Questions from the Field is a selection of questions the NPCA Technical Services Department receives from calls, emails and comments on blogs or magazine articles on precast.org. If you have a technical question that needs an answer, contact us by calling (800) 366-7731 or visit precast.org/technical-services.
Can you explain the procedures and methodology of hydrostatic tests for precast box culvert joints?
NPCA Technical Services engineers answered:
The use of preformed rubber gaskets for precast box pipe sections is not universal. A designer should check with local precast box manufacturers about availability. ASTM C1677, “Standard Specification for Joints for Concrete Box, Using Rubber Gaskets,” provides details regarding the joint design and plant performance proof-of-design testing criteria. The test involves placing hydrostatic pressure on a joint centered in straight alignment and/or in an off-centered alignment. The specific procedure on how to conduct the test is left to the precast manufacturer.
For small-diameter pipe and box sections, a common test method involved casting a bulkhead to be placed at the ends of at least two jointed pipe sections. The pipe was then filled with water to the prescribed test pressure and joints were observed for the specified holding time. However, this test method is not practical for large-span box sections and even circular pipe. The test method used for these sections is to provide an apparatus or setup to isolate the specific tested joint so the compressed gasket seal can be hydrostatically energized to the required testing pressure. This is also accomplished for circular pipe with the use of an internal or external joint testing apparatus that encapsulates the joint with a metallic can and seals the joint on each side with inflatable bladders. The annulus is then filled with air or water to test the gasket seal.
Unfortunately, rectangular geometry of box sections does not lend itself to standard joint testing. The best way to conduct a plant joint test is to use a secondary gasket of the homed joint, typically in the shoulder area of the spigot or end of the bell section. This creates a small isolated space confined by the spigot, bell-tested gasket and secondary backup gasket. Two small holes are then carefully drilled into the joint space and small-diameter copper or steel pipes with shut-off valves are grouted into the openings. One pipe allows water to enter and fill the annular space between the joint gasket and the backup gasket and serves to only isolate the test water. The second pipe serves as an air release (see diagram). This setup makes joint design testing simple and easy. However, plant employees are recommended – depending on the joint geometry, setup and hydrostatic forces – to hold the box sections together, as the pressure may push them apart.
I recently saw a TV program about a maximum security prison being built using precast concrete cells. The cement was especially strong and was coated using a special concrete primer and epoxy-resin paint. Are these materials suitable for building a ferro-cement sailboat?
NPCA Technical Services engineers answered:
Research shows a final seal coating should be placed on the exterior surface of the hull to add watertightness. However, research doesn’t say if the epoxy system used at prisons is an appropriate coating for boats. Coatings are developed for specific applications such as UV tolerance, temperature ranges, etc. Additionally, elasticity is an important characteristic to consider. Some epoxy coatings are hard like glass, but may not be best to use on a boat. Also, some elasticity could be beneficial in spanning any small curing shrinkage cracks that may occur.
One important consideration with a ferro-cement ship constructed today is the substantial advancement in concrete mix admixture and components technology. The use of special fiber reinforcement (thermoplastic or carbon) adds an increased level of ductility to a thin-shelled concrete structure. Additionally, admixtures can now allow concrete, known as self-consolidating concrete, to flow under their own head pressure without the need for traditional vibration consolidation. This is being used for intricate architectural components and difficult concrete pours with reinforcement constrictions. Another admixture technology is the crystalline sealants within the mix itself, which internally seal small cracks with cement hydration byproducts. This is successfully employed in many large concrete tank applications and concrete bridge decks.
Lastly, ultra high performance concrete with compressive strengths exceeding 15,000 psi is used to manufacture large, thin architectural sections. It currently is used without course aggregate and in conjunction with steel fibers. Good luck with the launch.