Sensor monitoring for concrete curing and early age strength

Concrete curing sensors are not a substitute for a curing regime. They are a way of finding out whether the regime you planned is actually being delivered inside the element, and whether the concrete has gained enough strength for the next operation. On a job where the programme is tight and the formwork is needed elsewhere, that distinction is the difference between stripping on evidence and stripping on hope.

The reason this matters is that concrete does not gain strength on a clock. It gains strength through a chemical reaction that depends on moisture and temperature. Two pours from the same truck, placed on the same day, can be at very different strengths a few days later if one is exposed to wind and sun while the other is covered and wet. A sensor reading taken at the right depth in the element is the only practical way to know which one you have.

This article deals with curing and early age strength monitoring specifically. Moisture monitoring more broadly — including the use of sensors to assess whether a slab is dry enough for a floor covering — is a separate subject with its own decisions and its own sensor types.

What curing actually is, and why it needs monitoring

Curing is the maintenance of moisture and temperature in concrete after placement so that the cement continues to hydrate. Hydration is the reaction between cement and water that produces the binding compounds in the hardened paste. If the water is lost, the reaction stops, and the strength that would have developed does not develop. If the temperature is too low, the reaction slows dramatically. If the temperature is too high, the reaction runs fast and the early strength may be high, but the long-term strength and durability can suffer.

The traditional approach on site is to cure for a nominated period and assume the concrete has reached a certain proportion of its design strength. That approach works when the conditions are close to the assumptions behind it. It fails when the pour is in hot weather, in cold weather, in wind, or when the element is thin and exposed. In those cases the concrete at the surface, and sometimes through the depth, is not where the calendar says it should be.

A sensor does not change the concrete. It tells you what the concrete is doing. That information lets you decide whether to extend curing, change the protection, delay stripping, or proceed. The commercial value is in avoiding two opposite errors: stripping too early and damaging the element, or leaving formwork and props in place longer than necessary while the programme slips.

Temperature, maturity and strength: what the readings can tell you

The most common early age monitoring approach is temperature measurement at one or more points in the element, combined with the concept of maturity. Maturity is a way of combining time and temperature into a single number that correlates with strength gain for a given concrete mix. The relationship between maturity and strength is not universal; it is established for a particular mix, usually by the concrete supplier or in a trial, and then used to interpret the temperature record from the site.

In practical terms, you place temperature sensors at the locations that matter, log the temperature over time, and use the maturity relationship to estimate the strength at those locations. The output is not a test result in the sense of a cylinder test; it is an estimate based on the temperature history. That distinction is important when you are using the data to make a decision, and it is why the method should be agreed with the engineer and the certifier before it is relied on.

Temperature sensors can also be used more simply, without a maturity calibration, to confirm that the concrete is being cured within the temperature range the specification requires, or to confirm that protection is working in cold weather. In that role the sensor is a compliance tool rather than a strength prediction tool.

Choosing sensors for the decision you need to make

There is no single sensor that answers every question. The starting point is the decision. If the decision is when to strip formwork from a suspended slab, you need to know the strength at the critical location, which generally means temperature or maturity monitoring at depth. If the decision is when a slab is dry enough for a moisture-sensitive floor covering, you need moisture or humidity readings, which is a different measurement and often a different device.

Embedded temperature sensors come in several forms. Thermocouples are simple and robust and can be read by a handheld meter or a logger. Resistance temperature detectors are more stable and are common in logging systems. Digital sensors on a bus can be daisy-chained through an element and give a profile of temperatures at several depths. The choice depends on how many points you need, how long you need to log, and whether you need to read the data remotely.

Wireless and remote logging systems have become common because they remove the need for someone to attend the site with a reader. That matters on a job where the critical period falls over a weekend or a public holiday, which is exactly when a stripping decision often needs to be made. The trade-off is that the system needs to be reliable, the data needs to be accessible to the people making the decision, and the sensors need to survive placement.

Surface moisture sensors and humidity sensors are used where the question is about drying rather than strength. A concrete element dries from the surface inwards, and the moisture condition at the surface is not representative of the condition at depth. If the floor covering manufacturer specifies a moisture condition at a particular depth, the measurement has to be taken at that depth, using the method the manufacturer recognises.

Placement: the reading is only as good as the sensor location

The most common failure in curing monitoring is not a sensor failure. It is a sensor in the wrong place. A temperature sensor sitting on the bottom of a slab, or tied to the top mat of reinforcement, will not tell you the temperature in the middle of the element. If the decision is about strength at the critical location, the sensor has to be at that location.

For a suspended slab, the critical location for stripping is often the point of maximum moment, which is typically near mid-span or over a support depending on the span arrangement. For a thick element, the core temperature can be significantly higher than the surface, and the difference drives both strength gain and the risk of thermal cracking. A single sensor gives you one point; a profile of sensors at different depths gives you the gradient.

Sensors must be fixed so they cannot move during placement. Vibration, the flow of concrete, and the actions of the placing crew can all displace a sensor that is only loosely tied. A displaced sensor may still read, but it will read the wrong location, and the error will not be obvious in the data. Fixing to reinforcement with the correct standoff, or to a purpose-made carrier, is standard practice.

Sensor cables and leads need protection where they pass through construction joints or out of the element. A cable that is damaged during finishing or by subsequent trades will take the monitoring data with it. Leave enough slack to reach the logger, label the leads, and protect the exit point.

Setting trigger values and deciding what to do with the data

Monitoring without a plan produces data, not decisions. Before the pour, the project team should agree what the readings mean and what will happen if they fall outside expectations. For temperature monitoring, that usually means a range within which curing is considered satisfactory, and a response if the temperature falls below or rises above it. For maturity monitoring, it means the maturity value or estimated strength at which stripping or loading can proceed.

The response to an out-of-range reading should be specific. If the concrete is cooling too quickly, the answer might be additional insulation. If it is getting too hot, the answer might be to reduce the heat input or adjust the mix for the next pour. If the maturity is lower than expected at the planned stripping time, the answer is to wait, and the programme needs to absorb that. The point of setting triggers in advance is that the decision is made calmly, with the engineer, rather than under pressure on the morning the formwork is due to come down.

The data also has a second life. Temperature and maturity records are evidence of how the element was cured, and they can be used to support a claim that the concrete reached the required condition before a load was applied. That evidence is more useful if it is stored with the pour record, referenced to the sensor locations, and retained in a form that can be produced later. A logger that is read and then wiped is a missed opportunity.

Worked example: a suspended slab and a stripping decision

A builder in western Sydney is constructing a multi-storey residential building with suspended post-tensioned slabs. The programme requires the formwork and props to be struck and moved up to the next level as quickly as the concrete allows, and the engineer has specified a minimum strength before stressing and before stripping. The pour is scheduled for a warm day with a forecast of strong wind in the afternoon.

Before the pour, the builder agrees a monitoring plan with the engineer. Temperature sensors are installed at mid-depth at the critical locations, a maturity relationship for the mix is provided by the supplier, and trigger values are set for both the minimum maturity before stressing and the maximum temperature differential across the element. The logger is started before concrete arrives, and the pour start time and mix details are recorded.

On the day, the wind arrives earlier than forecast. The surface of the slab dries faster than expected, and the curing compound is applied later than planned because the placing crew is stretched. The sensors show that the temperature at mid-depth is tracking as expected, but the maturity at the critical locations is slightly behind the curve used for the programme. The builder has two options: stress and strip on the planned day and accept a risk, or wait.

Because the trigger values were agreed in advance, the decision is straightforward. The engineer reviews the data and confirms that the concrete has not reached the specified maturity at the critical location. Stressing is delayed, the formwork stays in place, and the programme absorbs a short delay. The alternative — proceeding on the assumption that the calendar had been satisfied — would have risked a post-tensioning operation on concrete that had not reached the specified strength, which is a safety issue as well as a structural one.

The cost of the delay was a few days on one level. The cost of the alternative is not worth quantifying, because it involves the potential for a structural failure during stressing, or a permanent defect that would surface later. The monitoring did not create the delay; the weather and the curing conditions did. The monitoring allowed the builder to see it and act on it.

What sensors do not replace

A sensor reading is not a substitute for the specified curing regime. If the specification requires the concrete to be kept wet, or covered, or protected from wind, those requirements still apply. A sensor can confirm that the regime is working, but it cannot compensate for a regime that is not being implemented. A slab that is left exposed because the sensors show the temperature is acceptable is still losing moisture, and the strength near the surface will suffer.

Sensors also do not replace the specified testing regime. Compressive strength testing of cylinders or cores remains the accepted method for verifying that the concrete meets the specification. Maturity monitoring is an additional tool that can be used to make decisions in the period before test results are available, and to provide a continuous record of conditions in the element. It should be used alongside testing, not instead of it.

Finally, sensors do not remove the need for judgement. A monitoring system can fail, a sensor can be displaced, and a logger can stop recording. The data should be reviewed for consistency, and any gaps or anomalies should be investigated rather than ignored. The value of the system depends on someone looking at the data and understanding what it means.

Records, compliance and the evidence trail

For a licensed builder in NSW, the monitoring record is part of the evidence that the work was carried out in accordance with the plans and specifications. If a question arises later about whether an element was cured properly, or whether a load was applied before the concrete was ready, the monitoring data is the best available answer. That is why the record should be complete, readable and retained.

The record should include the sensor locations, the sensor types, the calibration information where relevant, the logger settings, the pour details, and the readings over time. It should be clear who installed the sensors, who read the data, and who made decisions based on it. If the monitoring is being used to support a departure from a default assumption in the specification, the engineer’s acceptance of the method should be documented.

Keeping up with the technology and the requirements is part of maintaining competence as a builder. The guide to how NSW builder CPD works sets out the framework, and the course catalogue includes options that cover concrete practice and construction technology. The PointCPD home page is the starting point for finding courses that match the work you are doing.

Monitoring systems are also relevant to other moisture and ventilation decisions on a project. The principles of using measurements to inform decisions, rather than relying on assumptions, are the same whether you are dealing with concrete, timber or the building envelope. Related articles on concrete curing in hot weather and roof ventilation and condensation risk cover some of those adjacent situations.

Practical steps for the next pour

The most useful thing a builder can do is decide, before the pour, what the monitoring is for. If it is a compliance record, the plan should capture the readings the specification requires. If it is a decision-making tool, the plan should identify the decisions, the sensor locations that inform them, and the trigger values that will drive action. If it is both, the plan should say so.

From there, the steps are straightforward: select the sensor type that matches the decision, place the sensors where the decision is made, fix them so they stay there, protect the cables, start the logger before the pour, record the pour details, and review the data against the triggers. Agree in advance who is responsible for reading the data and who is authorised to act on it. And keep the record, because the value of the monitoring is often realised months later, when someone asks how the concrete was cured.

Questions builders ask

• Concrete
• Sensors
• Structural

Published 26 July 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.