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.
Adam writes:
We’ve recently had some trouble with inconsistency in our fresh concrete’s air content (we use an air-entraining admixture) and are trying to figure out the cause. Do you have any advice on where to look first?
NPCA Technical Services engineers answered:
Remember that an air-entraining admixture is interacting with the cement paste, which consists of the cement, any supplementary cementitious materials (SCMs) and water.
We suggest examining the results of your air tests over time to pinpoint when fluctuations began and measuring the severity of those changes. Over that same time period, look at your cement mill certificates to verify you’re receiving the same cement type each time. If your mill certificates are consistent, are you tracking the proportions of C3A, C3S, etc. from delivery to delivery?
Sometimes, small changes in the phases of cement can affect concrete behavior, which is also true for any SCMs you are using, particularly fly ash.
If you are not operating on a municipal water supply, you likely have test data for your water source. Check this data for chemical variances in your water supply around the times you notice unexpected changes in air content.
Make sure your aggregate is consistently batched clean. Also check if your coarse aggregate is unusually dry due to hot weather conditions. Dry coarse aggregate when included within the batch can sometimes act like a sponge with admixtures, like air entrainment, absorbing them and not allowing them to do their chemistry magic.
Most importantly, work with your air-entraining admixture supplier throughout this process. Your supplier can troubleshoot and suggest adjustments to dosage rates of their particular admixture.
Dan writes:
If we ensure the concrete reaches the required compressive strength at 28 days, let’s say 5,000 psi, why is 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 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.
NPCA file photo
Ensuring concrete reaches the minimum required compressive strength is not enough to ensure it will perform as required for the duration of its service life. A concrete mix could achieve 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 w/c ratio that’s too high. This will make the concrete susceptible to watertightness and durability issues. It’s possible a mix could reach the required compressive strength while using inappropriately sized aggregates, which could increase paste demand and thus the cost of the concrete, as well as create potential shrinkage-related issues. The 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.
Where does the 5ksi 28 day compressive strength apparent requirement for electrical & telecom vaults come from? I’m a CE PE not an EE so unfamiliar with EE related codes & standards beyond NEC. Noticed ALL the precast power & telecom vaults I found are at 5ksi, but no one seems to have a reference as to why not 4ksi as for storm & sanitary sewer. Is there an ASTM or some other standard paragraph you can point me too? Reason is I want to flesh out the or equal req’t in our telecom trunk line connection vaults thru our airfield as really don’t ever want to revisit even tho’ designed for H80 load. Believe the density at 5ksi for precast is easy enough to achieve (like for tanks) & worth the cost for durability/life.
Hello David,
For underground utility structures ASTM C857 and ASTM C858 would be the applicable standards that apply unless there are additional requirements stipulated by the owner or authority having jurisdiction.
For the design and loading, ASTM C857 “Standard Practice for Minimum Structural Design Loading for Underground Precast Concrete Utility Structures” would apply and for the manufacturing of the product ASTM C858 “Standard Specification for Underground Precast Concrete Utility Structures” would be the appropriate guidance.
The 5,000 psi 28-day strength requirement you note likely originated in one of two places, since the ASTM C858 calls for a minimum of 3,000 psi in 28-days in Section 7.6.
The first consideration would be directly from project specific specifications stipulated by the owner; the second would be something that a precast manufacturer would use.
Utility structures can be cast-in-place in some cases, and I believe this is the primary reason for the f’c of 3,000 psi minimum. However, for a precast manufacturer to be able to ship a product within 3 to 7 days a mix design with a f’c of 5,000 psi minimum is very necessary and quite common.
Should you have any additional questions please let me know.
The fastest and easiest way to reach me is on my cell number provided below.
Best regards,
Phillip B. Cutler, P.E.
Director of Quality Assurance Programs
NPCA