Sacking bug holes in natural concrete

Bugholes! What to do?

Well – it depends on you; what do you want in your concrete? Read on to learn a little more about bugholes in concrete. At Sanderson Concrete, we strive to reduce the amount and size of bugholes where possible.

Concrete is one of the world’s oldest building materials. Strong and durable precast concrete products can be manufactured in numerous finishes, textures and colours. 

 Precast concrete is produced in a plant – or factory – where mix designs, reinforcing placement, vibration and finishing are tightly controlled and can far exceed what is possible in the field with Cast In Place concrete. Subsequent finishing, such as sandblasting of various depths, from light sandblast to exposed aggregate, allow the precaster to turn plain old grey concrete into beautiful accent pieces for buildings and landscapes. Polishing techniques, ranging from a light polish that shows just the aggregates and sands’ tips to a full terrazzo polish with tight aggregates and little paste showing, can create some stunning precast concrete.

 Natural – or plain – concrete, particularly in grey, is everywhere in our day to day environments. This ubiquitous material is in our sidewalks, driveways, curbs, buildings and occasionally even inside our homes. The visible surfaces are sometimes “finished” to varying degrees, while other times are left in concrete’s natural state. Depending on which surface you are looking at, this natural state will have a varying degree of “Bugholes” or voids in the concrete.

 To learn more click on the headings below

Bugholes are normal

In its elemental form, concrete is produced by putting raw materials – sand, stone, cement and water – in a concrete mixer and tumbling the ingredients until a consistent concrete mix in a pourable state is achieved. The action of mixing concrete produces bubbles or air pockets in the concrete. Unlike a smooth cake batter, concrete “holds” on to the air bubbles – they do not move much in the concrete mix once it stops being mixed or vibrated. At the microscopic level, these air pockets improve the concrete’s durability by increasing the concrete’s ability to withstand constant freezing and thawing – a property that concrete producers and engineers seek to increase by adding an air-entrainment admixture to the mix. These air-entrainment bubbles are microscopic and allow water to freeze and expand on the concrete’s surface without damaging the concrete itself.

 Naturally occurring bugholes – some of which can be quite large – do not make for good or bad concrete. The concrete’s strength, durability and functionality are determined by the mixed materials and proportions and the reinforcing. Good concrete is strong, performs its intended function and lasts. Just look at the Colosseum in Rome (yup, concrete).

Aesthetically, bugholes may not be desirable.

The size of the bugholes can be reduced, but as air is a natural part of the concrete, it cannot be entirely eliminated integrally and on some finished surfaces. Depending on the surface plane of the finished concrete piece (top, sides, ends, bottom), this final finish will start in one of five ways:

  • Adhered to a surface – up against a wall, on top of a subsurface (such as a crushed stone in the case of a driveway). In these cases, the surface is unimportant and will not be seen – the gravel or wall become both the form the concrete is poured onto and the connection to whatever is next to the concrete. There may be bugholes – but they are not visible or important.
  • A troweled surface – the top of the concrete is troweled smooth, possibly broom finished or sandblasted after finishing, but (aside from Self Consolidating Concrete or SCC) the concrete is always troweled. Properly poured concrete will not have “bugholes” – voids typical to vertical walls. When the concrete is vibrated, the air pockets migrate upwards – like any air bubble. When vibration stops, the aggregates will not allow most of the bubbles to move. Trowelling will smooth out the surface and bring the paste to the surface (the paste is the mixture of cement, water, and the sand’s fines parts – it’s the “glue” that holds the larger aggregates together when the concrete is cured). While this troweling with mask and holes – under the finish, there are still air holes. Sandblasting or wear will expose these holes; however, they tend to be smaller, and fewer of them are right at the surface.
  • A form finish – This is the surface that appears on the concrete when the forms or moulds are removed. Not only is it the final shape of the cured concrete – depending on the material used for the form, but it is also smoother and more uniform than any concrete finisher can produce with a trowel. These surfaces have more than one possibility and have varying degrees of bugholes:
  • The bottom of the form – THE SMALLEST AMOUNT OF BUGHOLES – often the top of the finished precast concrete piece. Many precast forms are designed, so the piece is poured upside down. Among other reasons, this allows the precaster to control the bugholes on the most likely visible surface on the final product. The concrete is poured down onto the bottom of the form. As vibrating the concrete moves, the air bubbles up, they naturally pull away from the form surface, and then paste (the cement and water that holds the aggregates together in the concrete) fills into this area. If zero vibration is applied to the concrete, it will leave bugholes on this surface – but the act of dropping concrete into the form will create some vibration, and this surface will have the least bugholes.
  • The sides of the form – THE LARGEST AMOUNT OF BUGHOLES– often the visible face of the finished product, this surface will always have bugholes, regardless of placement and vibration. Air is sticky and bugholes, as they move up through the concrete, tend to bind to the form surface. The longer a sidewall is on the form, the more bugholes will be visible moving up the wall or form surface. As the bugholes stick – they tend to stay, so as the concrete is placed, more concrete = more air bubbles = more bugholes. The troweled surface –Here, bugholes are minimized in the pouring process. They can be reduced or eliminated fully in the finishing process.

Bugholes! What to do?

Well – it depends on you; what do you want in your concrete?

We have three options when it comes to Bugholes – and it all comes down to us explaining what your concrete can look like and you deciding which you prefer for your precast concrete:

Au Natural or plain. 

Depending on the product and visible surfaces, this is likely all you need. You may accept that the concrete has bugholes – and some items will have very few bugholes on the surfaces that matter – and other surfaces will have few enough that this is not an issue for you. We strive to minimize bugholes using the constraints of making really good concrete that serves its required function, whether that is dictated but the aesthetics of the precast concrete piece or the engineered requirements.

Reduce the bugholes using a superplasticizer. 

Superplasticizers are a chemical admixture that concrete producers add to the concrete mix. These admixtures increase the flow of the concrete without adding additional water to the mix design. As the water to cement ratio in concrete is critical to the concrete’s final strength, only adding water. Simultaneously, it will improve the flow of the concrete and reduce the bugholes, significantly reducing the strength of the concrete. Adding a superplasticizer to the concrete mix allows for a reduction in bugholes and can allow the producer to reduce the water content, which increases the strength of the final product. As a matter of course, Sanderson Concrete uses superplasticizers in our concrete mix designs.

There is a “fourth option” – using superplasticizers for Self Consolidation Concrete or SCC – more on that below.

Patch the bugholes and sack the concrete to remove any visible holes. Depending on the product, sacking can be labour intensive, but it is a perfectly acceptable method of eliminating bugholes from the finished product’s surface. There are some limitations, and there is a cost associated with sacking; however, as producers or Architectural Precast Concrete, Sanderson Concrete’s sacking and finishing crew are efficient and able to overcome most challenges.

A couple of things to be aware of with sacking:

Sacking is not perfect!

Bugholes are a natural part of all concrete. The imperfections in concrete that we try to mask with a patch and sack are just that – masked. The precast concrete was poured before the patch was applied and a different mix design than the patch. The concrete has large aggregates in it, which are an integral part of the concrete’s strength. The patching material has no large aggregates so that it is able to fill in the imperfections and bugholes that you would like covered. As the precast concrete and the patch cure, they will cure at different rates – the concrete is newer, the patch has a bonding agent in it to stick to the precast, and the fine sand is a different grade than the concrete sand in the precast. At Sanderson Concrete, we use a patching compound blended specifically for us to match our concrete mix designs – patches eventually almost perfectly match the precast concrete and blend in or disappear entirely. A final sacking of the entire surface may be desired – typically, this is done to larger surfaces and reasonably quick. 

 At worst – patching bugholes and sacking the surface can be more aesthetically pleasing to some people than Au Natural. At its best – patching and sacking can disappear so that the concrete’s surface looks like it is machine-made. Some shadowing can occur in natural grey concrete and white concrete, but this is more visible in coloured concrete. The pigments’ inherent nature and the small quantities used in a regular concrete batch are impossible to get exactly right at the patch batch size. A 2,000 lb concrete mix may have 1 lb of pigment – a batch of patch will be a pound or two at most. Measuring pigment to 0.02 of a gram is not possible in a concrete plant, but our production managers have decades of concrete mix design experience, and we are rarely wrong on our cured concrete patches. 

And Self Consolidating Concrete (SCC)?

We can work with SCC. We rarely have to – we’re pretty good at creating a finished product that performs aesthetically and functionally according to engineered requirements. SCC is not a panacea – it is merely a mix design – a concrete manufacturer can pour you a product or wall using SCC, and it can still have bugholes. The bugholes will always be there, and they will likely be smaller – on some of the surfaces, they will be eliminated or at least significantly reduced. Still, there is a cost associated with SCC that you may be able to avoid by speaking with us about what you are trying to achieve.

So what about Costs? 

We have an SF (Standard Finish) price for most items we regularly produce, so this is usually simple to quote. We do need to know for custom products in advance – the earlier we can get at the green (fresh) concrete, the closer the precast concrete and the patch tends to be. We can still patch concrete within a few weeks of casting, but it does get a little more costly. Please discuss with our sales team regarding costs.