February isn’t just hot – it’s a battleground for concrete durability. In the relentless summer heat, a silent enemy known as plastic shrinkage can wreak havoc on freshly placed concrete, leading to unsightly micro-cracks that threaten long-term structural integrity. At Kraftur®, we’ve spent years perfecting our approach to this challenge, turning potential disasters into robust, long-lasting structures.
The Hidden Enemy: What is Plastic Shrinkage?
Plastic shrinkage in concrete occurs when water evaporates from the surface faster than it can be replenished by the bleed water rising from within. This phenomenon takes place within the first six hours after placement, while the concrete is still in its “plastic” state – soft, workable, and vulnerable. The rapid loss of moisture creates tiny, often barely visible cracks. Over time, however, these micro-cracks can develop into significant durability issues, compromising both appearance and structural integrity.
The Technical Side:
Several factors contribute to plastic shrinkage:
- Mix Design: The balance of water, cement, and additives determines how quickly water bleeds out, making the concrete mix design crucial to managing shrinkage.
- Evaporation Rate vs. Bleed Water: When the rate of water evaporation exceeds the rate at which bleed water reaches the surface, shrinkage occurs.
- Environmental Conditions: High temperatures, low humidity, and strong winds – common in Australia’s summer, exacerbate the problem.
The Aussie Climate: A Recipe for Risk
When concrete is cast externally, especially in the hot and windy months of the Australian Spring and Summer, every environmental variable counts. Extreme temperatures and strong breezes accelerate water evaporation, making plastic shrinkage a common and costly issue. While several preventative measures exist, each comes with its own challenges:
- Pouring in the Cooler Hours: Early morning or night pours can reduce evaporation but may incur extra costs due to concrete supply surcharges, penalty rates for sit crews, or restrictions from local councils.
- Forecasting Evaporation Rates: Accurately predicting the evaporation rate requires understanding complex variables such as wind speed, humidity, ambient and concrete temperatures. A miscalculation can lead to inadequate preventive measures.
- Reducing Concrete Temperature: Methods like adding ice or chilled water can slow evaporation but add to project costs and are often not budgeted.
- Aliphatic Alcohol Application: This evaporation retardant forms a surface membrane to trap bleed water, but timing is everything. Applied too early or only once in severe conditions, the membrane can break or be insufficient.
- Effective Curing: Same-day curing at the specified coverage rate is critical. Delays or the use of substandard curing compounds can leave the concrete vulnerable.
- Mix Design Considerations: The concrete mix must be designed with an appropriate bleed rate. A mix that bleeds too slowly may not provide enough moisture to counteract evaporation, while a quick bleed might not sustain the surface long enough for finishing operations.
- Incorporation of Synthetic Fibres: Adding fibres can help reduce shrinkage cracking, but only when integrated correctly into the mix.
The Kraftur® Advantage: A Proactive, Science-Backed Approach
At Kraftur®, our expertise isn’t just about following best practices – it’s about refining them. Our extensive experience with plastic shrinkage has taught us that every pour is a high-stakes operation where precision and adaptability make all the difference. Here’s how we turn potential disasters into durable successes:
01 Monitoring Evaporation Rates in Real Time
We install weather stations and monitoring equipment directly at the pour location. By measuring air temperature, relative humidity, concrete temperature, and wind speed every 30 minutes, we calculate the evaporation rate (expressed in Kg/m²/h). When readings exceed 0.5 Kg/m²/h, we know it’s time to take extra precautions.
02 Issuing Detailed Weather Reports
The day before a scheduled pour, we produce a comprehensive weather forecast that includes prospective evaporation rates. This information is critical for our Site Leaders, who use it during toolbox talks to ensure every team member understands the environmental challenges and the necessary preventive measures.
03 Coordinating Strategic Night Pours
For sizeable pours, we negotiate night pours to avoid peak daytime heat. This means concrete can be on site at 1 AM, poured at 6 AM, and finished by 9 AM – well before the heat of the day accelerates evaporation.
04 Controlling Bleed Rate
We work closely with our concrete suppliers to understand the proposed mix properties. During the pour, extra testing helps us measure both the duration and volume of bleed water. This data informs our decision on whether one or two applications of aliphatic alcohol are needed and ensures that the mix design is optimal for the prevailing conditions.
05 Investing in a Skilled and Committed Workforce
Ultimately, our greatest asset is our people. Our crews are not only skilled and well-trained but are also passionate about achieving positive outcomes. At Kraftur®, we invest in our team’s development, ensuring that every individual contributes to producing concrete slabs that stand the test of time.
Building Beyond the Cracks
Plastic shrinkage might be an invisible enemy, but at Kraftur®, we’re not in the business of letting unseen challenges undermine our work. By combining cutting-edge monitoring technology, strategic planning, and an unwavering commitment to quality, we transform the threat of plastic shrinkage into an opportunity for innovation and excellence.
We don’t just lay concrete, we deliver to our clients A Concrete Advantage®. Every project is an opportunity to innovate and improve, ensuring that our structures remain durable, reliable, and free from the hidden dangers of plastic shrinkage. With Kraftur®, you’re not just investing in concrete; you’re investing in a future built to last.