A chemical attack is caused by a dissolution of substances, or a reaction between a substance and components of concrete – the resulting products from these reactions have the potential to cause problems with structure, due to dissolution or expansion.
When concrete is exposed to sulphate, the penetration of aggressive agents and it’s reaction with the cement matrix can cause concrete deterioration. The three main reactions involved are:
Each of these chemical reactions can lead to expansion, cracking and the loss of strength and elastic properties of concrete. The form and extent of the damage to concrete will vary depending on the:
NZS 3101:2006 has prescriptive requirements in the durability section for concrete to be used in this environment for a 50 year design life. The exposure class is determined by the severity of the application. These are shown in the table below:
Additional protective measures are required for environments more severe than those listed in NZS 3101:2006, or in more highly acidic and permeable soils with a pH below 3.5 – in order to isolate the concrete from direct contact with the aggressive ground conditions.
Including Microsilica SF or EverPlus Fly Ash at the specified replacement levels complies to the binder requirements of NZS 3101:2006.
Alternatively, using a ternary blend of EverSure Type GP, Microsilica SF and EverPlus Fly Ash from our product range will offer the highest level of resistance against sulphate attack.
ASR is best pictured as a two-stage process:
It’s usual to distribute reactive aggregate particles throughout the concrete, in order to link the micro cracking from individual reaction sites into a network of cracking – meaning that the physical and mechanical properties of the concrete material can be impaired.
The CCANZ document TR3 – Alkali Silica Reaction provides guidance and notes on how to minimise the risk of ASR. The durability section in NZS 3101:2006 – Concrete Structures specifies the use of TR3.
To minimise the risk of ASR, including of a minimum 8% Microsilica SF will reduce the associated associated when reactive aggregates are used.
The mechanisms of including 8% Microsilica SF in concrete are:
Two factors determine the rate of an acid attack; concentration and strength of the acid, and the rate at which fresh acid is supplied to the concretes surface.
The acid will continually lower the pH of concrete. At pH below 7, the calcium hydroxide followed by the calcium silicate hydrate phases are readily decomposed.
The process can be slowed with the inclusion of Microsilica SF and EverPlus Fly Ash. Alternatively, a ternary blend of EverSure Type GP, Microsilica SF and EverPlus Fly Ash from our product solution range will offer the highest level of acid resistance.