Concrete is a popular construction material because of its durability and versatility. Unfortunately, it is also incredibly volatile in its plastic state, which makes it difficult to predict how the concrete might change during the trip between the ready-mix plant and the job site. This is especially true regarding the concrete slump. Concrete slump is one of the most challenging characteristics of concrete to control and is a common reason why concrete trucks get rejected from the job site. So why does concrete slump change so much, and how can you – the ready-mix producer – gain more control over it?

Temperature Changes

Temperature variations are perhaps the number one challenge when controlling the concrete slump. As the temperature rises, water evaporates from the mix. This causes a decrease in the workability and the slump of the concrete. Some loads lose up to one inch of the slump in warm temperatures.

Aggregate Type & Size

The aggregates used in concrete all tend to absorb moisture at varying rates. The amount of water absorbed by the aggregates depends on the type and size of the aggregate and the amount of moisture already present. Aggregates tend to be more moist first thing in the morning and lose moisture throughout the day. So, your first load may have a different slump from the last load, even if they have the same mix design. As for how the type of aggregate impacts the slump, the more fine materials present in the mix, the lower the slump will be. Fine materials absorb water faster because they have more surface area than coarse aggregates.

Mixing Time

Concrete slump decreases the longer the load is mixed. This is due to water evaporation over time and the development of fine materials during the mixing process. Some tests showed a nearly six-inch slump loss after 60 minutes of additional mixing time.

How Can You Control the Slump?

Adding a superplasticizer or viscosity modifying admixture at the job site is the easiest, most cost-effective way to adjust the slump at the job site. Fritz-Pak’s water-soluble packaging makes this easy and fast. For more information about our slump-change admixtures, check out our product video. Don’t forget to follow our social media channels for more information about how our products can improve your concrete.