The effect of cyclic loads on the surface profiles of ECC linings cast on foundations comprising crushed stone and compacted soil was investigated. A geotextile was embedded between the crushed stone and ECC lining for some of the samples. After 28 days of water curing, the hardened surfaces were loaded and monitored for roughness and crack development by measuring surface levels and crack widths, respectively. Neither cracking nor significant variations in the lateral profiles were observed on all the samples for all the loads applied. However, significant variations which depended on the foundation types were observed in the vertical profiles. It was concluded that while ECC can resist cracking due to its high strain capacity, its flexibility causes ECC linings to assume the shape of the foundation material, which can increase the surface roughness at certain loading configurations.
An open channel is medium in which a liquid flows with a “free surface,” which is defined as the interface between the moving liquid and an overlying fluid at a constant pressure [
Basically, while the selection of a suitable lining material is governed by both structural integrity and economic viability, the interaction between the subsurface drain and lining in response to imposed loads needs to be fully understood to avoid durability and structural failures. Failures emanate from various causes which include uncertainties in loading, deficiencies in construction materials, inadequacies in design, and poor maintenance [
Previous researchers have elucidated that cyclic loading, deficiencies in the construction materials as well as instabilities in the foundation culminate in durability problems for linings [
Efforts to address some of the shortcomings of concrete have led to the development of fiber-reinforced materials in recent years [
While concrete has been in use since the 19th century [
ECC-lining surfaces were monitored for variations in roughness and crack development. Three subsurface conditions were investigated, that is, compacted soil, crushed stone and crushed stone with a separator inserted between the ECC lining/crushed stone interface. The roughness of the lining and crack development were assessed by monitoring the levels on fixed nodes on each surface and the progression of crack widths, respectively. To simulate the fluctuations of actual loading during cycles of flow, the samples were subjected to cyclic loading.
ECC premix powder was used for the lining material while sandy soil, 20 mm angular crushed stone and a geotextile separator were used for the foundation. The premix comprised cement, sand, fly ash and PVA fiber 12 mm in length, 0.04 mm in diameter, 1690 MPa in tensile strength and 40.600 MPa in modulus of elasticity. The proportions of the mix components are shown in Table
Mix proportion of ECC (1 m3).
ECC premix* (kg) | Water (20°C)(kg) | Admixture-type A(kg) | Admixture-type B(kg) | Admixture-type C(kg) |
---|---|---|---|---|
1.562.50 | 350.00 | 16.88 | 15.25 | 3.13 (diluted 25 times) |
*ECC premix composition: sand/cement = 0.65; fly ash/cement = 0.3; PVA fiber volume fraction = 2%.
Samples M1, M2, and S1 were prepared in 250 mm × 250 mm × 50 mm forms as detailed in Figure
Description of samples and loads applied.
Load(t) | Stress (kPa) | Sample M1 | Sample M2 | Sample S2 |
---|---|---|---|---|
0 | 0 | M1-0 | M2-0 | S1-0 |
1 | 21 | M1-1 | M2-1 | S1-1 |
2 | 41 | M1-2 | M2-2 | S1-2 |
3 | 62 | M1-3 | M2-3 | S1-3 |
4 | 83 | M1-4 | M2-4 | S1-4 |
Description of samples used in this study.
Gridlines on the surface of each sample.
An automatic leveling device was used to measure the levels on each of the 25 nodes, while a crack viewing device was used to monitor surface cracks. Initial level and crack width readings were taken after which a 220 mm × 220 mm loading head was used to apply uniform stress onto the surface of the lining. Each sample was subjected to 4 loading cycles of stress values described in Table
The compressive and flexural strength of the ECC mix used in this study were determined by casting standard ECC prisms of dimensions 40 mm × 40 mm × 160 mm and water curing them at 20°C for 28 days after which the strength tests were carried out. All compressive strength tests adhered to the ASTM C349-02 standard, while the flexural strength tests were carried out in accordance with the ASTM C348-02 standard.
The data from the line A3–E3 marked on Figure
Surface profile along line A3-E3 of ECC lining cast on crushed stone (sample M1).
Surface profile along line A3-E3 of ECC lining cast on crushed stone with a geotextile embedded (sample M2).
Surface profile along line A3-E3 of ECC lining cast on compacted soil (sample S1).
From the graphs for the lining cast directly on crushed stone, that is, samples M1-0 to M1-4, it can be seen that while there was no significant change in the lateral profile of an individual sample with each loading cycle, a systematic pattern in the vertical displacement of the linings was observed. The surface levels generally increase with increase in cycles. Since an increase in the reduced level value indicates a compaction, the results show that the lining/crushed stone unit compressed with each loading cycle. Characteristically, it is impossible to totally eliminate voids in uniform size crushed stone despite prior vibration. As a result, the repacking of the crushed stones into existing voids in response to the applied load resulting in the compaction. Even though the loading was in the vertical direction only, the lining was free to move laterally and curve upwards since it was floating in the form. However, the graphs show insignificant variations in the lateral profile. This means that even though the crushed stone foundation beneath the lining comprised loose stones, the lining/foundation unit actually displaced vertically as a single unit. It is thought that at casting stage, due to the capability of ECC to flow under its own weight and fill in the form work in a process termed “self-compactibility” [
After the first loading cycle, the lateral profile of the ECC lining cast on compacted soil flattened out and maintained the same levelness with consecutive cycles. Unlike the ECC paste cast over crushed stone which flowed into the pores within the crushed stone and created an ECC/crushed stone composite, the ECC lining and compacted soil remained separate layers after solidifying. As a result, the lining could alter its lateral profile and flatten out in response to the first load. On the other hand, as in sample M1, a systematic pattern in the vertical displacement of the linings for sample S1 due to compaction of the surface was observed. However, the magnitude of the vertical displacements did not differ significantly from those of samples M1. Whilst sandy soil was expected to undergo higher compaction due to relatively lower densities to stone [
The behavior of the lining cast on crushed stone with a geotextile separator embedded in between was similar to that of sample S1 for the lower loads of 21 kPa and 41 kPa, but the pattern significantly deviated for the higher loads. In this case, the geotextile separator attached itself to the ECC paste and acted as barrier which prevented the ECC paste from flowing into the crushed stone pores. Therefore, like in the sample S1, the ECC lining and the crushed stone foundation remained separate after solidifying. Consequently, the first loading cycles only flattened out samples M2-1 and M2-2, and the lateral profile levelness was maintained with consecutive loading cycles as in samples S1-1 and S1-2. Moreover, as in samples S2-1 and S2-2, the levels in samples M2-1 and M2-2 generally decreased after application of the first loading cycle. It is thought that since both samples M2 and samples S1 remained as separate layers after solidifying, the lining separated itself from the compacted soil as it flattened out resulting in a protruded surface and consequent decreased levels. However, a marked deviation in behavior from sample S1 was observed with application of the 62 kPa loading. The ECC lining in sample M2-3 responded by deforming into an undulating profile as shown in Figure
Finally, at applied loading of 83 kPa, which was much higher than the strength of the ECC, the lining lost its ductility and neither undulations nor lateral profile variations were observed on sample M2-4. Rather, the lining underwent increased compaction as the cycles increased due to repacking of the crushed stone as observed in samples M1 and S1.
Cyclic loads affect the vertical profile of the ECC lining cast on both crushed stone and compacted soil by compacting the surface. The level of compaction depends on the compactibility of the foundation components.
The effect of cyclic loads on the lateral profile of the ECC lining depends on the composition of the underlying subsurface drain. Cyclic loads do not affect the lateral profile of the ECC lining cast directly on a crushed stone drain. ECC paste can flow and solidify in the crushed stone voids, resulting in an ECC/crushed stone composite with a higher compressive strength than the originally designed ECC lining. However, the solidification clogs the voids in the crushed stone and diminishes its function as a drain culminating in damage to the lining due to a buildup of ground water pressure.
The inclusion of a geotextile separator between the crushed stone subsurface drain and the ECC lining has a significant impact on the response of the ECC lining to cyclic loads. The geotextile effectively separates the subsurface drain from the lining and allows the lining to deform freely. By preventing the ECC paste from clogging the crushed stone pores and diminishing the capacity of the subsurface drain the geotextile prevents the buildup of ground water pressure. In addition, the geotextile also prevents the occurrence of cracks on the ECC lining by moderating the upward pin loads from the crushed stones in the foundation.
Cyclic loads do not affect the lateral profile of the ECC lining cast on compacted soil foundation. The lining and the subsurface also remain as separate layers after solidification allowing it to displace freely. However, the absence of a freely draining material below the ECC lining will cause damage to the lining due to build up of ground water pressure.
Whether it is used with or without a geotextile, the ductility of ECC enables it to respond to excessive loads by stretching-like metal rather than breaking-like concrete. Being flexible, ECC can assume the shape of the underlying foundation material and develop undulations. While undulations due to small sizes of crushed stone cannot significantly increase surface roughness of a water channel, larger undulations can cause significant increases which consequently increase Manning’s roughness coefficient and decrease the velocity of flow across a surface.
The characteristic high strain capacity of ECC enables it to restrict the crack widths in linings to less than 0.1 mm and satisfy serviceability limits in water storage facilities where according to British Code BS 8007(1987) [