How can we maintain the smile of concrete?

Introduction

Have you ever witnessed concrete "crying" for help to restore its original condition?

The answer, unfortunately, is Yes. In recent years, we have observed reinforced concrete structures showing signs of deterioration prematurely.

Taking inspiration from renowned author Brian Tracy, 10 Keys to a More Powerful Personality, ten C's ( refer to figure 1). This article puts together nine C's for durable concrete

Nine C's for Concrete Durability

The nine C's are divided into three stages, the Design stage, the Construction stage and the Service/Use stage

The ninth C is Concrete Assessment during the use/service stage. It will provide early warning for maintenance.

In the following discussion, we will delve into key considerations during the three pivotal stages: Design, Construction, and Service.

Design Stage

During the design stage, six critical aspects are established: cover, concrete grade/strength class, cement type, water-cement ratio, cement content and coating.

BS 8500 outlines a step-by-step procedure to specify concrete for durability

In highly corrosive environments the use of corrosion-inhibiting admixtures would delay process of corrosion.

Construction Stage

During the construction stage, water added to the concrete mix eventually evaporates, leaving behind pores. These pores can become a haven for deleterious gases in the environment, such as CO2, which reduces the alkalinity of the concrete. Fresh concrete possesses a high pH, typically above 13, which contributes to the formation of a protective passivation layer around the reinforcing bars (rebar), safeguarding them against corrosion.

To prevent this, it is crucial to ensure proper compaction and curing to render the concrete non-porous. Additionally, the site should have provisions for regular water content testing, such as the Microwave Oven Water Content Test (AASHTO TP 23), to maintain the quality of the concrete.

Use Stage

Concrete deterioration can be categorized into three distinct phases: Initiation, Propagation, and Delamination. Conducting regular condition assessments is crucial as it enables early intervention to prevent the deterioration from advancing to the propagation or delamination phases, thereby prolonging the structure's lifespan.

Defects in concrete can generally be classified under three broad categories:

1. Mechanical - This includes damage from impact, overloading, movement, and forces from vibration, earthquakes, or explosions.
2. Chemical - This encompasses deterioration due to aggressive chemical exposure, efflorescence, leaching, and alkali-aggregate reactions (AAR).
3. Physical - This covers degradation from freeze/thaw cycles, thermal movement, salt crystal expansion, shrinkage, erosion, abrasion, and wear.

Regular condition assessments are vital for monitoring the health of concrete structures and determining the necessity for preventive maintenance or repair work.

Summary

The nine C's keeping the smile of concrete forever are,

1. Cover
2. Concrete grade
3. Cement type
4. Water-Cement ratio
5. Cement Content
6. Compaction
7. Curing
8. Coating
9. Condition assessment

As Design Engineers, Construction Engineers, Supervision Engineer or Asset Owners/Managers, we have the responsibility to uphold these nine aspects without compromise.

References

1. <https://concrete.ie/wp-content/uploads/2018/12/specification_and_testing_of_concrete.pdf>
2. https://www.istructe.org/journal/volumes/volume-64-(published-in-1986)/issue-2/the-design-life-of-concrete-structures/
3. https://www.concrete.org.uk/fingertips-nuggets.asp?cmd=display&id=34