Concretes that can be reliably related to each other can be grouped into families and the combined strength data from the family can be used for conformity control. The whole family of concrete mixes are related back to a reference concrete mix design, which is used as the bench mark against which all other family of concrete mixes are related.

What is a Family of Concretes?

A family of concrete mixes may be based upon only two constraints and are mix designs of varying proportions for different applications:

• The same sourced constituent aggregates;
• The same cement type;

Example of a Family of Concrete mixes are (*All values are typical)

Approved Materials List

Admixtures

Special additives are used in concretes to perform specific functions or add a characteristic to the concrete. They are commonly referred to as admixtures and come in liquid form, powders, plastics and metals.

Air Entraining Agent

Air bubbles are introduced into a concrete mix to protect the hardened concrete from freezing and thawing cycles. If there are not enough voids in the hardened concrete water will permeate the surface and when it freezes the water will expand and cause spalling of the concrete. This is apparent when you can see exposed aggregate in a concrete road. When extra voids are artificially introduced it give space for the freezing water to expand into thus providing some protection against freeze / thaw cycles. Air entraining is usually used in road construction or any other large concrete surface area that is exposed to the elements.

Water Reducing Admixture

This is used to reduce that amount of water needed to achieve a specified consistency Class or more commonly used to achieve a specified water/cement ratio. Water cement ratio is directly related to compressive strength, the lower the water cement ratio the greater the concrete strength achieved within certain cement content ranges. Most proprietary water reducing admixtures will give greater strength to concrete mixes for the same cement content used.

Concrete Mix Design

All concrete mix designs in the UK are usually presented as One Cubic metre of fresh concrete using Saturated Surface Dry aggregate values.

*Free Water/Cement Ratio

It should noted that the Free Water/Cement Ratio can be directly related to concrete compressive strength performance, generally speaking the lower the free Water/Cement Ratio the higher the compressive strength should be. Compressive strength of concrete in the UK is commonly measured in N/mm2 (Newtons per millimetre squared) or KN/m2.

Examples of some typical concrete mix designs for one cubic metre of fresh concrete at the same cement content:

Examples of some typical concrete mix designs for one cubic metre of fresh concrete for the equivalent compressive strength of 42.0 KN/m2: (KN KiloNewtons or MPa Mega Pascal's)

Main Relationship Analysis

A strength relationship analysis needs to be created so that all the concrete mixes of the family can be assessed for strength compliance with EN206. This is done by starting with trial mix data from each of the mix design batchbooks. It is apparent from the example mix designs above that strength differs between mix design. So how do we relate all these different characteristics between designs?

Trial Mix data

Concrete trial mixes are performed on every mix design batchbook and 3 x concrete cubes samples are made from each trial mix. One cube will be tested at 7 days and the remaining 2 samples will be tested at 28 days using the average of the two as the reported 28 day result.

All the trial mixes are done at the same consistency class, usually S2 (70mm slump). The set of trial mixes are usually done at at least seven cement contents equally spread throughout a range of 100kg to 550kg.

The trial mix data will provide a complete mix design for every batchbook for material weights per cubic metre and a strength at each cement content throughout the batching cement content range in 5kg increments.

To minimise variation error and for the perfomace relationships to be accurate it is important that enough materials are gathered to complete ALL the trial mixes using the same materials batch.

A Typical Trial Mix Data set for Batchbook 2PCXX