CSL 316 Self Levelling Silicone Joint Sealant: Frequently Asked Questions

What is CSL 316 and what does it do?

CSL 316 is a one part self levelling silicone joint sealant from CSL Silicones. Contractors use it on horizontal concrete joints on roads, bridges, airport aprons and taxiways, harbours, industrial floors and logistics yards. The sealant flows level into the joint, bonds to clean concrete and steel without primer, and cures at ambient temperature and humidity.

In addition, it complies with ASTM D5893 Type SL and handles +100/−50% joint movement per ASTM C719. It also conforms to the COTO specification used on SANRAL projects. CSL reports 316 joints in service in North America for more than 20 years, far longer than polyurethane joint sealants usually last in exposed conditions. Full product details are on the CSL 316 product page.

What is a concrete expansion joint and why does it need a seal?

A concrete expansion joint is a gap that designers build into a concrete structure so the concrete can expand and contract with temperature, settle under load, and move without cracking. Without joints, concrete roads, bridges, airport pavements and industrial floors would crack at random as the structure moves.

An open joint, however, lets water into the subgrade. Water erodes the supporting layers, causes uneven settlement, leads to spalling at the joint faces, and on bridges reaches the reinforcing steel and starts corrosion. Stones and debris in an open joint also stop the concrete closing freely, which builds up compressive stress and can cause blowups in hot weather. A good joint sealant therefore keeps water and debris out while moving with the joint. Our complete guide to concrete joint sealants covers this in more detail.

What is the difference between silicone and polyurethane joint sealants?

The chemistry decides how long the joint lasts before it needs attention. In particular, three differences matter most on exposed infrastructure in Southern Africa.

First, UV resistance. Polyurethane is an organic polymer that breaks down under UV through photo oxidation. Over time it hardens, chalks and cracks, and in strong sun this can start within a few years. Silicone, by contrast, has an inorganic silicon oxygen backbone. CSL states that sunlight, ozone, rain and temperature extremes do not affect 316.

Second, movement. A standard polyurethane joint sealant carries a Class 25 rating, which means ±25% of the joint width. CSL 316, on the other hand, carries a +100/−50% rating per ASTM C719. So it can stretch to twice its installed width and compress to half of it.

Finally, service life. CSL reports 316 joints in service in North America for more than 20 years. Polyurethane joint sealants on exposed infrastructure, by contrast, need replacing far sooner. Read more in our silicone vs polyurethane comparison.

What do ASTM D5893 and +100/−50% movement mean?

ASTM D5893 is the American standard specification for single component silicone joint sealants that go on cold onto Portland cement concrete pavements. Its scope covers road and airfield joints. Within it, Type SL means self levelling, for horizontal joints. CSL 316 complies with ASTM D5893 Type SL.

ASTM C719 is the test that cycles a sealed joint open and closed while it stays bonded to the joint faces. A rating of +100/−50% means the sealant held its bond and did not tear when the test opened the joint to twice its width and closed it to half its width. A standard Class 25 sealant, however, only has to pass at ±25%.

Does CSL 316 meet SANRAL requirements?

SANRAL does not approve individual products. Instead, it refers specifiers to the COTO standard specifications, so contractors can use any product that conforms to COTO on SANRAL projects. CSL 316 conforms to the COTO specification for joint sealants used on SANRAL projects.

Why does polyurethane joint sealant fail so quickly in South Africa?

Two things work together.

South Africa has some of the highest UV levels in the world, and the Highveld gets intense sun all year. As a result, polyurethane on road and bridge joints starts to photo oxidise in direct sun. Over time the sealant hardens, cracks across its body and stops sealing. Water then gets into the subgrade, and the result is the potholes and broken pavement we see across the country.

Thermal cycling adds to it. On the Highveld, the difference between overnight lows and afternoon highs can exceed 20°C. Because the slabs move every day, the movement pushes a Class 25 polyurethane past its limit until it tears. CSL 316, however, has four times the extension and twice the compression of a Class 25 sealant, and keeps its flexibility in the sun.

What is self levelling and why does it matter for horizontal joints?

A self levelling sealant is thin enough to flow under its own weight when you pour it into a horizontal joint. It spreads across the joint, wets both faces and settles to a smooth, level surface.

Gun grade sealants for vertical and overhead joints, on the other hand, are too stiff to do this properly in a horizontal joint. As a result, they trap air, leave voids and do not wet the joint faces evenly. A self levelling sealant fills the joint completely and leaves a flush surface with no edge for tyres to catch. Even so, CSL still recommends tooling 316 against the joint faces after placing it, for the best adhesion.

What is the correct joint preparation before applying CSL 316?

Preparation decides whether the joint lasts. The joint faces must be clean, dry and free of dust, frost, oil, laitance, loose concrete and old sealant, because contamination on the faces is the main cause of adhesion failure.

CSL’s preparation steps:

  • First, allow new concrete at least 7 days to cure and dry
  • Then sandblast the top 25 mm of each joint face in a separate pass, with the nozzle close to the face
  • After that, blow out dust with oil free compressed air, moving in one direction only
  • If you use water to flush the joint, work in one direction and let it dry fully
  • Next, install a backer rod that is a snug fit on both sides, at the right depth
  • Finally, place the sealant at half the joint width in thickness, 6 to 12 mm, recessed 6 to 12 mm below the surface

316 needs no primer on clean, dry concrete or steel. For unusual substrates, however, contact Technical Solution Supplies before starting.

What is a backer rod and why does the joint need one?

A backer rod is a compressible foam rod, polyurethane or expanded polyethylene, that goes into the joint before the sealant. The rod does two jobs. First, it sets the depth of the sealant so the thickness is right along the whole joint. Second, it stops the sealant bonding to the bottom of the joint. Sealant bonded on three sides cannot stretch properly, so it tears early. With a backer rod in place, however, the sealant bonds only to the two joint faces and stretches and compresses as designed.

Can CSL 316 handle heavy vehicle traffic on roads and industrial floors?

Yes. 316 suits horizontal joints that carry traffic. Its elongation at break is 1000% and its hardness is 10 Shore A, so it gives under each wheel and recovers without fatigue. Polyurethane sealants, by contrast, stiffen as UV and thermal cycling age them, which puts more stress on the bond line until the sealant tears or lets go of the joint face. A properly recessed 316 joint can take traffic once installation and clean up are complete. See our article on joint sealants for heavy traffic.

Does CSL 316 resist jet fuel and chemicals?

Yes. CSL has tested 316 against jet fuel, hydraulic fluid and deicing fluid, and the test reports are under Supporting Documents on the CSL 316 product page. That makes it suitable for airport aprons and taxiways, fuel handling areas and industrial floors with oil spills. For heavy or continuous chemical exposure, however, contact Technical Solution Supplies with the details.

Is CSL 316 suitable for bridge deck joints?

Yes. Bridge deck joints take traffic loading, the thermal movement of the whole structure, and water, salt and grit from road drainage. On the coast they also face salt air.

The +100/−50% movement rating and low modulus suit the movement of bridge decks. In addition, UV resistance means the exposed sealant does not harden between the carriageway and the edge beam. Silicone also stands up to road runoff and salt.

Is CSL 316 suitable for airport apron and taxiway joints?

Yes. Fuel spills are the main problem on aprons, since polyurethane sealants swell and break down under repeated fuel contact. CSL has tested 316 against jet fuel, hydraulic fluid and deicing fluid without loss of adhesion or cohesion.

Aircraft loads are also far higher than road traffic. The high movement rating and good recovery after compression suit these joints, and a long service life means fewer shutdowns for joint repair on operational surfaces. Read more about silicone sealant for airport aprons.

Is CSL 316 suitable for harbour and port infrastructure?

Yes. Port pavements face salt water, heavy container handlers, cargo spills and constant wetting and drying. Because silicone does not break down in salt air or chlorides, and because the movement rating handles the loads from container handling equipment, 316 is a good fit for concrete paving joints in ports such as Durban, Cape Town and Richards Bay. See our article on joint sealants for harbours.

Is CSL 316 suitable for water infrastructure such as canals and reservoirs?

For horizontal joints that get wet, such as canal linings, reservoir roof decks and treatment plant slabs, 316 stays flexible and keeps its seal. However, 316 is a self levelling product, so it is not for vertical joints on dam faces or walls. Instead, a gun grade sealant is the right choice there. For joints that are permanently under water, or in contact with drinking water, contact Technical Solution Supplies before specifying.

How does thermal cycling affect joint sealants on South African roads?

Thermal cycling is one of the main causes of joint sealant failure on our roads. The Highveld sees daily swings of 20°C or more, and across the year Gauteng goes from summer afternoons above 35°C to winter nights below 5°C.

In fact, concrete moves with every cycle. For example, a 10 m slab moves about 3 mm at each joint for a 30°C change. On a 12 mm joint, a Class 25 sealant can take 3 mm either way, so it is working at its limit every day. CSL 316 on the same 12 mm joint, by contrast, can open to 24 mm and close to 6 mm, which leaves a wide margin.

What is the lifecycle cost advantage of CSL 316 over polyurethane?

316 costs more per litre than a standard polyurethane joint sealant. Over a 20 year asset life, however, the comparison changes.

Each time a polyurethane joint fails, the owner pays again for traffic management, lane closures, cutting out and cleaning the joint, new material and labour, plus any subgrade damage from water that got in before the repair. On a highway with thousands of metres of joints, that bill comes around again and again over the life of the road.

A correctly installed 316 joint, by contrast, pays those costs once. So for road authorities running large joint programmes, the saving grows with the size of the programme.

How long does CSL 316 take to cure?

316 is a moisture cure silicone. It cures by reacting with moisture in the air, so it needs no mixing and no heat. Tool the sealant against the joint faces within about 22 minutes of placing. After that, the surface turns tack free in about 90 minutes, the sealant cures by 48 hours, and full properties develop at 7 days. A properly recessed joint can take traffic once installation and clean up are complete.

Where is CSL 316 available?

CSL 316 comes from CSL Silicones. Technical Solution Supplies distributes CSL Silicones products across Southern Africa, including the DRC and the Seychelles. Contact us for specifications, application guidance, samples and pricing.

Phone: 031 002 7376 Email: sales@tssupplies.co.za

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