxy matrix, and the peak on the right represents the quartz sand.

It is clear from figures 3-25 through 3-31 that there is considerable variability within the individual standards, although the three distinct fluorescence levels attributed to air bubbles, cement paste, and aggregate can be distinguished in each of the standards. Assuming that the intermediate intensity levels represent the cement paste, channels 75 through 175 were used to quantify the cement paste fluorescence of the standards. The choice of channels 75 through 175, however, does not ensure that the pixels included in the measurement exclusively represent cement paste, nor does it ensure that some pixels that represent cement paste are not omitted. One concern is that the choice of channels 75 to 175 may exclude unhydrated cement grains, which appear as dark specks against the background of cement hydration products and capillary porosity. The overall fluorescence of the cement paste is related to both the capillary porosity as well as the quantity of unhydrated cement grains. In these measurements, variations in capillary porosity alone seem sufficient to distinguish variations in w/c (see figure 3-32). However, with the inclusion of unhydrated cement grains, the variation in intensity versus w/c would perhaps be more pronounced.

There is no doubt that it would be advantageous to employ a more rigorous method to ensure that only those pixels that represent cement paste are used in the measurements, but the simple method employed here performed adequately. The major weakness in the strict use of channels 75 to 175 is that the fluorescence of the cement paste and aggregates are both influenced to some degree by the fluorescence of the surrounding phases. For instance, the cement paste surrounding air bubbles often appears brighter than the rest of the cement paste due to the

Figure 3-25. Histogram of 30 fluorescence measurements from 0.38 w/c standard.

Figure 3-26. Histogram of 30 fluorescence measurements from 0.41 w/c standard.

Figure 3-27. Histogram of 30 fluorescence measurements from 0.42 w/c standard.

Figure 3-28. Histogram of 30 fluorescence measurements from 0.52 w/c standard.

Figure 3-29. Histogram of 30 fluorescence measurements from 0.56 w/c standard.

Figure 3-30. Histogram of 30 fluorescence measurements from 0.74 w/c standard.

Figure 3-31. Histogram of 30 fluorescence measurements from 0.80 w/c standard.

proximity to the air bubbles. For this reason, some researchers exclude cement paste that neighbors air bubbles from recorded fluorescence measurements (Elsen et al. 1995). Similarly, translucent aggregates, such as quartz, may appear brighter if surrounded by brightly fluorescing cement paste. Depending on the appearance of a series of images, it may be necessary to interactively change the interval of channels that is used to represent the fluorescence of the cement paste. However, this would require the operator to repeatedly make decisions that could introduce bias to the measurements, hence the rigid choice of channels 75 to 175 used here.

The first step in quantifying the fluorescence for each individual measurement was to normalize the number of pixels contained in channels 75 through 175 to 100. This step is necessary to account for differences in the volumes of cement paste between standards. For instance, the low w/c standards contained a higher volume of cement paste in order to maintain the workability of the plastic concrete. Next, an average intensity value was determined from the normalized number of pixels in channels 75 through 175. The average intensity value was used as a measurement of cement paste fluorescence. Figure 3-32 plots the average fluorescence intensity values versus the w/c of the standards, along with a best-fit line. The equation for the best-fit line from figure 3-32 can then be used to convert average fluorescence intensity measurements from unknown concrete samples to values for w/c.

The concrete samples analyzed here are from mid-panel of the traffic lane and from mid-panel of a left-turn lane. Thin sections were prepared from the cores at different depths to make cement paste fluorescence measurements. Figures 3-33 through 3-38 summarize the measurements from the old concrete and the new concrete.

Figure 3-32. Average cement paste fluorescence measurements versus w/c. Error bars represent one standard deviation.

Figure 3-33. Histogram of fluorescence measurements from 1950 concrete from mid-panel of the left-turn lane of site MN-065-064-001.

Figure 3-34. Histogram of fluorescence measurements from 1990 concrete from mid-panel of the traffic lane of site MN-065-064-001.

Figure 3-35. Distribution of the w/c values from the 1950 concrete from mid-panel of the left-turn lane of site MN-065-064-001.

Figure 3-36. Distribution of the w/c values from the 1990 concrete from mid-panel of the traffic lane of site MN-065-064-001.

Figure 3-37. The w/c values versus depth from the 1950 concrete from mid-panel of the left-turn lane of site MN-065-064-001.

Figure 3-38. The w/c values versus depth from the 1990 concrete from mid-panel of the traffic lane of site MN-065-064-001.

When using the fluorescent method of w/c determination, it is important to realize that the capillary porosity is not solely a function of the w/c. As both proponents and critics of the technique are quick to point out, the capillary porosity is influenced by a number of different parameters. For example, the degree of hydration, the use of cementitious additives such as fly ash, and the amount of weathering or leaching of the cement paste all influence the capillary porosity, just to name a few (Jakobsen et al. 1997; Neville 1999). Certainly, to make an accurate estimation of w/c, appropriate standards of similar hydration and composition should be used. Since it would be difficult to produce suitable standards for every situation, another approach would be to refer to the fluorescence measurement of w/c as an "equivalent w/c". This makes it clear that the w/c value determined for a sample of concrete is expressed in terms of an equivalent w/c as compared to the standards. In the case of site MN-065-064-001, the w/c standards came from concrete that had cured for 65 days. The concrete from the 1950's has had a long time to hydrate, and it might not be appropriate to directly compare it to the 65-day cure standards. The concrete from 1990 may have undergone some leaching and weathering, so again, it might not be appropriate to directly compare it to the standards as a measure of constructed w/c. However, if it is understood that the w/c measurement is an equivalent to the 65-day cured laboratory standards, then the results are not as misleading.

Scanning Electron Microscopy (SEM)

SEM was used to identify infilling material in voids and cracks. As indicated earlier, the principal materials identified were ettringite and hydrocalumite as shown in the typical SEM micrographs and x-ray analyses presented in figures 3-39 and 3-40.

Figure 3-39.Typical SEM micrograph and x-ray analysis for ettringite infilling air void.

Figure 3-40. Typical SEM micrograph and x-ray analysis for hydrocalumite infilling air void.

Table 3-15 presents the quantitative results from a single spectrum collected from an ettringite deposit, compared to a calculated composition for dehydrated ettringite. Table 3-16 is a summary of 10 analyses of hydrocalumite deposits, compared to calculated compositions for the 3 dehydrated end members of the hydrocalumite solid solution series. The ternary diagram presented previously in figure 3-15 shows the probable range of composition for the hydrocalumite deposits analyzed from the pavement. Hydrocalumite describes a solid solution series with 3 end-member compositions and a range in substitutions between Cl^-, OH^-, and CO₃^2-. The hydrocalumite analyses were low in chlorine as compared to the pure Cl- hydrocalumite end member shown above. Oxygen and carbon were not analyzed so the true composition of the hydrocalumite cannot be determined.

Table 3-15. Quantitative results from single spectrum collected from ettringite deposit, compared to a calculated composition for dehydrated ettringite.

Interpretation and Diagnosis

This test site illustrates a key point with the laboratory analysis of MRD. Namely, standard procedures will provide adequate data for diagnosing the majority of MRD cases. But in some cases, a more in-depth or "unique" analysis must be conducted to fully understand the MRD mechanisms identified. In this case, a more in-depth investigation of the effective w/c led to a better insight into why the distresses observed were occurring.

In the context of a guideline, it is impossible to design an analytical approach that will identify all possible types of MRD in every situation. For more difficult or complex cases of MRD, a State highway agency (SHA) may have to contract with outside labs for petrographic services if such services are not available within the organization. Even if an outside contract is required, the guidelines still assist the analyst or engineer in refining the questions that you want the external petrographer to answer.

Table 3-16. Summary of 10 analyses and theoretical composition of hydrocalumite deposits.

Having performed the described laboratory analyses and applied the diagnostic flowcharts reproduced in figures 3-41 to 3-45, two possible MRDs were identified in MN-065-064-001, including paste freeze-thaw and deicer attack. To finalize the diagnosis, the diagnostic tables were consulted. The diagnostic features identified in the analysis processes are listed below in table 3-17 along with their associated MRD type and significance as related to this pavement. A brief discussion follows of each possible MRD identified in the laboratory analysis:

Paste Freeze-Thaw - Clearly, there were two distinguishing features of this distressed concrete. The first was the softness and high effective w/c for this concrete. The porous paste was evident even in the mid panel when compared to the passing lane constructed with similar aggregates but approximately 40 years before the failed concrete. Of course, changes in cement characteristics and other factors may be contributors to the observed paste characteristics. However, it is unlikely that these factors would result in a difference of the magnitude seen in these pavement sections. It is most probable that a high w/c contributed to the observed distress. In addition, the original air system was marginal and once infilling occurred; the hardened air system became inadequate to protect the weakened paste.

Deicer Attack - This MRD may be opportunistic or it may be a contributor. Given the degree of distress seen near the joint, it is reasonable to infer that deicer attack was a factor. Also, the presence of hydrocalumite is an indicator that this concrete was exposed to a high chloride environment, given the necessary high chloride concentration required for this phase to precipitate.

 

Figure 3-41. Flowchart for assessing the likelihood of MRD causing the observed distress in the pavement as applied to MN-065-064.

 

Figure 3-42. Flowchart for assessing general concrete properties based on visual examination as applied to MN-065-064.

Figure 3-43. Flowchart for assessing the condition of the concrete paste as applied to MN-065-064.

 

Figure 3-44. Flowchart for assessing the condition of the concrete aggregates as applied to MN-065-064.

Figure 3-45. Flowchart for identifying infilling materials in cracks and voids as applied to MN-065-064.

Table 3-17. Diagnostic features identified along with their associated MRD type and significance as related to MN-065-064.

In summary, a combination of heavy deicer use, an inadequate air-void system, and a weak paste resulting from a high w/c all combined to cause the distress in this pavement. With care in batching future concrete mixes and possibly alternative deicers, this type of distress may be minimized in future construction.

Recommended Treatment/Rehabilitation Alternatives

The application of the procedures presented in Guideline III in Volume 2: Guidelines Description and Uses is not straightforward in this case since the high w/c is a major factor in the observed failure, yet it is not considered a common MRD. Even so, the guideline can be used to select feasible treatment and rehabilitation alternatives for paste freeze-thaw deterioration and deicer attack. Based on the visual assessment, Section 001 is clearly suffering high-severity distress in the vicinity of joints, with patching observed at every joint. Section 002 is in slightly better condition, but still would be considered high severity based on the depth of spalling and the number of patches observed. It was also noted by maintenance crews that the deterioration extended through the full depth of the slab. Based on this assessment, the following treatment/rehabilitation options are available:

• Full-depth repairs
• Reconstruction/recycling

The use of full-depth patching is still feasible, although it should only be used as a stop-gap measure with the full realization that deterioration will likely continue at the patch boundary. Ultimately, as the pavement continues to deteriorate, a reconstruction/recycling option becomes more viable.

Recommended Prevention Strategies

For the distresses noted, the best preventative strategy is to ensure that the concrete specified is constructed. It seems evident that the w/c as constructed was well in excess of that specified and that the as-constructed air-void system was inadequate to protect against paste freeze-thaw damage. Further, the deicer attack was also a result of these two factors. If this concrete was constructed as specified, it is unlikely that deterioration would have occurred. Thus, the use of a w/c equal to or less than 0.45 and the addition of an effective air entraining admixture at a dosage sufficient to create an adequate air-void system would be all that was needed to address the observed MRD.

2.3 Near Raleigh, North Carolina (NC-440-015)

Project Description

Pavements with durability problems in the wet-nonfreeze climatic region were not as easy to locate. However, the North Carolina DOT did provide a viable section that was selected as the primary site for the wet-nonfreeze region (this area receives approximately 1070 mm of annual precipitation and has a freezing index of 58°C-days). The section is located on I-440 near Raleigh and exhibits surface cracking over the entire slab area. The project extends approximately 3 km from Pool Road to Raleigh Boulevard in both directions. The highway is a divided roadway with a minimum of three and in some cases up to five lanes in each direction.

The project was constructed in 1982 and consists of a 250-mm JPCP and a 100-mm cement-treated base. A 25-mm AC separator layer is located between the PCC slab and the base course. The transverse joints are doweled, sealed with silicone, and employ a variable joint spacing pattern of 7.6-7.0-5.8-5.5 m. The longitudinal joints were formed using a plastic joint insert and have not been sealed. AC shoulders are located along the inside and outside edge and are 1.8 and 3.0 m wide, respectively. The design information is summarized in table 3-18.

Distress Survey Results

An initial windshield survey was conducted over the entire project. Following this cursory survey, two sections-one in each direction-were selected for more detailed surveys. Both sections are located in the outer traffic lane. Table 3-19 presents a summary of the distress survey results for Section 001; the results for Section 002 are summarized in table 3-20.

Both sections are in similar condition. The predominant distress is map cracking over the entire pavement surface. Several flexible patches (low to medium severity) have also been placed on each section. Faulting is not to the extent where it is creating any reduction in ride quality. The only observed difference is the presence of two transverse cracks on Section 002. Due to the surface cracking, these cracks have begun to deteriorate and have been patched with AC. The transverse joints, which have been sealed with a silicone sealant, are in fair condition. However, it appears that as this MRD continues to progress, it could result in problems in the future.

Table 3-18. Summary of design features for NC-440-015.

Category	Design Feature	Description
General Information	Project limits	Pool Road to Raleigh Blvd.
	Highway type	Divided
	Number of lanes	6
	Direction	Eastbound/westbound
	Construction date	1982
	Cumulative ESALs
Pavement Cross Section	Pavement type	JPCP
	PCC slab thickness	250 mm
	Base	100-mm cement-treated base (CTB)1
	Subbase
	Subgrade type
Transverse Joint	Joint spacing	7.6-7.0-5.8-5.5 m
	Joint skew	None
	Load transfer	Dowels
	Sealant type	Silicone
Longitudinal Joint	Load transfer
	Sealant type	None
Outer Shoulder	Surface type	AC
	Width	3.0 m
Inner Shoulder	Surface type	AC
	Width	1.8 m
Climatic Conditions	Region	Wet-nonfreeze
	Annual precipitation	1070 mm
	Freezing index	58 °C-days

¹ Also includes 25-mm AC separator layer between the CTB and PCC slab.

Table 3-19. Summary of pavement condition surveys for NC-440-015-001.

Distress Type		Distress Measure	Severity Level			Comments
			Low	Moderate	High
Cracking	Corner Breaks	number	0	0	0
	Longitudinal Cracking	linear meters	0.0	0.0	0.0
	Transverse Cracking	number of cracks	0	0	0
		linear meters	0.0	0.0	0.0
		percent of slabs	0
Transverse Joints	Sealant		fair condition			silicone sealant
	Spalling	number	2	0	0
		linear meters	0.5	0.0	0
	Faulting	millimeters	n/a1
		millimeters	1.2
	Width	millimeters	8.1
Long. Joints	Sealant		n/a			not sealed
	Spalling	linear meters	0.0	0.0	0.0
	Shoulder Dropoff	millimeters	n/a			not measured
Surface Conditions	Map Cracking	number of slabs	24			all slabs affected
		square meters	578.0			entire area
	Scaling	number of slabs	0
		square meters	0.0
	Polished Aggregate	square meters	0.0
	Popouts	number/sq. meter	0.0
Other	Blowups	number	0
	Flexible Patches	number	5	0	0
		square meters	0.8	0.0	0.0
	Rigid Patches	number	0	0	0
		square meters	0.0	0.0	0.0
	Pumping/Bleeding	number	0
		linear meters	0.0

¹ Faulting could not be measured due to high traffic volumes.

Table 3-20. Summary of pavement condition surveys for NC-440-015-002.

Distress Type		Distress Measure	Severity Level			Comments
			Low	Moderate	High
Cracking	Corner Breaks	number	0	0	0
	Longitudinal Cracking	linear meters	0.0	0.0	0.0
	Transverse Cracking	number of cracks	2	0	0
		linear meters	7.4	0.0	0.0
		percent of slabs	8
Transverse Joints	Sealant		fair condition			silicone sealant
	Spalling	number	0	0	0
		linear meters	0.0	0.0	0.0
	Faulting	millimeters	0.6			measured at 0.30 m
		millimeters	0.7			measured at 0.75 m
	Width	millimeters	7.6
Long. Joints	Sealant		n/a			not sealed
	Spalling	linear meters	0.0	0.0	0.0
	Shoulder Dropoff	millimeters	3.0
Surface Conditions	Map Cracking	number of slabs	24			all slabs affected
		square meters	576.0			entire area
	Scaling	number of slabs	0
		square meters	0.0
	Polished Aggregate	square meters	0.0
	Popouts	number/sq. meter	0.0
Other	Blowups	number	0
	Flexible Patches	number	0	2	0
		square meters	0.0	0.4	0.0
	Rigid Patches	number	0	0	0
		square meters	0.0	0.0	0.0
	Pumping/Bleeding	number	0
		linear meters	0.0

MRD Field Characterization

An evaluation of the MRDs is a key component of this project, so they are thus examined closely in the field. This detailed investigation, along with the comprehensive laboratory investigation, will help diagnose the type and cause of the distresses. The findings from this evaluation are summarized in table 3-21 and typical photos are shown in figure 3-46.

The primary distress, and the reason this section was selected as part of this project, is the extensive surface cracking exhibited over the entire pavement area. These hairline cracks run parallel to the longitudinal joints and are more prevalent near the outer edge of the slab (i.e., near the longitudinal joints). Some cracks are stained immediately around the cracks, which further highlights the presence of the cracks. The staining is typically dark gray areas around the crack; no exudate appears to be present in the cracks.

Table 3-21. Summary of MRD characterization for NC-440-015.

Description		Section 001	Section 002	Comments
Cracking	Location	Entire slab	Entire slab
	Orientation/shape	Parallel to longitudinal joint	Parallel to longitudinal joints
	Extent	Entire slab	Entire slab
	Crack size	Hairline	Hairline
Staining	Location	Around cracks	Around cracks	Not all cracks have staining
	Color	Dark gray	Dark gray
Exudate	Present	None	None
	Color	n/a	n/a
	Extent	n/a	n/a
Scaling	Location	None	None
	Area of surface	n/a	n/a
	Depth	n/a	n/a
Vibrator Trails	Visible	None	None
	Discolored	n/a	n/a
	Distressed	n/a	n/a
	Change in texture	n/a	n/a

(a)	(b)

Figure 3-46. Typical cracking pattern at NC-444-015. Note distress over entire slab length with spalling occuring at joints.

For the most part, these surface cracks are not causing any immediate concerns or problems. At a few joints and at the two transverse cracks, the surface cracks have begun to deteriorate and create loose pieces and some large spalls were seen on the side of the road. These areas have been patched with AC. Otherwise, the cracks are not adversely affecting the ride quality.

Laboratory Analysis

Core Selection/Visual Inspection

Based on the field survey and site inspection by researchers, the distress was determined to be widespread and uniform. Therefore it wasn't necessary to emphasize the joints as compared to the mid-panel concrete in selecting cores to analyze. As a result only two cores were examined, cores A and E. In addition, a large spall found by the side of Site 1 was selected for examination. Pictures of the cores and spall are shown in figure 3-47.

Figure 3-47. Cores and specimens evaluated from NC-440-015.

As with the other sites, mix proportions were not documented in the construction records and were only noted in the visual inspection as being normal. The concrete cores were 265 mm in depth, considerably greater than the design thickness of 250 mm. The concrete was well consolidated with no apparent segregation or parallelism of the aggregates. No scaling was present and there was no evidence of sub-parallel cracking or entrapped water voids under aggregates and surface cracking was seen in the cores that was thought not to be shrinkage cracks. Cracks extended from aggregates and through the paste. Secondary deposits were seen in some entrapped air voids and darkened rims were seen on coarse aggregate particles.

Stereo Optical Microscopy

As with the other sites, the stereo optical microscope was used to inspect the concrete in greater detail and perform the ASTM C 457 analysis. Infilling in cracks and voids was common with some entrapped air voids exhibiting large amounts of gel products. Figure 3-48 shows examples of infilling. The aggregate shown stained in figure 3-48 was prepared in thin section for additional study. Coarse aggregates in contact with the cement paste showed darkened rims.

(a) Desiccated ASR gel lining large entrapped air void.	(b) Granite coarse aggregate that picked up the stain, especially in cracks within the aggregate. Matching face from this aggregate prepared in thin section to examine material in cracks.

Figure 3-48. Stereo optical micrographs of ASR gel and reactive coarse aggregate from NC-440-015.

The coarse aggregate type was identified as granite and the fine aggregate appeared to be the same rock type. The fine aggregate was very angular, further indicating it may be a crushed version of the coarse aggregate used. The paste was hard and varied in color from yellow on one site to green on the other. This was attributed to different cement types being used in different sections, a fact later confirmed by observation of the unhydrated cement grains in the concrete using the petrographic optical microscope.

The results of ASTM C 457 are shown below in table 3-22. Both the air-void parameters and phase abundance results are presented. As can be seen, the value of Power's spacing factor measured for this concrete exceeds the maximum recommended to ensure freeze-thaw protection. In general this is not a concern for this pavement as the environment is considered a wet/no-freeze zone. However, the fact is that some freezing does occur and there may be some minor contribution of paste freeze-thaw damage to the overall observed distress.

Table 3-22. Results of ASTM C 457 on concrete from NC-440-015.

	Original		Existing		Volume Percent
Core	Air Content (vol. %)	Spacing Factor (mm)	Air Content (vol. %)	Spacing Factor (mm)	Paste (vol. %)	Coarse Aggregate (vol. %)	Fine Aggregate (vol. %)
Site 1 Core A	6.6	.278	6.4	.320	29.6	41.7	22.2
Site 1 Core E	6.5	.252	6.5	.276	30.0	44.4	21.2

Staining Tests

The results of staining indicate that both sulfate minerals and ASR gel are present as infilling material in cracks and voids. Figure 3-49 below shows some typical results for these tests. The characteristic yellow of the sodium cobaltinitrite stain is seen on the left and the brilliant purple color associated with the barium chloride/potassium permanganate is seen on the right. The phenolphthalein stain indicated a depth of carbonation of approximately 5 mm at the road surface and at the bottom of the slab.

(a) Sodium cobaltinitrite stain	(b) Barium chloride/potassium permanganate stain

Figure 3-49. Typical stereo optical micrographs of stained concrete. Note that the broad banding occurs from the montage process to create a single image.

Petrographic Optical Microscopy

The petrographic microscope further confirmed dense reaction rims on coarse aggregates in contact with the cement paste. Cracks within the aggregate were filled with crystalline ASR product and ettringite. The ettringite was generally very dense and tightly packed. In addition to the granite aggregates reacting, some reactive chert was seen in the fine aggregate. Also, the fine aggregate contained some reactive particles that were a red to tan color and occurred as a "coating" on other particles. The reaction seems to convert the coating to a gel product, which appears desiccated in thin section. Apart from consuming the tan to red coating, the reaction appears to cause no damage to the surrounding concrete. Some typical petrographic optical micrographs of reacting aggregate and ASR gel "blobs" are shown in figures 3-50 through 3-54.

(a) Transmitted plane polarized light	(b) Epifluorescent mode	(c) Transmitted cross polarized light

Figure 3-50. Petrographic micrographs from spall obtained from NC-440-015 showing ettringite filled entrained air voids. Ettringite growths are unusually dense.

(a) Transmitted plane polarized light	(b) Epifluorescent mode	(c) Transmitted cross polarized light

Figure 3-51. Petrogrpahic micrograph of a tan to red coating observed on some of the fine aggregates that appears to undergo ASR from spall obtained from NC-440-015.

(a) Transmitted plane polarized light	(b) Epifluorescent mode	(c) Transmitted cross polarized light

Figure 3-52. Petrographic micrograph of ettringite and "ASR gel blob" from spall obtained from NC-440-015. The term ASR gel is misleading in this example, since the reaction product is slightly birefringent and, therefore, crystalline.

(a) Transmitted plane polarized light	(b) Epifluorescent mode	(c) Transmitted cross polarized light

Figure 3-53. Petrographic micrograph of ettringite intermixed with ASR gel. As in figure 3-52, the ASR "gel" in this image is birefringent and crystalline.

Figure 3-54. Petrographic micrograph of ASR gel in crack within coarse aggregate. The gel deposit in this image was analyzed with the SEM. Again, the ASR "gel" in this image is birefringent and crystalline.

Scanning Electron Microscopy

Using the SEM. quantitative measurements of ettringite and ASR "gel" compositions were made. Ettringite deposits were extensive and found both in entrained air voids and in cracks in the cement paste and in cracks along the contact with coarse aggregates. Measurements were made of the coarse aggregate ASR product pictured in figure 3-54, both within the crack in the coarse aggregate, and in the region where the crack comes into contact with the cement paste. Measurements were also made of the gel blobs associated with the fine aggregates. The presence of small amounts of iron in the ASR product is unusual. Figures 3-55 and 3-56 are SEM micrographs of ettringite and ASR reaction product. Figure 3-57 shows the typical spectra for ettringite and ASR reaction product.

(a) Ettringite in entrained air void, and in crack along contact between cement paste and coarse aggregate.	(b) Ettringite in entrained air void, and in crack along contact between cement paste and coarse aggregate. Large relict cement grain also visible.

Figure 3-55. SEM micrograph of ettringite filling air voids and crack.

(a) Two ettringite filled air voids connected by a crack, also filled with ettringite.	(b) ASR "gel blob" associated with fine aggregate. Spectrum shown in figure 3-57 contain data collected from this deposit.

Figure 3-56. SEM micrograph of ettringite and ASR reaction product.

Interpretation and Diagnosis

This test site was a good example of an "obvious" MRD, if there is such a thing. The visual inspection indicated a classic case of ASR distress. Further examination led to no other likely distress other than the potential of some paste freeze-thaw damage as a result of an inadequate air-void system. It should be pointed that with the visuals and stereo optical microscope inspection alone, a diagnosis of ASR is tempting. However, to be certain of your diagnosis, all other avenues must be explored. In this context the guidelines proved useful in keeping the analysis on track and not stopping at the first MRD identified.

(a) Typical spectrum from ettringite deposit.	(b) Typical spectrum from ASR reaction product deposit.

Figure 3-57. Typical spectra for ettringite and ASR reaction product.

Having performed the described laboratory analyses and applied the diagnostic flow charts as shown in figures 3-58 through 3-62, two possible MRDs were identified in NC-440-015, including paste freeze-thaw and ASR. To finalize the diagnosis, the diagnostic tables were consulted. The diagnostic features identified in the analysis processes are listed in table 3-23 along with their associated MRD type and significance as related to this pavement. A brief discussion of each possible MRD identified in the laboratory analysis:

Paste Freeze-Thaw - The probability of paste freeze-thaw damage is low. The fact that the air void system was inadequate and was further compromised by infilling should be noted. Microcracking resulting from paste freeze-thaw may have accelerated the ASR reaction by providing a path for water ingress into the concrete.

ASR - This MRD was clearly dominant in terms of extent. It is most likely that the major contributor to the overall observed distress is ASR.

In summary, ASR is the most likely cause of distress in this pavement. Any steps to remediate should focus on ASR mitigation. In future construction, more attention to the quality of the entrained air system may help improve performance.

 

Figure 3-58. Flowchart for assessing the likelihood of MRD causing the observed distress in the pavement as applied to NC-440-015.

Possible Distress	Present		Additional Information
Error in Mix Proportioning	Yes	No	See Recommended Literature
Poor Placement	Yes	No	See Recommended Literature
Poor Finishing/Curing	Yes	No	See Recommended Literature
Poor Steel Placement	Yes	No	See Recommended Literature
Carbonation at Depths > 5-10 mm	Yes	No	See Recommended Literature

 

Figure 3-59. Flowchart for assessing general concrete properties based on visual examination as applied to NC-440-015.

Possible Distress	Present		Additional Information
Shrinkage Cracks or Sample Preparation Cracks	Yes	No	See Recommended Literature
Paste Freeze-Thaw	Yes	No	Table II-2
Aggregate Freeze-Thaw	Yes	No	Table II-3
Sulfate Attack	Yes	No	Table II-4
Deicer Attack	Yes	No	Table II-5
Secondary Deposits	Yes	No	Figure 3-62

• Application of a lithium compound.
• The application of a HMWM.
• Overlay.

• Heavy media separation
• Blending with non-susceptible aggregates
• Washing

• Reducing the total mixture alkalinity to less than 3 kg/m³
• Using a blended cement containing a pozzolan or ground slag
• Through supplementation or addition, use pozzolans or ground slag in the mixture
• Add lithium nitrate

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