Strength and Rigidity

The strength of aggregate will influence the strength of concrete made from it. High-strength concrete requires aggregates of high strength. However, weaker aggregates may be satisfactory if the strength of concrete is not expected to exceed that of the aggregate. The strength of aggregates is likely to vary considerably with their structure and mineral composition.

Aggregates influence the drying shrinkage of the concrete by restraining the shrinkage of the cement paste. The rigidity of the aggregate will influence its restraining effect. Thus, the higher the modulus of elasticity of the aggregate, the more effective it will be in reducing the shrinkage of the concrete.

Aggregate strength is generally gauged by a crushing test in accordance with NZS 3111, Section

14 Crushing Resistance of Coarse Aggregate. This test determines the crushing force which, when applied to a known mass of coarse aggregate, will produce fines amounting to 10% of the original mass. There are no specific limits set by NZS 3121.

Typical values range from 50 kN to 100 kN depending on the concrete exposure condition, the lower value being for aggregates to be used in concrete in protected conditions. Higher wet strengths are required for aggregates to be used in more adverse conditions.

3.3.10 Reactivity

General

Aggregates that are chemically stable will neither react chemically with cement in a harmful manner nor be affected chemically by normal external influences. Reactive aggregates may result in serious damage to the concrete by causing abnormal expansion, cracking and loss of strength.

Alkali-aggregate reactions

Some aggregates containing reactive silica will react with the alkalies in cement – sodium and potassium oxides – to form an alkali-silica gel which takes up water and swells. This causes abnormal expansion and map-cracking of the concrete

Figure 3.6.

Figure 3.6 Typical map cracking caused by alkali-aggregate reactions

The situation in New Zealand is that considerable investigations were carried out by the DSIR (now IRL) over a long period of time to establish rock types that were prone to alkali reaction.

This work was coordinated together with other test work from other researchers into a publication TR3 Alkali Aggregate Reaction and Tables 3.7 and 3.8 (page 3.12) categorise the principal rock type into non reactive and reactive types.

Chapter 3 Aggregates for Concrete

Table 3.7 Aggregates known to be non- reactive from field experience and testing

Greywacke Schist

Basalt <50% Si02 Quartz sands Phonolite Rhyolitic pumice

Granite Perlite

Vermiculite Limestone

Table 3.8 Aggregates or minerals known to be potentially reactive either from field experience or laboratory testing

Basalt >50% Si02 Christobalite

Andesite Tridymite

Dacite Quartzite

Rhyolite Amorphous and

cryptocrystalline silicas Volcanic glass (including opal and

chalcedony)

From experience gained by examining a limited number of structures that had experienced the problem, it was concluded that if the alkali content in the concrete could be kept no higher than 2.5 kg/m3 then the risk of expansive reactions was significantly lowered when potentially reactive rocks are used. New Zealand cement manufacturers assist with this requirement by voluntarily keeping the alkali level of the cement to below 0.6%.

One key factor that has often been overlooked is that it is the sand that can be an important trigger mechanism in the reaction. This has been particularly so in New Zealand which perhaps

explains why the mortar test method set out in NZS 3111, Section 11 has been successful in predicting problems. Typical traces of results for rhyolite and andesite compared with non reactive aggregates clearly demonstrates the relative reactive risks between materials, see Figures 3.7, 3.8 and 3.9 (page 3.13). (Tests based on ASTM C289).

One topic not easily understood is that different reactive aggregates produce different amounts of expansion in concrete as their proportional content changes. Figure 3.10 (page 3.13) shows how, for example, having just 12% of rhyolite causes 0.55% expansion yet using a concrete that has been made with 100% rhyolite materials the expansion is less than 0.1%. It is important therefore in any analysis of structural expansion to consider whether pessimum levels of reactive materials were in use. Another kind of harmful reaction, which also results in abnormal expansion and cracking of the concrete, occurs between the alkalies in cement and dolomitic limestone. This is known as alkali- carbonate reaction but is rare in New Zealand and Australia.

Other Reactions

Other damaging chemical reactions involving aggregates include oxidation or hydration of certain unstable mineral oxides. Pyrites (ferrous sulphide), for example, can oxidise and hydrate to form brown iron hydroxide which causes unsightly stains. The presence of magnesia (MgO) or lime (CaO) in the aggregate may also cause pop-outs or cracking due to their hydration and expansion.

Tests and Testing

Field service records, when available, provide

Figure 3.7 Results of testing rhyolite, dacite, and some alluvial materials containing these rocky types by ASTM C289

Figure 3.8 Results of testing Egmont andesite from Taranaki by ASTM C289

Chapter 3 Aggregates for Concrete

Guide to Concrete Construction 3.13

Figure 3.9 Results of testing greywacke samples by ASTM C289

Figure 3.10 Typical pessimum proportion curves for three New Zealand rock types (expansion at 12 months at

1.5% Na20 equivalent)

A Whakamaru rhyolite (Samples 80-88) B Tongariro andesite (Sample 30) C Egmont andesite (Sample 456)

information for the selection of non-reactive aggregates. If an aggregate has no service record, a petrographic examination can be useful by providing a description of its mineralogical and chemical constituents. It involves an examination of the aggregate particles with a microscope, together with other procedures for determining the constituents present, and in the hands of an experienced person can identify potentially reactive