Mythbusting concrete: Does high strength equate to high performance?

Video

Abstract

The compressive strength of concrete is arguably the most important parameter in the structural design and construction of reinforced concrete (RC) elements. It is a quick, easy, and cost-effective test, making it an appealing characteristic for quality control and conformity assessment of concrete during construction. While a higher strength class indicates a higher structural capacity, it is also often mistaken to be a sign of higher quality. Conversely, compressive strengths higher than that required for the ultimate limit state (ULS) have no structural benefit and often reduce the structure's sustainability, lead to an increase in material costs, and increase the risk of concrete cracking. In the context of structural repair to damaged RC members, European standards often list compressive strength as one of the top requirements. However, these requirements are no more than arbitrary lower-bound limits that have led to the production of commercial products with incredibly high strengths that often over-exceed these lower limits.

The role of compressive strength in the structural design and construction of RC structures is discussed, presenting a summary of the research on compressive strength and its influence on concrete repair conducted by the Concrete Materials and Structural Research Unit at the University of Cape Town. The first study delves into the relationship between compressive strength and shrinkage cracking tendency using the ring test. It was found that a higher compressive strength resulted in a significantly higher tendency to crack under the effects of restrained shrinkage. This higher tendency resulted from the increased compressive strength, which increased the elastic modulus and reduced the tensile creep and relaxation of the mortar, generating higher stresses when the shrinkage deformations are restrained. The second study developed an analytical model investigating the long-term effects of high-strength mortars on structurally repaired members under compressive loads. The results from the model revealed that while a high-strength mortar can contribute to the load-bearing capacity of a repaired concrete member in the short term, its contribution rapidly declines in the first days of loading. The reduction in the repair mortar's structural contribution was mainly due to its shrinkage and creep, resulting in the externally applied loads being transferred to the concrete substrate over time.

The presentation concludes by stating that more emphasis is placed on the repair materials' durability and limiting their susceptibility to cracking. A recommendation to place upper limits to the compressive strength in standards and specifications and pay greater attention to parameters that influence crack sensitivity (particularly shrinkage) are considered applicable measures to attaining a more durable repair.