Investigating the causes and mechanisms of a concrete pavement distress in Hawaiʻi

Abstract

The soils underneath a PCC pavement on the island of Oahu, Hawaii were sampled and tested extensively because adjacent slabs had experienced vertical and horizontal separation. Based on the site geology, historical sea level rise and ebb may have caused the alluvial soils of volcanic origin to mix with calcareous fines (calcite), the source of which are the remains of dead coral giving rise to possible cementation in the soil. Hydrochloric acid and XRD tests suggest the presence of calcite in the upper reaches of the soil deposit and halloysite throughout. Index tests criteria proposed by the U.S. Army Corp of Engineers to assess soil collapsibility showed that the soils are collapsible based on dry unit weight, porosity and plasticity index values. However, the liquid limit test criterion was not consistent with the other criteria suggesting that the U.S. Army Corp of Engineers liquid limit criteria may not be as reliable in identifying collapsible tropical soils. The soils in the median are not as collapsible compared to soils directly underneath the pavement possibly due to the fact that the soils in the median have been subjected to a decade more exposure to atmospheric conditions and possibly because the soils below the pavement are constantly moist due to the canopy effect. The matric suction values under the pavement indicate that the soils below the pavement are saturated or nearly saturated. In contrast, the matric suction of the soil in the median fluctuated depending on the precipitation and the weather. Another notable feature is that the moisture content is higher at 0.5 ft than at 2.5 ft below the pavement but the reverse is true in the median . This phenomenon can also be attributed to the canopy effect at the pavement location and conversely, there is no canopy effect in the median. Suction controlled consolidation tests ran using a constant rate of strain consolidometer indicated soil collapse during three different phases of testing: saturation, drying and cyclic loading. A batch of samples collapsed by about 4% to 6% when saturated at a seating stress of only about 5 kPa. This occurred in the topmost samples below the pavement. The samples were first saturated so that the starting point on the soil-water retention curve is known. Since these samples are from the shallows, it can be implied that the shallowest soil is highly collapsible. It is postulated that the collapse in these samples is due to the breaking of calcite cementation bonds upon dissolution. A second batch of samples did not collapse during saturation. Instead, they collapsed by about 1% during suction application. In both samples, the suction applied was 200 kPa. One possible explanation for this is that the halloysite present loses the layer of water molecules in between the kaolinite units upon drying or application of suction. As a result, the soil collapsed when the water layer is removed irreversibly. Admittedly, the collapse due to suction application is smaller than that during saturation. The third batch of samples collapsed due to load-unload-reload cycles on saturated samples. The rationale for this is that PCC pavements heat up during the afternoons causing the pavement slab to curl downwards. They cool down in the early hours of the morning causing the pavement slab to curl upwards. These temperature cycles cause stresses to escalate at the edges and center of the pavement slab during the day and night, respectively. To simulate this in a consolidometer, the soil is loaded to a target vertical stress and then unloaded at a rate of 1 cycle per day for many cycles to simulate the repeated upward and downward curling. It was found that 60 to 80 cycles were adequate to yield the asymptotic strain. The collapse settlement or permanent deformation due to repeated loading and unloading depends on factors such as the number of cycles, duration of loading, and magnitude of stress difference in a cycle. A methodology is presented to estimate the long-term collapse settlement due to this mechanism. A hyperbolic curve can be fitted through each collapse versus cycle number curve, from which the asymptotic value of collapse strain can be estimated. The asymptotic strain can then be plotted versus the stress difference to allow an engineer to easily estimate the long-term load-unload-reload collapse for any value of stress difference. Curling-induced stresses were estimated utilizing first a finite difference analysis to estimate the temperature profile and then applying the temperature profile in a finite element analysis. The diurnal curling-induced subgrade stress can be as high as 25 kPa at the pavement edges with truck loading during the hottest time of the day in relation to an unloaded pavement at the coldest time of the morning. Based on a diurnal cyclic stress difference of 25 kPa and the cyclic consolidation test results, the collapse settlement due to curling-induced subgrade stress cycling is estimated to be about 2%. In sum, the 3 different sources of collapse strains are as follows: a. Collapse Strain due to Curling-Induced Subgrade Stress Cycling ≈ 2%. b. Collapse Strain due to Saturation and Loss of Cementation ≈ 4 to 6%. c. Collapse Strain due to Drying or Loss of Water Between Kaolinite Units ≈ 1%. Summing the 3 sources of collapse strain leads to a total collapse of 7% to 9%. According to Jennings and Knight (1975), the severity of the problem for these values of strain is considered “Trouble.” These stresses induced in a collapsing soil can lend an explanation of why the pavement distress was observed in the concrete pavement. Finally, some methods of mitigating collapse of similar soils are proposed in the Implementation section of the report (Section 5.3). They include dynamic compaction, pre-wetting and pre-loading, partial excavation and replacement, and a variety of soil improvement options.