Curing Practices for Concrete Pavements (2023)

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51   C H A P T E R   4 Introduction In this chapter two case examples (at three project sites) are presented. These examples illus- trate various aspects for concrete paving related to type of curing compound, application rate and quantity, climatic region, method of construction, monitoring of curing, and related sensing instrumentation, including noted effects of curing on slab behavior. Field Testing—Cleveland and Roseville Projects The Cleveland, Texas, and Roseville, California, projects (41), which consisted of roller com- pacted concrete (RCC) pavement construction, were instrumented to evaluate curing quality and are reported here as examples of curing quality monitoring, including the sensors involved and the analysis of the data collected from the sensors. The illustration of these two sites shows the effect of climatic conditions on curing quality and the need for consideration of weather conditions in concrete pavement construction. Cleveland, Texas Project The instrumentation was installed in July 2017 during a roadway construction project con- ducted in Plum Grove, Texas, a small community near Cleveland, Texas. Before and after the placement of the RCC, a series of instruments were installed to monitor the curing quality. Although the instruments included a range of sensors, only the data related to the dial gauges and the dew point and dry bulb temperature sensors described in Chapter 2 are addressed in this discussion. The instrumented section was placed in the afternoon at approximately 5:30 p.m. on a hot summer day (air temperature was approximately 97°F (36°C). The RCC was placed directly on a subgrade that had been stabilized using fly ash to a depth of six inches. The design thickness of the RCC was 5 in. (12.5 cm) and the width of the entire roadway was 22 ft (6.70 m). Soon after concrete was placed, a resin-based curing compound was sprayed on the pavement surface about 2.5 hours after paving, transverse joints were sawcut at every 15 ft (4.6 m), and a longitu- dinal joint was sawcut down the center dividing the roadway into two equal lanes, each with a width of 11 ft (3.4 m). Roseville, California Project Curing-related instrumentation was installed on a segment of an RCC construction project conducted in Atkinson St. in Roseville, California (Figure 45). Before and after the placement of Case Examples of DOT Curing Practices

52 Curing Practices for Concrete Pavements the RCC, a series of instruments was installed to monitor vertical movement of the pavement. This section presents key background information on this study and a portion of the associated data collected with it. The segment of paving instrumented was approximately one mile in length. The RCC pave- ment was 5 in. (12.5 cm) thick and 22 ft (6.7 m) wide and constructed in September 2018. The location and street layouts are presented in Figure 47. The natural subgrade was clayey soil, of which the top 6 in. (15 cm) was stabilized before placement of the RCC. Soon after placing concrete, a layer of resin-based curing compound was sprayed on the pave- ment surface; about 2.5 hours after paving, transverse joints were saw-cut every 15 ft (4.6 m), and a longitudinal joint was saw-cut down the center equally dividing the roadway into two lanes (each with a width of 11 ft). All slabs were cured with a resin-based Type II curing compound, which appears white in color when wet, but dries clear. The application rate was 155 ft2/gal (3.8 m2/lit). EI and Potential Evaporation Rate Data Data from this study indicated that the performance of curing is affected by the prevailing weather conditions during construction. The ACI 308 Manzel nomograph (Figure 3) was used to characterize the effect of weather with respect to potential evaporation (PE) for both project sites. Figure 3 considers ambient temperature, relative humidity, concrete temperature, and wind velocity. This nomograph can also be represented by the equation: PE T RH T WVc a( )( )( )= − × × + × ×100 1 0.4 0.0000012.5 2.5 where PE = potential evaporation rate, lb/ft2/hr RH = relative humidity of ambient conditions, % Figure 47. Project location and layout of streets.

Case Examples of DOT Curing Practices 53 Tc = temperature of the concrete, °F Ta = temperature of ambient conditions, °F WV = wind velocity of ambient conditions, mph To quantify the eects of weather on the vertical setting characteristics of the concrete through the thickness of the pavement, PE is useful to account for the eects of ambient temperature, relative humidity, concrete temperature, and wind velocity on the development of a vertical set prole. Figure 48 shows the PE traces for the project sites in Roseville, California (in September) and Cleveland, Texas (in July) for comparison. Dial Gauges and Corner Displacements Environmental and curing-related eects in terms of curling and warping behavior of a slab were highlighted with respect to the vertical displacements detected by the use of dial gauges at the slab corners. e dial gauges were mounted along the slab edges and corners as shown in Figure 49. e dial gauges were placed approximately two inches away from the transverse joints to monitor vertical displacements near the pavement corners. Figure 49. Measuring vertical displacement at corners using offset and corner dial gauges (41). 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0 2 4 6 8 10 12 14 16 18 20 22 24 PE Time (hr) Cleveland Roseville Figure 48. PE values for three different location placements (41).

54 Curing Practices for Concrete Pavements The collected data indicated that the concrete slab started to curl (induced by temperature differential) and warp (induced by moisture differential) as the concrete approached final set (as defined by ASTM C403) where a substantial amount of upward corner displacement occurred initially. The curling/warping was detected using the displacement data from both the offset and corner dial gauges. From a daily and over-time perspective, the vertical movement of the slab due to the negative gradient became more pronounced, as shown in Figure 50. The black line shows the differential absolute curling upward displacements (induced by temperature and/or moisture differential) between the two gauges’ locations. The absolute value of a verti- cal displacement measurement represents the distance from the current slab configuration to the flat slab condition. The differential displacements were almost always positive, suggesting that the corner movements were largely in an upward configuration. These trends indicate the dominance of the moisture gradient on the behavior and the final set condition of the slab. The illustrated trends suggest the role that curing effectiveness may have on the degree of set that is built into a concrete pavement. Figure 51 shows the vertical displacements measured at the Cleveland site using dial gauges (which were installed in the same manner as those in Roseville). Note the difference in mag- nitude between the corner movements of the two sites, which again shows the effect of both climatic and curing conditions on the behavior of a concrete slab. EI Determinations The EI parameter is useful to represent the effects of curing. To evaluate the effectiveness of curing compounds, EI is introduced as an index based on the adjusted maturity model (40): = − − EI t t t t f a s a where tf = equivalent age of concrete under filtered curing conditions; ts = equivalent age of concrete under sealed curing conditions; and ta = equivalent age of concrete under ambient curing conditions. -1.5 -1.0 -0.5 0.0 0.5 V er tic al C ur lin g/ W ar pi ng D isp la ce m en t (m m ) Time (Day, Hr) Offset Corner Time (Day, Hr) Offset Corner -5 -4 -3 -2 -1 0 1 2 V er tic al C ur lin g/ W ar pi ng D isp la ce m en t (m m ) Figure 50. Daily vertical movement for offset and corner dial gauges placement (Roseville, CA) (41).

Case Examples of DOT Curing Practices 55 e temperature data for both project sites were monitored and incorporated into the EI computations. In the placement at Roseville, the curing compound was sprayed at a 155 2/gal (3.8 m2/lit) application rate aer several hours of placement and aer the sheen had disappeared. Figure 52 shows that the relative humidity at the sealed and surface locations progressively increased to 90% aer approximately four to ve hours. However, the surface relative humidity started to reduce aer a given period of time, which greatly depended on the quality of the curing conditions. Due to the nature of the curing and climatic conditions, the monitoring data collected at the Cleveland site indicated a higher EI at the initial hours. Here a white-pigmented curing compound was sprayed at 170 2/gal application rate aer a few hours. e relative humidity at Figure 51. Vertical curling displacement versus time measured for corner and offset dial gauges (41). 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% EI R el at iv e H um id ity Time Sealed RH RH Surf AmRH EI 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% EI R el at iv e H um id ity Time Sealed RH RH Surf AmRH EI Figure 52. The EI trends in Roseville, CA (left) and Cleveland, TX (right) (41).

56 Curing Practices for Concrete Pavements the sealed and surface positions progressively increased to 90%. The surface relative humidity started to reduce after a given period resulted in a lower EI of 0.69, as shown in Figure 52. The higher EI, such as that measured in Cleveland, showed higher quality of curing at the surface, which again was influenced by the type and the amount of the applied curing compound and the ambient environment. Case Example of Florida DOT Field Test Slabs In order to assess the effect of curing practice on pavement performance, a study was under- taken at the Florida Department of Transportation (FDOT) near Gainesville, Florida, to inves- tigate the effect of the curing method on a set of newly constructed concrete pavement slabs (40). This investigation studied early-age concrete pavement properties with variable thick- nesses, curing practices, and times of placement. Accordingly, four test slabs were placed and cured under different climatic conditions; the resulting data substantiate the significant effect of concrete curing conditions on the final configuration of a slab and demonstrate that early-age concrete temperature and moisture conditions are key factors affecting pavement performance. Field Testing and Data Collection The field investigation conducted at the FDOT State Materials Office examined the behavior of early-age concrete pavement behavior with respect to variable thicknesses and curing prac- tices. There were four slabs, which were 12 ft ×15 ft (3.66 m x 4.57 m) with three of them 10 in. (30 cm) and one 7 in. (18.5 cm) thick, placed on an asphalt layer. A curing compound was used as a bond breaker between the slabs and the asphalt base layer. The test slabs were instrumented to support obtaining sufficient data to understand the behavior of a concrete slab under differ- ent curing conditions. The instrumentation included vertical displacement measurements using LVDTs at the slab corners. Curing monitoring, as previously described, was employed to assess the effectiveness of curing under the different placement times and condition. To evaluate curing compounds’ effective- ness, two application rates at different placement times were used to assess sensitivity to a change in the curing method. One slab was left uncured to control the effects of curing practices. A summary of the used application rates and curing compounds is presented in Table 3. Vertical Displacement and EI Data Corner displacement data are shown in Figure 53 and the corresponding EI data are shown in Figures 54 through 57. Careful examination of the illustrated data clearly indicates the Slab/ Thickness Curing Compound Application Rate (ft2/gal) Batch Mix Time Placement 1/7 Resin Based (Type II) 250 2/21/2017 9:20 10:00 2/10 Wax Based 250 2/21/2017 13:40 14:00 3/10 Wax Based 180 2/21/2017 16:30 17:00 4/10 None None 2/22/2017 9:20 10:00 Table 3. Summary of used application rates and curing compounds.

Case Examples of DOT Curing Practices 57 -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 0.020 Time (hr) D is lo ca tio n (in ) SLAB 1 -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 0.020 Time (hr) D is lo ca tio n (in ) SLAB 2 -0.015 -0.010 -0.005 0.000 0.005 0.010 0.015 0.020 Time (hr) D is lo ca tio n (in ) SLAB 3 -0.015 -0.01 -0.005 0 0.005 0.01 0.015 0.02 Time (hr) D is lo ca tio n (in ) SLAB 4 Figure 53. Dislocation (lift-off) of slabs at corners.

58 Curing Practices for Concrete Pavements 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% EI R el at iv e H um id ity Time Sealed RH RH Surf AmRH EI Figure 54. RH and EI for Slab 1. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% EI R el at iv e H um id ity Time Sealed RH RH Surf AmRH EI Figure 55. RH and EI for Slab 2. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% EI R el at iv e H um id ity Time Sealed RH RH Surf AmRH EI Figure 56. RH and EI for Slab 3.

Case Examples of DOT Curing Practices 59 significance of curing quality and key factors associated with it. Although three of the four slabs placed were cast on the same day, the ambient and placement conditions varied enough to cause the slab movements at the corners to differ. The corner displacement data are a direct reflection of the curing regimen associated with each slab placement. The greater the EI, the less amount of movement occurred at the slab corners, which results in reflection on the built-in curl/warped shape as the slab reaches its final set. Work was undertaken to collect data to allow for the examination of how EI was tied to the built-in curl/warp that develops during and after the curing of the concrete slabs. The dew point and dry bulb temperature data were monitored on all four slabs and was used in EI determinations. Data Summary Table 4 shows a partial summary of the data assembled for the four test slabs. Slabs were placed at different thicknesses, curing compound application rates, and times of day. The data listed in the table show that the ambient conditions at the time of placement affects the tempera- ture and moisture distribution inside a concrete pavement, as well as how set develops through its cross-section and its subsequent behavior relative to relationship. The potential evaporation (PE) data indicate the range of ambient conditions that existed at the time of placement, which is pertinent to the resulting built-in curl/warp. The total equivalent temperature differences 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% EI R el at iv e H um id ity Time Sealed RH RH Surf AmRH EI Figure 57. RH and EI for Slab 4. Slab EI Curing Compound AR* (ft2/gal) Time of Placement PE Equivalent Set Gradient Temperature (ºF/in.) Total Equivalent Temperature Difference at Set (ºF) Slab 1 0.568 250 10:00 0.081 -2.44 -16.8 Slab 2 0.595 250 14:00 0.115 -1.10 -10.8 Slab 3 0.783 180 17:00 0.076 -0.67 -6.4 Slab 4 0.388 None None 10:00 0.075 -1.87 -18.2 Table 4. Comparison of curing-related parameters as represented in each slab.

60 Curing Practices for Concrete Pavements between the bottom and the top of the slab after 72 hours from the placement are also listed in the last column. This parameter was determined as follows: ( )∆ = × − × α Equivalent T r v MoR C E set2 1 . . where MoR = the modulus of rupture of concrete rset = the stress ratio at set (i.e., the ratio of the environmental stress to the MoR of the concrete) C = the Bradbury coefficient at the slab corner E = the elastic modulus of concrete α = the thermal coefficient of expansion ν = the Poisson’s ratio Using EI to Control Curing This case example included key data used to illustrate the role of curing on pavement perfor- mance. The key factors associated with the performance of curing compounds in terms of EI were also well illustrated. EI is useful when choosing the most appropriate application rate for a given curing compound. The direct relationship between weighted EI and the rset parameter for the slabs and the differ- ent curing conditions prevalent during curing are shown in Figure 58. A higher EI is generally associated with a lower set, which represents lower irreversible shrinkage. It should be noted that EI is a function of PE and AR. Therefore, in a certain set (either low or high state), the distribu- tion of EI (material-driven and weather-driven) should be considered. A protocol for evaluating concrete curing effectiveness is based on sensor data collected from the test slabs’ placement at the FDOT State Materials Office. The evaluation principally consisted of the EI to evaluate the curing quality. EI also facilitates the determination of the application rate for a given curing compound with a given ambient environment condition. It appears to be a viable method of monitoring curing quality performance under different conditions and over broad areas of concrete pavement construction. y = 2.87x + 1.00 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 -0.2 -0.15 -0.1 -0.05 0 Ev al ua tio n In de x, E I rset Figure 58. EI versus set gradient in slabs.