by James Mackechnie

The sustainability of concrete mostly focuses on the contribution of cementitious binders since these materials have higher emissions and energy requirements.

James Mackechnie - Education, Training & Research Manager

Concrete aggregates are generally shown to have much lower embodied energy and carbon in comparison but this assumes optimum materials that minimise binder contents in concrete without compromising strength, dimensional stability and durability.

This assumption is not always true especially as concrete suppliers sometimes have to use more marginal deposits due to economic, logistic and/or environmental reasons.

When new concrete aggregates are being considered there are several performance requirements that need to be taken into account.

Typical characterisation of aggregates properties for consideration in concrete are shown in Figure 1.

Figure 1: Characterisation of concrete aggregates for use in structural concrete

FRESH CONCRETE PROPERTIES
The quality of aggregates has a direct effect on the fresh concrete properties and this in turn affects hardened properties such as strength and dimensional stability. Examples of this include the following:

• Variable moisture content of fine aggregate makes consistent concrete production more challenging, especially when not using moisture probes. These fluctuations may not just affect slump tolerances but often affect strength and yield of concrete supplied.
• Grading of aggregates needs to be consistent to produce uniform workability and strength of concrete. This is particularly important for fine aggregate where extra fines and silt contamination can have a dramatic effect on water demand affecting not just workability but also compressive strength.
• Shape of aggregate particles is an important but often overlooked aspect of concrete aggregates. Figure 2 shows how this affects water demand and in turn increases cement content and drying shrinkage of concrete.

Figure 2: Typical water demands used for structural concrete in New Zealand

HARDENED CONCRETE PROPERTIES
Some properties of concrete aggregates have a more obvious relationship with the hardened properties of concrete such as density, strength and stiffness.

While most aggregate sources have consistent strength, stiffness and dimensional stability this cannot be assumed for all materials due to the relatively young and diverse geology of New Zealand.

This is illustrated when the elastic modulus of concrete made with different geological types is compared as shown in Figure 3.

Figure 3: Elastic modulus versus compressive strength at 28 days

CONCRETE AGGREGATE QUALITY AND SUSTAINABILITY
When concrete suppliers view quarry resources there is sometimes the misconception that the cheapest materials produce the most cost-effective concrete.

Pressure on quarries to deliver low-cost concrete aggregates often end up costing more for concrete since extra cementitious material is required to compensate for sub-optimal fresh and hardened properties of structural concrete.

Financial incentives need to be built into supply agreements that encourage optimum aggregate properties such that moisture contents are controlled, grading is consistent, aggregate shape is optimised and aggregate strength and stiffness are monitored.

Experience in New Zealand has shown that when concrete aggregates are optimised there is the potential to reduce the cementitious content by 15-20 percent compared with material that is poorly produced.

This would benefit not just material costs of concrete but also reduce emissions associated with Portland cement.

PDF - Mackechnie's Lab 6