Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and comments on blog posts or magazine articles posted on precast.org.
If you have a technical question, contact us by calling (800) 366-7731 or visit precast.org/technical-services.
If we ensure the concrete reaches the required compressive strength at 28 days – let’s say 5,000 psi – why are M10, M15, M20 and M25 specified? Why are different proportions specified as long as the concrete reaches the necessary strength?
NPCA Technical Services engineers answered:
M10, M15, M20 and M25 refer to different mix design proportions. Various mix designs can achieve a 5,000-psi compressive strength using entirely different proportions of raw materials. It’s important to not only ensure the concrete mix design will provide the necessary compressive strength, but also that the raw materials and the proportions in which they are batched into the mix provide the desired fresh and hardened concrete properties. The economy of the mix is another important consideration in proportioning.
Confirming that concrete reaches the minimum required compressive strength is not enough to ensure it will perform as required for its entire service life. A concrete mix could achieve a 5,000-psi compressive strength but may not perform well in service because of the type, quantity or proportions in which the other raw materials are used. For example, a mix could reach the required compressive strength while having a water-to-cement ratio that’s too high. This could make the concrete susceptible to watertightness and durability issues. A mix can reach the required compressive strength while using inappropriately sized aggregates, which could increase paste demand and the cost of the concrete while also potentially creating shrinkage-related issues. Mix proportions affect all fresh and hardened concrete properties, and compressive strength is only one of many important factors to consider. This necessitates defining specific mix proportions.
We manufacture retaining wall blocks indoors in a cold climate. Sometimes, we notice small bugholes on the top surface of some of the blocks. In service, those small bugholes could fill with water and then freeze, potentially causing the concrete around the bughole to spall. The specification calls for 4,000-psi concrete with an air-entraining admixture. Our overnight breaks are over 3,000 psi and 28-day breaks are over 8,000 psi. Do we have any reason to worry about the small bugholes or the durability of the concrete?
NPCA Technical Services engineers answered:
This is unlikely to be a cause for concern. However, you should first revisit your concrete placement and consolidation practices to minimize the frequency and size of any bugholes. NPCA has resources on both topics available on our website at precast.org. In some cases, performing a minor repair on the bugholes may be advantageous. In other scenarios, leaving them alone is best.
Consult the project specification for guidance regarding bughole acceptance and minor repairs.
While there is little consensus throughout the industry regarding acceptable criteria for bugholes, AASHTO R73, “Standard Practice for Evaluation of Precast Concrete Drainage Products,” evaluates bugholes for certain products and is used as a guide by some agencies – even for products outside the scope of R73.
Additionally, the Portland Cement Association states, “These surface voids are primarily an aesthetic problem for exposed structural concrete. However, problems do arise if the concrete surface is to be painted or if the voids reach a larger diameter (typically greater than 1 inch).” 1
Because these small bugholes are on the top exposed surface of the precast product, water may collect there and freeze in your cold climate. However, the water would not be trapped in the bugholes. If the water does freeze, it will have space outside of the precast block into which it can expand as it freezes.
Additionally, the air-entraining admixture used in the concrete mix design will help protect the blocks against freeze/thaw damage. In short, any water collected within small bugholes like the ones you’re referencing is unlikely to be confined during freezing and expanding, and consequently is unlikely to cause spalling. PI