By Johan van Wyk MICT, N.Dip Civil Engineering | Technical Engineering Support, Penetron Africa
Durability resides where specification, mix design, and workmanship intersects.
Concrete durability cannot be achieved by specification and mix design alone. The influence of workmanship on the durability of structures is something I see every day. A project may have the correct specification, a sound mix design and the right durability-enhancing technology, but if the concrete is not placed, compacted, protected and cured correctly, the performance of the structure will be compromised.
The South African National Standards are clear on the compaction, protection and curing of concrete structures. Not only is this a standard or a Penetron specification, but it is just good concrete practice. Good concrete practice minimises shrinkage and cracking, and it helps ensure that the concrete structure performs as it was designed to perform.
All concrete elements are designed with the understanding that some cracking may occur. The objective is not always to eliminate every crack, but to ensure that crack widths remain within acceptable limits. In the case of Penetron Admix, this is especially important because Penetron specifies self-healing of cracks up to 0,5 mm. When cracks remain within this limit, and good concrete practice has been followed, the crystalline reaction can continue to support long-term durability and waterproofing performance.
All too often, Penetron technical services are called out to a project “because the concrete leaks”, only to find that the concrete has cracked. It is the crack that leaks, whereas the concrete is waterproof with the addition of Penetron Admix, provided that the product has been used correctly and good concrete practice has been followed.
This distinction matters. Penetron Admix enhances the concrete, but it does not excuse poor workmanship. It cannot compensate for concrete that has been badly placed, poorly compacted, exposed to rapid drying or inadequately cured. Durability is not only a product outcome. It is a workmanship outcome as well.
Compaction And Vibration
Compaction, also referred to as consolidation, and mechanical vibration are required to eliminate entrapped air. This ensures that the concrete achieves maximum density and structural strength.
Not only does every 1% of air in the concrete reduce strength by approximately 5%, but it also leaves voids that lead to water ingress, reduced durability and deterioration of the reinforcing steel. I have seen this on the site more times than I can count. The voids are not visible once the formwork comes off, but the water finds them. It always does.
Vibration is the primary defence against honeycombing, which compromises durability. Honeycombing is not just a surface defect. It is a durability problem, and it tells you that the concrete did not properly consolidate around the reinforcement or within the formwork. Where there are open spaces, water gets in. Where water gets in on a reinforced structure, you have a long-term problem.
The purpose of a poker vibrator is not to move concrete around the formwork. It is to pull the air out and let the concrete settle into a dense, continuous mass. The spacing of insertions, the layer depth and the withdrawal rate all matter. Get any one of those wrong and the compaction is not what it needs to be.
Over-vibration causes its own problems. Segregation, where the coarse aggregate separates from the paste, is a real risk if the vibrator is left in one place for too long. So the answer is not simply to vibrate more. It is to vibrate correctly.
In my experience, poor compaction is one of the most common ways a good specification gets undone on site. The mix design can be right. The product can be right. But if the compaction is wrong, the concrete will not perform the way it was designed to.
Protection Is Not Curing
It is important to understand that there is a difference between protection and curing.
Protection refers to the prevention of concrete from drying out rapidly before it is fully set. The most dangerous period is between the time of placement and the time the concrete reaches sufficient strength to resist tensile stresses, due to shrinkage. During this early stage, the concrete is particularly vulnerable to plastic shrinkage cracking.
If there is any delay between placing and finishing the concrete, it needs to be protected against evaporation in hot, dry and windy weather. This is especially relevant in South African conditions, where high temperatures, low humidity and wind can cause rapid moisture loss from the surface. Anyone who has poured in a Highveld summer knows exactly what this looks like.
Covering with plastic sheeting is recommended until finishing occurs. Spraying water into the air is also very effective in maintaining a high-humidity environment around the fresh concrete. The objective is to reduce evaporation from the surface while the concrete is still vulnerable.
The standard also emphasises that the formwork and the subgrade should be pre-moistened before pouring. This prevents them from drawing moisture out of the fresh mix. A dry subgrade or dry formwork can remove water from the concrete at exactly the wrong time, increasing the risk of early shrinkage and cracking.
Protection is essential to reduce plastic shrinkage cracking. Many concrete durability problems begin in the first few hours after placement. They are not always caused by the mix design. They are often caused by the way the concrete was handled, exposed and protected on site.
Curing Develops The Concrete
Curing refers to the maintenance of moisture and temperature to facilitate full hydration. The American Concrete Institute describes curing as maintaining satisfactory temperature and moisture conditions long enough for hydration to develop the desired concrete properties.
In order for concrete to develop strength, it should be kept damp for an adequate period after placement. Hydration requires water. If the concrete is allowed to dry out too early, hydration is interrupted. Once that happens, the concrete may not achieve the density, strength and durability that were expected from the specification.
Water curing should be continued for at least 7 days. Curing compounds such as Peneseal FH will suffice if applied correctly. Peneseal FH is a clear, reactive penetrating sealer that acts as a floor hardener, dustproofer and curing aid. Like any site-applied solution, it must be used correctly and at the appropriate stage of the work.
Personally, I prefer that the work is covered as soon as surface texturing is completed, with plastic sheeting that is weighted down to ensure that wind does not get in under the sheeting. If wind gets under the sheeting, the concrete is no longer properly protected. If the sheeting lifts, rapid drying begins again.
The aim is straightforward: keep the concrete damp and protect it from conditions that will interrupt hydration.
Temperature And Thermal Shock
SANS 10100-2 provides guidelines for placing concrete in extreme heat, which is common in South African summers, as well as in cold conditions.
Temperature affects concrete behaviour in both directions. Hot weather increases the rate of moisture loss and can accelerate setting. Cold weather slows hydration and strength development. Both conditions must be managed if the concrete is expected to perform properly over time.
The standard also warns against using curing water that is significantly colder than the concrete itself, as the resulting thermal shock can cause surface crazing and structural cracks. The FHWA Guide for Curing Portland Cement Concrete Pavements also notes that curing water should be near the temperature of the concrete, cautioning against water that is more than 10 degrees Celsius cooler than the concrete surface.
This problem is often evident when the concrete is not kept continuously wet, but rather sprayed with water first thing on arrival on site and then just before leaving the site.
That is not proper curing. It creates cycles of drying, wetting and temperature shock. Concrete requires consistent moisture during curing, not occasional attention.
A common misunderstanding on site is that spraying concrete once or twice a day means the concrete has been cured. It has not. Curing is a continuous process. If the concrete is allowed to dry between applications, the damage may already have occurred. Good curing is not complicated, but it does require discipline.
Durability Is A Shared Responsibility
It is clear from conversations on sites and with engineers that Penetron Admix can ensure durability, but not overcome bad concrete practice.
This is the central point, and it is one I find myself repeating often.
Penetron Admix works from within the concrete matrix. It uses moisture and the unhydrated cement that is always present in concrete to form crystals that block the pores and capillaries through which water would otherwise move. It keeps working as long as there is moisture present. That is what makes it effective over the long term.
But it needs something to work with. If the concrete has been poorly compacted, if it has honeycombed, if it dried out too fast and cracked beyond the self-healing threshold, then no admixture is going to fix that. The structure has already been compromised.
Specification, mix design and product selection are important. So are placing, protection and curing. One cannot be expected to carry the other.
The durability of a concrete structure is determined by the total process. What is placed, compacted, protected and cured. All of it counts.
We cannot make bad concrete good, but we can make normal concrete ultra durable.
About the Author
Johan van Wyk holds a MICT and a National Diploma in Civil Engineering and serves as a Technical Engineering Support specialist at Penetron Africa. With extensive field experience across South African construction projects, Johan works directly with engineers, contractors and site teams to ensure that Penetron products are applied correctly and that good concrete practice is followed from placement through to curing. His work is grounded in what he sees on site every day.