The Challenges of High Slump, High Early Strength Concrete

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One of the products that the energy transition relies on is high quality, low carbon concrete; no one wants cracks in wind turbine foundations or pre-stressed girders leading to post-stressed project managers!

In today’s blog, Arche’s guest author Forbes Day, shares some of his experiences from 23 years manufacturing pre-stressed precast concrete elements.

Forbes was directly involved with concrete suppliers Hanson and Holcim in refining the high slump, high early strength mixes for pre-stressed precast girders, deck units and octagonal piles.

Keep reading to find out about some of the technical points of using of low-carbon cementitious materials such as fly ash and slag from blast furnaces used in steel making.

Why do we need high slump, high early strength concrete?

To meet the tight timeframes of Infrastructure projects, it is often necessary to maintain a daily casting cycle, therefore efficient casting are critical in maintaining production schedules.

High slump concrete is required to ensure that the concrete flows around the highly congested areas of pre-stressing strand and reinforcement within the mould and a high early strength is required to facilitate the transfer of the prestress and allow the element to be removed from the mould.

Both of these requirements are met by:

  • blending it with slag and fly ash
  • adding a superplasticiser and water reducing agent 
  • applying heat-accelerated (steam) curing. 

Due to fluctuations in slag quality, it becomes necessary to switch between a binary blend of cement and fly ash and a ternary blend of cement, fly ash and slag. The binary blend generally performs better with compressive strength, but is not as effective at reducing the concrete core temperature during curing.

Alkali-Silica Reaction (ASR) and Delayed Ettringite Formation (DEF)

Blended cements also help reduce alkali–silica reaction (ASR), which is the expansive reaction of the alkaline cement paste and silica in the aggregate. This is what is commonly known as “concrete cancer”.

To consistently achieve the minimum transfer strength (usually 40MPa) and keep the core concrete temperature below 80℃, as per Queensland’s Transport and Main Roads Specification MRTS70) is a real balancing act.

Ettringite is a mineral that forms within the concrete as it cures. Research suggests that when the concrete is subjected to temperatures above 80 degrees, the formation of Ettringite is delayed, this is known as Delayed Ettringite Formation (DEF).

DEF creates minor cracks in the concrete , especially when the hydration process is accelerated by external heat. This ultimately leads to long term durability issues as moisture can enter the concrete via these micro cracks, shortening the service life of the product.

Variable quality materials

Throughout the year, the quality of bi-product slag made in the production of steel changes. As stockpiles get depleted, steel production is moderated to match demand in China. Usually around April and then again around October we experience a drop in performance of the compressive strength of the concrete.

Usually a small increase in steam curing can compensate for this drop in strength, however, great care must be taken to avoid overheating the concrete and risking DEF. An increase in the proportion of cement, within the allowable design parameters, in conjunction with some modulation of the steam curing regime can assist in meeting both criteria.

There are also aftermarket products that can be applied before the concrete has fully hardened, these penetrate the surface of the concrete and seal its pores to prevent moisture ingress and improve overall durability of the finished element. If applied early enough, these products can also assist earlier formation of Ettringite.

About the Author

Forbes Day is a civil engineer with 30 years’ experience. His experience in pre-cast concrete manufacturing and quality management is second to none, having been responsible for the supply of pre-cast components to most of the major transport infrastructure projects in south-east Queensland over the past 23 years.

Forbes is available to consult as an associate of Arche Energy in green and low-carbon concrete, concrete substitutes, concrete quality, civil works inspections and civil project engineering.

Contact us to discuss how Forbes and the Arche team can help your project.

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