Hydrologic survey for a dam site | Civil works and studies

Construction of curved ogee spillway profiles demands precise concrete casting techniques to achieve the required hydraulic efficiency and structural integrity. The process begins with thorough preparation of the foundation surface, which must be cleaned, roughened, and moistened to ensure proper bonding with the new concrete. This initial step proves crucial as it directly influences the long-term durability of the spillway structure.

The formwork system serves as the backbone of successful ogee profile construction. Traditional wooden formworks, while economical, often fall short in achieving the precise curvature required for optimal hydraulic performance. Steel formwork panels, designed specifically for curved profiles, provide superior dimensional accuracy and surface finish. These panels must be assembled with particular attention to the transition points where the curve radius changes, as these areas often present the greatest challenge in maintaining profile accuracy.

Concrete mix design plays a pivotal role in achieving the desired results. The mix must balance workability for proper placement with strength requirements and surface durability. A typical mix ratio of 1:2:4 with a water-cement ratio of 0.45 to 0.50 has proven effective for most spillway applications. The addition of superplasticizers enhances concrete flowability without compromising strength, particularly beneficial when working with complex curved formwork.

The casting sequence requires careful planning to prevent cold joints and ensure uniform concrete placement. The spillway profile is typically divided into manageable sections, with each pour carefully timed to maintain concrete consistency throughout the structure. Vertical casting segments should not exceed 2 meters in height to maintain proper concrete consolidation and prevent formwork deformation under hydraulic pressure.

Vibration techniques require special attention when working with curved profiles. Internal vibrators must be inserted at regular intervals, maintaining consistent spacing to ensure uniform consolidation without creating voids or honeycombing. The vibrator should be inserted vertically at intervals of approximately 450mm, with each insertion lasting 10-15 seconds. This systematic approach prevents the formation of weak zones that could compromise the spillway's structural integrity.

Surface finishing demands particular care to achieve the required smoothness for optimal hydraulic flow. The initial screeding follows the formwork profile, but the final finishing must wait until the concrete reaches the right degree of hardness. Power floating proves especially effective for larger surfaces, while hand troweling may be necessary for areas with tight curves or limited access. The timing of finishing operations is critical – too early can trap bleed water beneath the surface, while too late can make it difficult to achieve the desired smoothness.

The curing process becomes particularly crucial for curved surfaces where differential shrinkage could lead to cracking. Immediate application of curing compound or wet burlap helps maintain uniform moisture content across the profile. The curing period should extend for a minimum of 14 days, with special attention to areas exposed to direct sunlight or strong winds. Installing temporary shading may prove necessary to prevent rapid moisture loss during initial curing phases.

Quality control measures must be implemented throughout the casting process. Profile checking using templates at regular intervals ensures adherence to design specifications. Concrete testing, including slump tests and cube strength tests, should be conducted for each batch. Surface regularity measurements, taken after form removal, help identify any areas requiring remedial work before the structure enters service.

The formwork removal sequence demands careful planning to prevent damage to the concrete surface. Forms should remain in place for a minimum of 24 hours, with longer periods recommended for complex curves or during cold weather. The removal process must proceed gradually, starting from the top and working downward to prevent sudden stress concentrations that could damage the fresh concrete.

Surface treatment applications may be necessary depending on specific project requirements. These might include the application of surface hardeners, waterproofing compounds, or erosion-resistant coatings. Such treatments should only be applied after the concrete has fully cured and achieved its design strength, typically after 28 days.

Maintenance considerations should be incorporated into the casting process. Including inspection points and access provisions in the original construction makes future maintenance more practical. Regular inspection and maintenance of the spillway surface ensure continued hydraulic efficiency and structural integrity throughout its service life.