Chapter 2

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DESCRIPTION OF FIELD SECTIONS

The Georgia site consists of three test sections located on eastbound U.S. Interstate 20 (I-20), near Augusta, GA, between mileposts 189 and 192. The existing pavement at this site is 230-millimeter (mm) jointed plain concrete pavement (JPCP) with 9-meter (m) joint spacing. The two-way annual average daily traffic (ADT) at this location was 25,750, with 19.5 percent trucks. The average truck factor during the time of field testing was 1.37.

The materials tested at this site include Georgia Department of Transportation’s 4-hour (h) mix (GADOT), Fast Track I (FT I), and Very-High-Early-Strength (VES). The mix designs for these materials are given in table 1. GADOT has extensive experience with fast-track full-depth repairs, and the SHRP C-206 repairs were included as a part of an on-going repair project. FT I is a 12- to 24-hr opening PCC mix developed by the Iowa DOT (Grove, Jones, and Bharil, 1993). VES is a 4-hr h opening PCC mix that utilizes a nonchloride accelerator (calcium nitrite) developed under SHRP C-205.

1 yd³ = 0.7645 m³; 1 lb = 0.4535 kg; 1 gal = 3.78 liter (L);
1 oz = 29.57 millileter (mL)

w/c = water to cement ratio

The Georgia sections were placed between about 10 p.m. and 2 a.m. and opened to traffic at 6 a.m. At this site, a typical day’s work started at 6:30 p.m. with the closing of the work area to traffic. Concrete removal started as soon as enough of the work area was blocked off, and concrete placement typically started at 10 p.m. (Whiting et al., 1994).

Table 2 provides a summary of Georgia sections, including the strength at opening for each repair and calculated fatigue damage due to early opening. Fatigue damage due to early opening was defined as the damage incurred within the first 14 days of service in this project, based on typical curing time and the strength at the opening time for conventional projects. Fatigue damage was calculated on an hourly basis for the first 3 days, then on a daily basis for the remaining 11 days.

^*Miner’s fatigue damage.

1 m = 3.28 ft;
1 megapascal (MPa) = 145.04 pounds per square inch (psi)

PERFORMANCE OF GEORGIA SECTIONS

In general, the Georgia sections provided excellent performance. After 6 years of service, the only patches that developed any distresses were the 4.6-m repairs in the GADOT section and one 2.4-m repair in the same section. Faulting was minimal (less than 1 mm) and remained unchanged (within measurement error) throughout the monitoring period, although the LTE was somewhat less than that found in new pavements with an effective load transfer design. A summary of joint performance data is given in table 3; detailed data are given in appendix B. The Georgia sections also did not exhibit any material-related distresses.

ANALYSIS OF CRACKING IN GADOT SECTION

The cracking of the longer (4.6-m) repairs in the GADOT section is consistent with expectations. The Georgia sections were placed at night and opened to traffic at 6 a.m. At the time of concrete hardening, there was no difference in the temperature between the top and bottom of the PCC slab. Thus, the repair slabs were built flat with no built-in temperature gradients. Although an excessive built-in negative temperature gradient is undesirable because it contributes to top-down cracking, a moderate amount of built-in negative curling is beneficial because it moderates the effects of high daytime temperature gradients (Yu et al., 1998). Without any built-in curling, the 4.6-m repair length is excessive for a 230-mm repair slab. Differential shrinkage does provide equivalent built-in temperature of about –2.2 °C (28.04 °F), but that is not enough to counter sufficiently the effects of high daytime temperature gradients.

^*Average absolute faulting.

1 in = 25.4 mm

The cracking problem was analyzed using the fatigue calculation procedure and model developed by Yu and colleagues (1998). The traffic data needed for the fatigue analysis are shown in figure 1. The traffic on I-20 through the project site is relatively high. The annual traffic is about 1.1 million equivalent single axles (ESALs) in the design lane.

This project did not include a material-testing program; therefore, certain assumptions had to be made regarding long-term strength development of the GADOT mix. Under SHRP C-206, extensive laboratory testing of repair materials was conducted to develop correlation relationships for maturity and pulse-velocity methods of determining in-place material properties (Whiting et al., 1994). Those tests showed that for the GADOT mix, the 90-day compressive strength is about 9 percent greater than the 28-day strength. Also under SHRP C-206, cores were retrieved from the test sections during a 2-month evaluation and tested for strength. Figure 2 summarizes the available information on Georgia site materials.

Figure 1. Estimated traffic on Georgia site.

Figure 2 shows lower strength for cores retrieved at 2 months, but this is likely caused by damage during coring or shipping of the cores and testing variability. Extensive material testing was conducted under task C of this project for VES and HES mixes, and the results of those tests showed that the long-term (5-yr) strength of those mixes is up to 18 percent greater than the 28-day strength. A comparison of early-age (up to 90 days) strength-gain characteristics of these mixes suggests that the long-term strength-gain characteristics of the GADOT mix should be similar to those of VES and HES. Based on this assumption, figure 3 was developed to show the long-term strength development of the GADOT mix.

Figure 2. Available material strength data for the Georgia sections.

1 megapascal (MPa) = 145.04 psi

Figure 3. Estimated strength development of GADOT mix.

1 MPa = 145.04 psi

The results of fatigue analysis are shown in figure 4, along with the observed slab cracking. The model was run with built-in curling of –2.2 °C (28.04 °F), which accounts for differential shrinkage. The effects of damage due to early opening are illustrated in figure 5. As shown in table B1 in appendix B, the amount of fatigue damage attributable to early opening was minimal (Miner’s damage of 0 to 0.07, except for 0.10 for the last patch). The damage due to early opening has the effect of shifting the time (or traffic) versus cracking curve upward, as illustrated in figure 5. However, for the Georgia sections, early opening to traffic had a negligible effect on the fatigue life of the repairs, even for the 4.6-m repairs. In fact, the last three 4.6-m repairs placed in the GADOT section (i.e., the repairs that were opened to traffic at the lowest strength) did not crack. For shorter repairs, the effects of early opening (and fatigue damage in general) are inconsequential.

Figure 4. Comparison of field performance of 4.6-m repairs in the GADOT section and predicted cracking using the rigid pavement performance/rehabilitation (RPPR) 1998 model (Yu et al., 1998).

Figure 5. Illustration of the effects of damage due to early opening.

Figure 6 shows the effects of repair length and built-in curling on the fatigue performance of full-depth repairs. For bottom-up transverse cracking, the GADOT section represents the most adverse condition, because the repairs hardened with zero built-in temperature gradient.

However, even under this condition, the 3.7-m repairs have an expected fatigue life of 10 or more years; shorter repairs should have practically unlimited fatigue life. Figure 6 also shows that if moderate built-in curling were introduced, the fatigue life of the 4.6-m repairs would increase dramatically. In this example, a total effective built-in temperature gradient of –5 °C (23 °F) was used, including the effects of temperature and differential shrinkage. This is a typical magnitude of built-in curling for PCC pavements in wet-nonfreeze climates. In general, for daytime construction, excessive built-in curling is more of a problem than is an inadequate amount of built-in curling.

Figure 6. The effects of repair length and built-in curling on performance of full-depth repairs.