CSA Cement for Non-Shrink Grout
The Real Specification Is Effective Bearing Area
A non-shrink grout exists to do one thing: transfer load from a baseplate into a foundation through continuous contact. Compressive strength is easy to achieve and easy to test, which is why it dominates datasheets. But a 60 MPa grout that has pulled away from the underside of the plate is carrying load through a fraction of the intended area. This page works from that requirement back to what the formulation has to deliver.
What the Grout Is Actually For
Under a machine base, a column plate or an anchor, the grout is a load path. The plate is set on shims at a design elevation, and the gap beneath it is filled. Everything the plate carries passes through that fill into the foundation.
Two failure modes follow directly, and neither is a strength failure:
- Loss of contact at the top. If the grout shrinks after hardening it pulls down and away from the underside of the plate. The load then concentrates on whatever contact remains, often the shims, producing stress concentrations the design never contemplated.
- Voids from incomplete filling. If flow was inadequate, or placement trapped air, the grout never reached full contact in the first place. The material may test perfectly in a cube and still be carrying load on 60% of the intended area.
So the property to design for is effective bearing area: the proportion of the plate underside in genuine contact with sound grout. Compressive strength matters, but it is necessary rather than sufficient, and it is not the property most likely to be missing.
Why CSA Chemistry Suits This Job
It addresses the contact problem directly
The ettringite reaction can be balanced to produce a controlled expansion after set, which maintains contact against the plate instead of allowing the grout to withdraw from it. This is the property the application is named after.
Early strength shortens the critical path
Grouting is usually on the critical path: machinery cannot be commissioned and columns cannot be loaded until the grout has strength. Our CSA Cement 42.5 reaches not less than 30.0 MPa at one day and 42.5 MPa at three, so waiting weeks for a Portland grout to mature is often the real cost being avoided.
Severe confinement suits the mechanism
Expansion needs restraint to become useful pre-stress. Grout under a baseplate is restrained on nearly every face, which is close to ideal, and the opposite of a thin screed with one free surface.
The Central Tension: Flow Against Stability
A grout has to be fluid enough to travel under a plate and fill every corner, and stable enough that nothing separates while it sits there. These pull in opposite directions, and the way formulators usually lose is by solving the first with water.
Add water and flow improves immediately. Then three things degrade at once. Bleeding brings water to the top and leaves a weak, laitance-rich layer exactly at the plate interface, which is the surface that most needs to be sound. Settlement lets solids sink, creating a gap at the top even before any drying shrinkage occurs. Drying shrinkage rises with water content, working against the compensation the expansive agent is providing.
The result is a grout that flowed beautifully and carries load on a fraction of the plate. The correct route to flow is the dispersant and the particle packing, with water held to what the specification allows.
How a CSA-based grout is usually built
Non-shrink grout is one of the applications where CSA binder and CSA expansive agent are used together. Our supplied product information gives a starting direction of 10 to 15% CSA Binder of the OPC blend for non-shrink grout with an expansive agent, and 6 to 8% expansive agent for shrinkage-compensating systems, both subject to trial.
The interaction between them is the main design problem: both are ettringite formers, and they draw on the same sulfate pool. Dosing each by reference to its own datasheet, without checking that the system can supply sulfate for both, is the most common way a grout formulation goes wrong.
What to Test, and Why
| Property | Why it matters for a grout |
|---|---|
| Flow and flow retention | Must remain placeable for the whole pour, not only at mixing. Measure at intervals, not once. |
| Bleeding and segregation | Bleed water collects at the plate interface and forms a weak layer where strength is most needed. |
| Settlement / height change, plastic and hardened | Captures loss of contact from settlement as well as from later shrinkage. |
| Restrained expansion | The property the compensation depends on. Our product information references CSA-I / CSA-II / CSA-III direction. |
| Effective bearing area | The specification that actually reflects the job. Assess on a representative trial plate rather than inferring from cubes. |
| Compressive strength at the relevant age | Necessary, but choose the age the schedule depends on, not only 28 days. |
| Behaviour at placement temperature | Flow, setting and expansion all move with temperature. Qualify at the extremes of the intended range. |
Placement Practice Decides as Much as Formulation
More grout problems come from placement than from the powder. A formulation that cannot survive normal site practice is not a good formulation, but the practice matters too:
- Place from one side only, letting the grout push air ahead of it. Introducing it from two sides traps air in the middle, under the plate, where it cannot escape.
- Maintain head with a head box or dam so the grout is pushed rather than poured. Continuous head is what drives it into the far corners.
- Do not stop mid-pour. A restart against partly set CSA grout creates a cold joint in the load path, and a fast-setting system is less forgiving of interruption than a Portland one.
- Control substrate suction. A dry, absorbent foundation draws water from the grout, starving both hydration and the expansive reaction. Pre-saturate as the specification requires, without leaving free water.
- Cure the exposed edges. Only a narrow perimeter is exposed, but it dries fast and it is the region most visible to the client.
- Do not over-vibrate. A fluid grout does not need it, and vibration promotes segregation and bleeding.
Common Problems and Likely Causes
| Symptom | Check first |
|---|---|
| Gap under the plate after hardening | Water content too high causing settlement or bleeding; expansive agent dosage or sulfate balance; curing at the exposed edge |
| Weak, dusty layer at the plate interface | Bleeding; excess water; segregation from over-vibration |
| Voids found on sounding or coring | Placement from two sides; insufficient head; flow lost before the pour completed |
| Flow adequate at mixing, unplaceable by the far corner | Flow retention not measured over time; temperature accelerating the system |
| Cracking at the exposed perimeter | Edge drying; inadequate curing; excessive water |
| Expansion lower than the trial | Substrate suction removing water; curing shortened; sulfate shared with the binder and under-supplied |
| Strength fine, performance disappointing | The specification being met is not the one that matters; assess effective bearing area |
Frequently Asked Questions
What makes a grout “non-shrink”?
A controlled expansion after setting, which offsets later drying shrinkage so the grout maintains contact with the baseplate. It is a system outcome that depends on dosage, sulfate balance, confinement and curing, not a fixed property of a powder.
Should I use CSA cement or CSA binder for grout?
Use CSA cement when you want a rapid-hardening binder that works as delivered. Use CSA binder when you are dosing CSA reactivity into an existing Portland-based grout and want to control the balance yourself; our starting direction there is 10 to 15% of the OPC blend with an expansive agent. See CSA Binder vs CSA Cement vs Belite Cement.
Can I add water to improve flow on site?
Adding water beyond the specified range is the most common cause of grout underperformance. It causes bleeding and settlement precisely at the plate interface and increases drying shrinkage. Improve flow through the dispersant and packing at formulation stage instead.
How soon can the equipment be loaded?
That depends on the strength the design requires and the grout’s development at your placement temperature. Our CSA Cement 42.5 reaches not less than 30.0 MPa at one day under standard conditions, but the governing figure is your engineer’s requirement verified on site conditions.
Does a non-shrink grout still need curing?
Yes. The expansive reaction consumes water, and although only the perimeter is exposed, it dries quickly. Inadequate curing is a frequent reason measured expansion falls below the trial result.
Why is my grout strong but the plate still moves?
Almost always a contact problem rather than a strength problem: settlement, bleeding, voids from placement, or shrinkage that opened a gap. Assess effective bearing area rather than testing more cubes.
Can I use the same product for anchor grouting and machine bases?
Often, but the confinement, section and placement method differ, so flow requirements and expansion behaviour should be verified for each case rather than assumed.
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View SolutionTechnical Note
This article is intended for technical communication and product selection discussion. Values and dosage directions quoted are typical data from supplied product information and starting points for formulation testing, not a specification. Actual performance depends on formulation design, raw materials, sulfate source, water amount, additives, confinement, substrate condition, curing regime, temperature and placement method. Trial mixing and local verification are required before commercial use, and design should follow the applicable standards and project specifications.
Developing a Non-Shrink Grout?
Tell us the application and confinement, the flow and strength you need and at what age, your base cement and sulfate source, and the placement temperature. We can suggest a starting direction and support trial mixing.
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