Quality control in concrete blocks

If you want to reduce claims, scrap and stoppages in your plant, quality control for concrete blocks can no longer be just “testing a few pieces in the lab”. Today it needs to be an integrated system: in-line quality control for blocks and pavers, with dimensional and color SPC, complemented with structured sampling, test benches and clear acceptance criteria. All of it connected to your real production, lot by lot.

In-line SPC, color, sampling and acceptance

Picture the scene: early Monday morning and a client calls because the grey paver delivered for today’s job site “doesn’t look like” the one they received three months ago. You compare photos and see differences in tone and edges. The block machine hasn’t changed, but curing, aggregate moisture or pigment have.

Without in-line data and well-defined acceptance criteria, you can only react… and absorb the cost. That is exactly what an advanced quality control system helps you avoid.

At CBM Experts we work every day with concrete block and paver plants that want to take the next step: moving from reactive control to a stable, traceable and automated system, supported by instrumentation, software and standardized testing. Let’s look at how to design it.

What does quality control in concrete blocks really mean today?

When we talk about quality control in concrete blocks, we are not just referring to “reaching the required strength” or “making a block that looks good”. We are talking about:

• Meeting the specifications of standard UNE-EN 771-3 for concrete blocks (strength classes, densities, product categories, dimensional tolerances, etc.).

• Verifying properties through UNE-EN 772 test methods: dimensions, strength, density, absorption, etc., as summarized in the Normabloc Code of Good Practice for block factories.

• Ensuring that your process is capable and stable over time, not just that an isolated sample has passed.

• Translating all of the above into operational decisions: stopping, adjusting the mix, changing moulds, reviewing curing or rejecting lots before they leave the plant.

In other words: quality control is no longer a department, it is a plant-wide system that combines people, procedures, sensors, software and machinery.

How to design in-line quality control for blocks and pavers

In-line quality control for blocks and pavers starts with a basic question: what do you see about your process while it is producing… and what do you only discover when it’s already too late?

In a modern plant, it makes sense to measure three “layers”:

• Process: moisture, mixing times, vibrocompression parameters, curing curves.

• Product in line: key dimensions, appearance, color, edge integrity.

• Laboratory tests: strength, absorption, density, etc.

Defining critical-to-quality characteristics (CTQ)

The first step is deciding what you are really going to control. For concrete blocks and pavers, typical CTQs are:

• Dimensions (length, width, height, flatness): affecting installation on site and jointing.

• Strength and density: linked to standards and to structural/non-structural use.

• Absorption and durability: especially for pavers, exposed-face units and aggressive environments.

• Color and surface texture: key in architectural precast and visible pavements.

Each CTQ should have:

• A specification (e.g. dimensional range, maximum color ΔE against pattern).

• A measurement method (in-line, spot checks, laboratory).

• Frequency and responsibility.

In-line control points

At CBM Experts we usually work with three main control points in the plant:

• At the block machine outlet: quick inspection of geometry and edges (samples per pallet).

• At curing exit: check of dimensions and color, where you already see real effects of shrinkage and moisture.

• End of line / palletizing: reinforced visual control, with the option of automatic rejection if you integrate machine vision.

This scheme integrates very well with broader plant improvement projects. If you are in the middle of a modernization phase, our guide on how to upgrade a concrete block plant can help you align sensors, SCADA/MES and OEE targets without stopping production.

Statistical Process Control (SPC) in block plants

Once you are measuring, the most important part is still missing: interpreting the data. That is where Statistical Process Control (SPC) in block plants comes in.

SPC is not “doing lots of spreadsheets”: it is about applying statistical tools (control charts, capability indices, detection rules) to see whether your process is:

• Stable (only random variation).

• Out of control (mix changes, mould wear, curing issues).

The NIST/SEMATECH e-Handbook of Statistical Methods provides an excellent summary of how to use X̄–R control charts, signal rules and capability analysis in industrial processes.

From “averages” to control charts

Instead of looking only at weekly averages, configure for each key CTQ:

• A subgroup: for example, 5 blocks measured every “n” pallets.

• An X̄–R (mean and range) or X̄–S chart.

• Control limits calculated from your own data (not from the standard).

What do you gain?

• Detecting a change in block height before it drifts outside standard tolerances.

• Seeing whether a new aggregate or pigment batch is shifting the process.

• Differentiating between natural noise and real problems.

Process capability: Cp and Cpk

Here you connect specifications with real variation:

• Cp tells you whether, in theory, your process fits within tolerances.

• Cpk tells you whether it is also centered (not pushed against one limit).

In concrete blocks, targeting Cpk ≥ 1.33 on critical dimensions is a reasonable goal to reduce scrap and jobsite complaints, provided your specification limits are aligned with the standard and with what the client actually perceives.

Dimensional control of concrete blocks and pavers

Dimensional control of concrete blocks and pavers is the foundation: even if the color is perfect, if the height varies by several millimetres, the jobsite will suffer.

Standard UNE-EN 771-3 for concrete blocks (dense and lightweight aggregates) defines how to declare dimensions, categories and permissible tolerances for length, width and height, differentiating between unit types and uses.

In turn, the UNE-EN 772 family defines test methods for the measurement of dimensions, flatness, density and other properties, compiled very clearly in the Normabloc Code of Good Practice.

In-line measurement vs. standardized testing

En In the plant it makes sense to combine:

Fast in-line measurement:

• Calibrated metal gauges for length and width.

• Height gauges at critical points.

• Automatic vision or laser systems that measure height and flatness at high speed.

Standardized dimensional testing:

• Samples taken according to a sampling plan (e.g. n blocks per production lot).

• Measurement on a flat test table, with calibrated instruments, following the corresponding UNE-EN methods.

The trick is to link the two: the quick in-line reading gives you the early warning; the standardized test confirms and documents for audits, certification or customers.

Using SPC on dimensions

Integrate your dimensional data into control charts:

• Choose variables: height, length, width and, if possible, face flatness on exposed-face blocks or pavers.

• Set up X̄–R control charts for each format.

• Monitor trends (mould wear, slack in the block machine) before you “touch” the UNE-EN limits.

If you detect continuous drift in height, it may be time to review moulds, stops and clearances. In our article on maintenance of concrete block making machinery you will find a practical guide to tackle exactly those critical points in the block machine.

Color control in concrete blocks and pavers

In architectural precast and pavements, color control in concrete blocks and pavers is as sensitive as strength. The human eye immediately picks up changes between lots, especially on large surfaces.

The de facto standard for quantifying color is the CIELAB (Lab)* color space, where:

L* represents lightness (from black to white).

a* the green–red axis.

b* the blue–yellow axis.

The difference in color between a pattern and a sample is expressed by ΔE. Spectrophotometer manufacturer Konica Minolta explains this very clearly in its guide on color tolerances, where it details how to calculate ΔL*, Δa*, Δb* and ΔE to assess whether the difference is acceptable or not. You can find it in their technical article “Understanding CIE Lab* Color Space”.

How to measure and specify color

A practical scheme for your plant:

• Define a pattern block for each reference (or a pattern plate) measured with a spectrophotometer in CIELAB.

• Measure production samples at curing exit at representative points on the face.

• Set a maximum acceptable ΔE against the pattern (for example, different for premium exposed-face units than for hidden structural pieces).

• Integrate the data into your SPC: a control chart for L* and for ΔE will help you see whether the problem comes from pigment, curing or mix variation.

In many plants we see that “unstable” color is a symptom of curing without fine T and RH control. If that is your case, it is worth digging deeper into how to design T-RH profiles and heat recovery, as we explain in detail in our article on curing chambers for vibrocompacted blocks.

Sampling, test benches and acceptance criteria for concrete blocks

So far we have talked about in-line measurements. The other pillar remains: sampling, test benches and acceptance criteria for concrete blocks.

Sampling plan: what, how much and when

Based on the recommendations of standards and sector guides such as the Normabloc Code of Good Practice, it is usual to define sampling plans by production lots (e.g. by shifts or by number of pallets).

Conceptually:

• Lot: number of blocks manufactured under relatively homogeneous conditions (same dosage, same line, same unit type).

• Sample size: number of blocks you test per lot for each property (dimensions, strength, absorption, etc.).

• Acceptance criteria: rules that set how many results may be out of specification (and by how much) before rejecting or reclassifying the lot.

In practice, you can combine:

• More intensive sampling at start-ups, raw material changes or new mixes.

• Reduced sampling when the process shows stability (high Cpk and no out-of-control signals).

Test benches: what you need in the plant

Typical test benches (or lab equipment) for blocks include:

• Compression testing press (usually 2,000 kN) with suitable platens for blocks.

• Tables and fixtures for dimensional and flatness tests according to UNE-EN 772 methods.

• Equipment for absorption and density (tanks, ovens, balances, water level gauges, etc.).

If you manufacture structural or special units, additional equipment for flexural tests or specific tests as required by the project.

The critical point is not having the largest array of machines, but ensuring their calibration, traceability and that procedures follow the test standards. This is where a good maintenance and calibration plan makes the difference.

Acceptance criteria: bringing together standards, SPC and business

For each CTQ, define:

• Standard and project specification: what UNE-EN 771-3 and UNE-EN 772 test standards require for strength, density, dimensions, etc.

• Internal safety margin: somewhat stricter than the standard to protect you from undetected variations.

Acceptance/rejection rules:

• What to do if a block is out of dimensional tolerance.

• What percentage of results may exceed the limit before considering a lot “at risk”.

• What events trigger an action plan (review of mix, curing, moulds, etc.).

When this system is combined with your SPC, you start making decisions before the problem, not after.

And if you are going to invest in new machinery or lab equipment, it makes sense for your choice to already incorporate these control requirements.

In our article on how to choose machinery for concrete blocks you will see which technical and automation criteria you should take into account so that the line and quality control go hand in hand from day one.

Implementing an advanced quality control system with CBM Experts

All of this may look like a lot if you are just starting out or if you are coming from a more “craft-based” kind of control. The good news is that you do not have to do everything at once.

At CBM Experts, with more than 30 years of experience and projects in over 60 countries, we help block and paver plants set priorities: identify the quality bottleneck, choose the right instrumentation and deploy it without stopping production.

A reasonable roadmap would be:

Diagnosis

• Analyse your current situation: scrap by family, claims, tests, stoppages.

• Identify which CTQs are costing you most today (dimensions, color, strength…).

Quick wins in line

• Install simple measures (gauges, patterns, visual checklists) where budget is limited.

• Add key sensors (moisture, curing temperature, vision for color/dimension) where the impact is greatest.

Basic SPC and structured sampling

• Start with 2–3 critical variables and simple control charts.

• Formalise the sampling plan and acceptance criteria.

Integration and automation

• Connect measurement to your SCADA/MES and to the end of line, especially if you are considering projects such as automatic palletizing in precast.

• Define dashboards that show you, on a single screen, how your quality is performing lot by lot.

Our role is to help you choose the right combination of machinery, peripherals and control systems for your specific context, not to push you towards the “biggest” solution.

FAQs – Dudas clave sobre control de calidad en bloques de hormigón

How often should I test strength and absorption?

There is no single frequency: it depends on volume, whether the product is structural, the plant’s life stage and what the project specifies. As a reference, many plants combine more frequent testing at start-up or when changing dosage, with a “steady-state” frequency linked to lots (for example, per X m² or X pallets). The important thing is that the plan is documented, risk-based and aligned with the applicable standards.

What is the difference between in-line control and final control?

In-line control measures while you are producing and allows you to correct the process (adjust mix, curing, moulds). Final control (sampling, lab tests, inspection at dispatch) only allows you to accept or reject product. Without in-line control, you are always chasing the problem; with it, you start to prevent it.

What is the minimum equipment needed to start with SPC?

You can start with very little: systematic dimensional measurements with basic calibrated instruments, an organised way of recording the data (even in spreadsheets) and simple X̄–R charts for 1 or 2 critical CTQs, for example block height and L* color value. Later you can add dedicated software and connect it to your SCADA/MES, but the value of SPC lies in how you make decisions with the data, not in the tool itself.

How does curing affect color and dimensions?

Much more than is usually thought. Strong temperature and humidity gradients in the curing chamber can cause tone differences within the same load, darker or lighter edges and deformation and shrinkage variations that translate into greater dimensional scatter. That is why it is so important to control T-RH profiles, uniformity and ventilation in curing chambers and to monitor them continuously, as we explain in our specific article on curing chambers.

What if I cannot always meet the theoretical tolerances?

First, it is worth checking whether the problem lies in the process (mould wear, mix, curing) or in the specifications themselves (perhaps they were defined without considering real capability). SPC will help you see whether the variation is inherent or due to special causes.

From there, you can act on the process to reduce variability and, when appropriate, review the tolerances agreed with the client (especially for non-structural products), always supporting your decisions with standards and real performance on site.

Take the next step in your plant’s quality control

The market no longer rewards whoever makes the most units, but whoever delivers consistent concrete blocks and pavers, with fewer incidents, better image and controlled costs.

Good quality control in concrete blocks, with SPC, dimensional and color measurement, smart sampling and well-defined test benches, becomes a competitive advantage… and a shield against claims.

At CBM Experts we can help you:

• Diagnose the current status of your quality control.

• Prioritise investments in machinery, sensors and lab equipment.

• Design a phased implementation plan that does not slow down your production.

• Support you during commissioning and in training your team.

If you would like to discuss your specific case, we invite you to contact us and start building a quality system that lives up to your blocks:

👉 Fill out our form for an initial technical assessment.

👉 Request a quote and a phased plan from CBM Experts to optimize your end of line without stopping the plant.