How can you determine the mix design of an existing concrete?

When an existing structure needs to be strengthened, extended, repaired, or reproduced, a simple compressive strength measurement is not enough. In the construction sector, design offices, method engineers, heritage architects, and precast concrete manufacturers need to identify the actual constituents of the material: hydraulic binder, the nature of the cement paste, the water/cement ratio, the type and proportion of aggregates, and any additions or special elements.

Determine the mix design of an existing concrete makes it possible to go beyond a simple mechanical assessment to understand how the material was formed and secure calculation assumptions in line with current standards.

An assessment of existing concrete can make it possible to identify the cement family or the hydraulic binder, estimate the cement content in hardened concrete, observe the microstructure of the cement paste, characterize the aggregates by petrographic nature, shape, and size classes, and reconstruct a particle size distribution curve.

Depending on the condition of the sample, it is also possible to approximate the water/cement ratio in existing concrete, detect certain mineral additions or heterogeneities, and guide interpretation with regard to porosity, compaction, and expected behavior in an aggressive environment.

Characterizing an existing concrete relies on several families of analytical tools. Visual and microscopic observation makes it possible to distinguish the cementitious matrix, the paste-aggregate interfaces, and heterogeneities. Concrete petrographic analysis helps identify mineral constituents and the overall texture. Electron microscopy and microanalysis techniques can contribute to concrete binder assessment and the identification of mineral phases.

Separation, sieving, and particle size measurement tests make it possible to reconstruct the particle size distribution curve and refine aggregate identification. Chemical and thermal approaches can also be used to estimate the constituent fractions and support the cement content in hardened concrete.

A compressive strength measured at a given time does not, by itself, tell you enough about concrete durability. Two materials with similar mechanical performance may behave very differently when exposed to water, freeze-thaw cycles, salts, carbonation, or chemical attack. Knowing the mix design means anticipating how the concrete will behave in its environment over the next 50 years.

Identifying the hydraulic binder, the water/cement ratio, the likely porosity, and the nature of the aggregates provides decisive information for assessing the compatibility of a repair, the feasibility of an extension, or the relevance of a recalculation.

The laboratory approach consists of breaking down hardened concrete as a multiphase system. We do not simply describe the concrete; we deconstruct it to understand how it was formed. From a core sample, a block, or a representative sample, the study combines macroscopic observation, concrete petrographic analysis, separation and aggregate identification, binder characterization, estimation of the cement content in hardened concrete, and assessment of the water/cement ratio in existing concrete.

The goal is to establish a composition sheet that can be used for recalculations, technical expertise, or reproducing a formula as closely as possible to the original material.

As with any de-formulation process, the analysis of hardened concrete aims at a technically usable reconstruction, not an absolute reproduction of every ingredient at the exact percentage. Aging, carbonation, advanced hydration, previous repairs, or sample heterogeneity can limit certain conclusions.

However, a well-designed analytical protocol makes it possible to obtain a composition profile precise enough to support a decision on calculation, repair, or industrial reformulation.

The laboratory first defines a study plan tailored to the client's objective: load recalculation, building extension, pathology, historical compliance check, or exact reproduction.

After the sample is received, the concrete analysis are sequenced to produce converging results. The data are then interpreted from a materials perspective to deliver a clear summary: type of concrete, probable composition, critical parameters, and associated confidence level.

This report gives the client a usable technical basis without being limited to the strength value alone.

For a structural design office, this data helps strengthen the input assumptions for a diagnosis or recalculation. For a heritage architect, it helps preserve the appearance and behavior of an old material. For a precast concrete manufacturer, it makes it easier to reproduce a reference concrete as closely as possible to the existing one.

In all cases, the challenge is to turn an unknown material into actionable technical information.

FILAB supports manufacturers and construction professionals with a tailor-made analytical approach. The laboratory develops a study plan based on your technical objective, deploys complementary analysis methods, and delivers interpreted results for practical use: authorizing new load calculations, preparing an extension, comparing several areas of a structure, or reproducing concrete identically with a new supplier or a new process. The added value lies not only in measurement, but in the ability to connect composition results to an engineering decision.