How To Seal Precast Concrete Joints

How to seal precast concrete joints


The main reason for applying mastic to the joints of precast concrete is to create an impermeable barrier between the units, this could be for preventing dirt and water getting into a construction joint or to make a weatherproof seal – Sealants can also improve the aesthetics of the wall giving the joints a smooth finish.

The benefits of using mastic rather than mortar are that the mastic will remain soft and be able to deal with movement between the precast units. It will also cope with expansion and contraction of concrete joints if being used for floors.

Mastics are supplied in 380ml tubes or 600ml sausages and are applied using a mastic or caulking gun. For large quantities of mastic, we recommend using a battery-powered gun, this will reduce the strain on your hands and arms and also provide a more constant flow of mastic for a better finish.

We supply a range of Sika products for sealing concrete joints and can advise on the most appropriate product for your project or application.

The Benefits of Elastic Joint Sealants

  • Flexible mastics will move with the concrete joint and increase the life-span of the seal.
  • They will adhere to the precast concrete to ensure the joint remains sealed at all times.
  • Mastic is easy to apply without the need for specialist equipment or training.
  • The mastic application is fast and cost-effective.
  • A sealant will improve the visual appearance of concrete joints and be more appealing to architects and customers.
  • The high mechanical strength, chemical resistance and durability of the mastic, will ensure excellent performance.



Step 1 – Concrete Joint Preparation

  1. Before installing a sealant, the joint must be dry and free of dust, dirt and release agent.
  2. Using a stiff brush, clean the joint and surrounding areas.
  3. If excessive release agent is apparent or if sealing a weathered concrete surface, it may also be necessary to clean the surfaces using a pressure-washer or cleaning solution.


Step 2 – Foam Backing Rods

  1. To prevent the sealer from sagging into floor joints or reducing the amount of sealant needed in wide gaps in wall joints, a foam backing rod can be used.
  2. Joints between precast retaining walls are typically 10mm wide. For a joint this size, we recommend using a 15mm diameter rod.
  3. The closed-cell foam rod is placed into the joint and pushed into the gap to fill the void. The exact depth required will vary depending on the mastic being used but is typically ½ the width of the joint – We suggest checking the product datasheet to confirm.
  4. Make sure that the tool used to insert the foam rod does not have sharp edges to prevent tearing the foam.


Step 3 – Priming Concrete Joints

  1. Your concrete joint should now be relatively clean and ready for mastic application. However, to ensure a good bond between the precast concrete units and the mastic, the surfaces should be primed before continuing.
  2. Care should be taken when using these products as they are often solvent-based. Ensure you are wearing the correct PPE and refer to the product data sheet for more information.
  3. Apply the Sika® Primer-3 N (or similar), using a paint brush – note only a small amount of primer is required and should only be used on the joint itself.
  4. The application of the primer before sealing the joints will ensure maximum adhesion to the concrete and enhance the life-span of the product.
  5. For a 1 Litre bottle, you would expect to prime approximately 40 linear metres.


Step 4 – Concrete Sealant Application

  1. Prepare the sealant tube/sausage by attaching the application nozzle to the tube or cut the end of the sausage. If using a mastic tube, cut the nozzle to the required size to enable enough mastic to flow in one movement to fill the joint.
  2. Starting at one end of the joint, apply the mastic using even pressure on the mastic gun or adjust the flow rate of powered mastic to the desired amount.
  3. Move the gun along the joint in one smooth action an even quantity of mastic is in the joint.


Step 5 – Finishing Concrete Joints

  1. Once the joint has been filled with the sealant, you can then smooth the edges, gently pressing into the mastic to get maximum adhesion to the concrete – a rounded edge plastic or wooden tool can be used to do this.


Prestressed concrete panel joint sealing

Maintenance & Repair

It may be necessary to repair or replace the mastic joints after time. This could be because of damage to a precast unit or if the sealant has started to crack or peel away from the concrete.

  1. The first step is to remove or cut away the existing mastic in the joint and if required, remove the foam backing rod if this is also damaged.
  2. Once the existing mastic has been removed, clean the joint thoroughly and follow the original 5-step process to re-seal the joints.
  3. In the event of damage to the edges of the precast concrete units, the concrete should be repaired before sealing. If the damage is minor, then a repair mortar, such as Oscrete – Renocem FC30 Light can be used.
  4. If the damage is more substantial and reinforcement is visible, this will require additional work to ensure the unit is structurally sound. If in doubt, please contact us for advice or speak to a structural engineer for guidance.