Precast concrete can present problems. However, there are proactive measures that can be taken and several solutions to combat many of these challenges.
Here are some of the most common challenges we hear associated with precast concrete structures:
- Sealing the Joints
- Shipping the Product
- Offloading & Rigging Concerns
- Preparation of Subgrade
- Lack of Flexibility
- Repairing Spalls or Cracks
SEALING THE JOINTS
Problems can arise with the sealing of precast concrete joints. Over time, joints can separate from one another which can compromise and weaken the structure. This problem can occur when joint sealants are not applied correctly. This usually occurs if the joint sealant instructions are followed incorrectly. If the joints are not properly prepared or cleaned prior to applying the joint sealant this can weaken the adhesion between joints and prevent the joints from properly sealing with one another. However, joint sealant problems are usually very preventable. If the joint sealant instructions are followed and all appropriate preparations are made prior to applying the joint sealant such as: ensuring that the joints are clean and free of any dirt or debris along with checking that the joints are properly formed to fit together correctly by conducting a dry fit or checking the alignment prior to the joint sealant application, then the application of precast joint sealants should be successful and problems with the joint sealants should not occur. See more about joint sealant options and installation in our article.
SHIPPING THE PRODUCT
Shipping precast concrete can be challenging and it is should be a coordinated effort between the manufacturer and job site contact. Precast concrete can be difficult to transport due to the weight and potential because of how large the structures can be. For larger structures, specialized trucking and various preparations could be needed to transport the structures. The greater the height, width, and weight of the structures, the greater number of arrangements that are possibly needed to facilitate the transportation. Structures over 8 feet 6 inches wide will require a permit. Structures that are over 14 feet wide require a permit and an escort. Structures that are over 16 feet wide will require a permit and two escorts. Precast concrete structures that are over a certain weight will also need a weight permit. For shipments taller than 13 feet, route inspections must take place and potential bucket lifts may be required to add in avoiding powerlines. The route will be surveyed to ensure it is safe for the structures to travel. Logistically, transporting precast concrete can seem challenging, but utilizing expert hauling companies who understand the DOT rules and who are equipped with the right trucks and trailers can make the delivery seamless. In the event the structure is too heavy, too tall, or too wide to be transported, precast designers can normally split the structure into multiple sections to reduce the weight or size of any individual precast section. There are several methods for creating structural connections between sections of precast in the field. This makes transporting these large structures possible and reducing the lifting capacity needed on site. See more about shipping best practices in our article.
OFFLOADING AND RIGGING CONCERNS
Another challenge with precast concrete is offloading and properly rigging the structures. Normally, cranes or large equipment are needed to pick up and move the structures. As with any process involving cranes, there is a risk associated with understanding the capacity of the crane, rigging, and lifting devices. It is important that the correct rigging is utilized so that the structure is secure before lifting.
Improper shoring of an excavation can lead to catastrophic failures and should be inspected and engineered if necessary. In the event that a job site has overhead obstructions or there is a question regarding the shoring, it is a good idea to involve the crane company and walk the site before installation. For extremely large structures, lifting diagrams and rigging plans should be created by certified engineers to ensure the structure is lifted properly. Understanding how to correctly rig a precast structure, recognizing the lifting capabilities of the equipment and rigging devices, and knowing which equipment is most appropriate for each individual lift will facilitate a safer and more efficient offloading and setting process.
PREPARATION OF SUBGRADE
Preparing the subgrade is one of the most important elements of a successful precast concrete installation. Problems can arise if this is done incorrectly. A subgrade that is weak can settle incorrectly and shift the structure causing it to crack or sink. For example, if electrical cables are running through any of these structures while they shift underground, cables can break creating significant problems. The surest way to prevent this is to make sure that the subgrade is properly prepared for the structure. To do this, the ground must first be excavated correctly. Any unwanted debris or materials should be removed. By doing this, it will help to create a level and firm subgrade and it will help to stop the settlement or any shifting that would cause the concrete to crack. The subgrade must also be compacted and completely flat. When backfilling the subgrade, it is important to know the permissible backfill material. The backfill should be compacted and evenly distributed. Properly preparing the subgrade will help to ensure that the structure will stay in place and greatly minimize any issues from occurring.
LACK OF FLEXIBILITY
Precast concrete can be challenging with its lack of flexibility once the structure has been built and delivered to the jobsite. Sometimes, precast structures are designed based on as-built drawings of what is expected to be encountered below ground. If a precast structure is built to tie into existing piping, there is the possibility of the existing piping to not be in the location expected, possibly rendering the precast structure useless.
To avoid this situation, partial excavations can be performed in advance to confirm the location of utilities prior to manufacturing the precast structure. Structures can also be designed utilizing thin wall knockouts, which are sections of the precast wall specifically designed to be thinner and allowable to break through the area needed. This provides more flexibility in where the location of the pipe penetration occurs in the precast wall.
REPAIRING SPALLS OR CRACKS
Precast concrete structures can sometimes get cracks or spalls. Structural spalls can occur from poor form construction, rough removal from forms, improper storage, early removal of the structure, and poor handling methods of the structure. There are three different types of concrete repairs: Structural Repairs, Cosmetic Repairs, and Architectural Repairs. Structural Repairs involve repairs around any lifting areas or structural connections. Cosmetic Repairs are used to repair the outside appearance of the concrete including filling in any bug-holes, honeycombing, or exposed rebar. Architectural repairs are more focused on high visibility architectural concrete requiring more stringent color and texture matching.
Cosmetic repairs can be performed with a cement and sand mixture called grout and can be applied with a sponge float or a steel trowel.
Structural repairs should be analyzed by a structural engineer and a detailed procedure should be developed to ensure a proper repair is completed. Often, an epoxy mortar or polymer-modified cement-based grout is used to repair structural cracks or spalls. In general, the damaged concrete should first be removed, then the surface prepared for the new repair material. Steel reinforcement should be cleaned and exposed and a primer should be used to create the proper adhesion between materials. Next, the new mortar should then be applied and left to set and cure.
Using precast concrete in your construction process has its challenges, but if handled properly, the benefits to your schedule and installation costs can far outweigh these challenges. Consult with your local precaster if you have any other concerns or questions on your next project.
Stay tuned for the next article!