Concrete and Rubber: Distant Cousins with Much in Common

By Mike Miller

Most everyone in the precast concrete industry knows concrete and rubber share a complementary relationship. In very simple terms, concrete structures form our stormwater, wastewater and traffic systems, while rubber products provide the sealing elements between these structures. Concrete brings strength, rigidity and permanence to the equation. Rubber provides the flexibility needed for rigid elements to be sealed at their junctions. Beyond these functional differences, the two materials share far more similarities than are immediately apparent.

A closer look inside

Concrete and rubber are both complex mixtures of a variety of different materials, many of which serve similar structural purposes and are used in somewhat similar ratios. If we compare these, they look something like this:

Table 1

In both, some ingredients serve more than one function. For concrete, water is both a processing fluid and the primary curing additive. For rubber, oils aid processing and also moderate physical properties of the finished product.

Permanence

Both concrete and rubber are thermoset materials. This means heat is required for them to achieve their final state, which is permanently fixed on a molecular level. Concrete develops its own heat through hydration while rubber must have heat provided. Both materials use a small quantity of binders, distributed completely throughout the mixture, that react to heat and form permanent bonds.

In concrete, the bonds and the final material are rigid. The cementitious material surrounds all elements in the mix, allowing the mixture to become one unit. In rubber, bonds are created only between nodes in the polymer molecules. This permits temporary displacement of these molecules while retaining the connections among them. This difference is what gives concrete and rubber unique and very different characteristics in their final states.

Processing similarities

The similarities between the materials continue with processing. Concrete and rubber require the constituent materials be mixed in a similar manner. Both materials are precisely batched to ensure the required ingredients are incorporated together consistently and at the proper time. There are practical and technical limits to this process in both instances. Concrete mix times are limited to prevent over-mixing, while rubber mixing times are limited to prevent over-heating.

For both materials, mixing equipment, mix designs and production processes vary but continue to share many similarities. Rubber and concrete are typically processed in forming equipment, prior to the introduction of heat. Concrete is typically placed in molds before heat, through hydration, occurs. Once this begins, the material hardens to its final state over time. Rubber processing usually starts at room temperature and processing equipment warms it so it’s easier to form. Then it is placed into a mold and additional external heat is applied. After a sufficient application of heat, the material, even immediately after being removed from the mold, displays its final characteristics and cannot be changed.

Material testing

Material properties of both rubber and concrete are interpreted from pre-process testing and confirmed with destructive testing of samples. Since the properties of both materials change during processing, the properties determined from lab tests correlate with actual outcome. For example, concrete mix design may be tested by slump or spread. Rubber may be tested by measuring its cure on a device that shows the expected cure curve. Both tests imply the suitability of materials and may also provide guidance for processing.

The results of destructive testing are then used to demonstrate the outcome of the materials and processes. Both rubber and concrete are commonly produced to exceed minimum requirements to protect against unexpected variables and provide consistent performance.

Advancements

Both rubber and concrete have been commonly used materials for many decades, and both have undergone dramatic changes and improvements in that time. It’s not always easy to see these improvements on the exterior, but performance is greatly impacted. The age and experience of both industries is an asset. Concrete and rubber have been tested extensively and proven to be trusted materials through decades of performance. Both the precast and rubber industries have bright futures ahead as they continue to advance and work together to create infrastructure we all depend upon.