Have you noticed concrete pitting or "bughole" defects on the surface of your poured concrete foundation walls? These imperfections, while often cosmetic, can sometimes be an indication of an issue with the overall quality of the concrete. If you aren't sure, don't hesitate to contact one of our structural engineers.
What is concrete Pitting (Bugholes)?
Bugholes are small voids on the surface of concrete structures, most commonly observed in vertical surfaces. These defects result from the migration of air and, to a lesser extent, water from the concrete to the interface with the formwork used during pouring. The issue primarily arises during the concrete's consolidation phase, where the goal is to densify the mixture and eliminate entrapped air and excess water. If the air bubbles don't find their way out of the concrete before it sets, they leave behind the small cavities known as bughole defects.
Causes of Bughole Defects
Several factors contribute to the formation of bughole defects, with improper vibration during the consolidation process being a significant one. Adequate vibration is essential to encourage the movement of entrapped air and water to the surface. However, excessive or insufficient vibration can either leave voids or cause segregation and bleeding of the concrete.
The type of formwork material and the use of form-releasing agents also play roles in bughole formation. Nonpermeable forms and incorrect use of releasing agents can hinder the escape of air voids, leading to more pronounced bughole defects. Additionally, the concrete mix design, including the choice of aggregate and admixtures, affects the likelihood of bughole occurrence. Stiff or sticky mixtures are particularly prone to surface void formation.
Solutions and Prevention
To minimize bughole defects, several best practices should be followed:

1. Proper Consolidation: Ensure adequate vibration with each concrete lift to facilitate air and water movement towards the form's surface.
2. Use of Permeable Forms: Permeable forms allow air to escape more easily, reducing bughole defects. Correctly choosing and applying form release agents also improves surface quality.
3. Optimal Mix Design: A workable, flowing concrete mixture with properly graded aggregate and suitable admixtures can significantly decrease the risk of bughole formation.

While bughole defects are primarily an aesthetic concern for exposed structural concrete, their presence can indicate underlying issues with concrete consolidation, formwork, and mix design. For home buyers, understanding the nature of these defects helps in making informed decisions about the quality of a concrete foundation. If concerns about the structural integrity of a concrete foundation arise, consulting with a structural engineer is the best course of action. By focusing on quality materials, proper construction techniques, and diligent supervision, the impact of bughole defects can be minimized, ensuring a durable and visually appealing concrete foundation.

The hollow cores of concrete blocks allow an easy path for insects to reach the the wooden sole plate at the bases of walls and floors and begin feasting away.

A better method of building the foundation is to have at least the top course of block to be of solid or “termite” blocks. Sometimes, even many years ago, many higher quality builders additionally capped this top course of concrete block with a metal “termite” shield – copper being one of the best and longest lasting materials. This shield is typically a little wider than the block and the overhangs are bent downward to become an inedible and troublesome mechanical obstacle to insects.

The wood frame garage in this picture was built on a foundation made entirely of hollow core concrete blocks. An expensive repair was required which involved cutting out and replacing large wall areas, a section at a time. At the same time the foundation was "upgraded" -- some of the concrete blocks were replaced with solid block and some hollow blocks were filled with a mortar grout. The photo was taken at an early stage of repair and shows the insect tunneling in the wooden sole plate.

The homeowner had purchased the house about 15 years earlier without the advantage of a thorough pre-purchase buyer's home inspection.