Bonding New Concrete to Old: SBR Bonding Slurries and Micro-Concrete Pours

Bonding new concrete to old sounds simple, yet it is where many repairs fail. In Egypt we see it on raised floor toppings, thickened slabs, rebuilt stair nosings and new sections cast against existing members. The new layer looks fine for weeks. Then it sounds hollow under a hammer, cracks along the edge, or lifts off in one piece. This guide explains why that happens, and how a prepared surface, the right bonding slurry and careful curing prevent it.

Contents

• Why New Concrete Does Not Stick to Old by Itself
• Bonding Agents for Bonding New Concrete to Old
• Preparing the Old Surface
• Mixing and Applying the Bonding Slurry
• Practical Steps for Bonding New Concrete to Old
• Curing When Bonding New Concrete to Old in Hot Weather
• Common Mistakes
• FAQ
• Conclusion

Why New Concrete Does Not Stick to Old by Itself

Old concrete is not a clean, fresh surface. It usually carries laitance, dust, oil, old paint or a weak carbonated skin. Fresh concrete placed on top bonds to that weak skin, not to the sound concrete below it.

Water is the second problem. A dry, absorbent slab sucks water out of the new mix at the interface. As a result, the first millimetres of the new layer never hydrate properly. Shrinkage then adds stress, because the new layer shrinks while the old one has already stopped moving.

So bonding new concrete to old depends on three things working together:

• sound, rough and clean concrete;
• the correct moisture state at the interface;
• a bonding layer that is still wet when the new material arrives.

Bonding Agents for Bonding New Concrete to Old

Most site work in Egypt relies on SBR (styrene butadiene) latex. It goes into a bonding slurry and, where specified, into the repair mortar itself. Two Sika products of this type are covered here. In addition, a flowable micro-concrete suits thicker sections cast in formwork.

Product	Type (data sheet)	Bonding use stated	Key limits stated
SikaBond LA	Synthetic rubber (styrene butadiene) emulsion; complies with ASTM C1059-91	Bonding of old to new concrete; screeds, renders, repairs	Bond coat 1:1 with water plus fresh cement; never emulsion and water alone; minimum +5 °C
Sika Latex	Modified SBR emulsion and mortar additive	Bonding mortar between old and new concrete	Grout of 1 latex : 1 water : 1 cement : 1 washed fine sand; up to 40 mm per layer
Sikacrete-114	Free-flowing, cement-based micro-concrete, 10 mm maximum aggregate	Casting sections and re-profiling members with formwork	50 mm minimum and 500 mm maximum per pour

The SikaBond LA and Sika Latex sheets also state 0 g/l VOC under SCAQMD Rule 1168. The Sika Latex sheet adds a WFBS listing for cement mortars in contact with drinking water.

Which one fits the job?

For thin toppings, renders and patching, a latex-modified mortar over a latex bonding slurry is the usual choice. Thicker sections behave differently. When the new element is too large for a grout but too small for normal concrete, Sikacrete-114 is poured into formwork instead.

Preparing the Old Surface

Preparation decides most of the result when bonding new concrete to old. Both latex sheets ask for a surface that is sound, clean and free from oil, grease, laitance and loose particles. The Sika Latex sheet suggests a scrabbling machine or mechanical abrasion to remove weak material.

In practice, our crews follow this order:

1. Break out hollow, cracked or contaminated concrete until sound material shows.
2. Roughen the surface mechanically, then remove all dust.
3. Expose and clean any reinforcement that will be embedded.
4. Saturate the concrete with clean water, ideally the day before.
5. Remove standing water, so the surface is saturated but dry to the touch.

The SikaBond LA sheet calls this state saturated surface dry (SSD). Likewise, the Sika Latex sheet warns against leaving puddles. Too dry and the old slab steals water; too wet and the bond coat dilutes.

Testing the substrate

Where the topping is large or loaded, a pull-off test helps. ASTM C1583 describes how to test the tensile strength of the prepared surface and, later, the bond of the new layer. It is a quick way to confirm the substrate is sound before the main pour.

Mixing and Applying the Bonding Slurry

The bond coat is a thin slurry, not a paint. Each sheet gives its own recipe, so we follow the one for the product in use.

For Sika Latex, the grout is one part latex, one part water, one part cement and one part washed fine sand. The sheet gives a coverage of 4 to 5 m² per litre of Sika Latex. It must be brushed generously and vigorously into the pre-wetted surface with a stiff brush.

For SikaBond LA, the sheet describes a 1:1 mix of SikaBond LA and water, mixed with fresh cement and brushed in. However, both sheets give the same warning: never use the emulsion with water alone as a bond coat.

Timing matters most. The first layer of new mortar or concrete must go on while the slurry is still wet. If the bond coat dries, it becomes a weak film instead of a bridge. Therefore, we coat only the area that the next layer can cover within minutes.

Practical Steps for Bonding New Concrete to Old

Target crews work through this sequence on toppings and repairs, adjusted to each specification.

1. Agree the design first, including any dowels or shear connectors set by the structural engineer.
2. Prepare and pre-wet the substrate to SSD.
3. Mix the bonding slurry to the product recipe.
4. Brush the slurry into the surface over a small area.
5. Place the new mortar or concrete onto the wet slurry and compact it well.
6. For latex mortars, keep the water/cement ratio low; the Sika Latex sheet advises below 0.4.
7. Build thick latex mortars in layers of 40 mm at most, keying and grouting each hardened layer.
8. Cure immediately and protect the work from heat.

Pouring micro-concrete against old concrete

Sikacrete-114 needs 1.9 to 2.1 litres of water per 20 kg bag, mixed by drill or forced-action mixer, never by hand. The surface is dampened thoroughly first. The sheet also lists a pull-off bond strength of about 3 N/mm². On large areas or very dense substrates, it says a bonding agent such as Sikadur-32 or SikaTop Armatec 110 EpoCem will improve adhesion. Formwork must be secure and watertight, with channels for trapped air to escape.

Curing When Bonding New Concrete to Old in Hot Weather

Egyptian summers are hard on thin toppings. Wind and sun dry the surface within hours, and the new layer cracks or curls at the edges. Consequently, curing is part of bonding new concrete to old, not an afterthought.

The Sika Latex sheet asks for protection from rapid drying with polythene, damp hessian or a curing compound. In the same way, the Sikacrete-114 sheet recommends a curing compound, polythene or wet hessian. For hot conditions, it advises chilled mixing water to keep the mortar below 30 °C. It also asks for shading base plates and foundations from direct sunlight.

Common Mistakes

• Casting onto dusty, oily or laitance-covered concrete.
• Placing new material on a bone-dry slab, or on standing water.
• Using latex and water alone as the bond coat.
• Letting the bonding slurry dry before the topping goes on.
• Pouring latex mortar thicker than the sheet allows in one layer.
• Skipping curing on hot, windy days.

FAQ

Is a bonding agent always needed when bonding new concrete to old?

Not always. Some designs rely on a rough, clean, saturated surface with a cement slurry. However, a latex bonding slurry adds a margin of safety on thin toppings and repairs.

Can I pour a new floor topping on an old tiled floor?

No. Remove the tiles, adhesive and any weak screed first. The new layer can only be as strong as the surface it bonds to.

How thick can a latex-modified topping be?

The Sika Latex sheet allows up to 40 mm per layer. For thicker sections, either build up in keyed layers or use a micro-concrete such as Sikacrete-114.

Does the bond coat replace dowels or steel connectors?

No. The bond coat improves adhesion at the interface. Structural connection between old and new members is a design decision for the engineer.

Conclusion

Bonding new concrete to old works when the substrate is sound, rough and saturated, and when the new layer meets a wet bonding slurry. After that, low water content and immediate curing protect the bond while it gains strength.

Target carries out concrete repair and strengthening work across Egypt. See our repair and rehabilitation services, our jacketing works, and the guide to concrete spalling repair. Contact Target to inspect your structure before the next pour.

For the standard behind latex bonding agents, see ASTM C1059, the specification for latex agents used to bond fresh to hardened concrete.