Ready-mix delivery times are a constant battle for producers.

{QUESTION} We are constantly battling time constrains on ready-mix deliveries and need help extending the time limits on some projects. We can maintain slump with water reducers easily beyond the 60 or 90 minutes allowed in the specifications. What advice can you offer?

{ANSWER} This is not a simple issue. Most states limit concrete delivery to 60 minutes from the time water hits the cement to the time of discharge. ASTM C94, “Standard Specification for Ready-Mixed Concrete,” allows for 90 minutes until discharge.

These are relatively safe time frames considering the impact on the quality of the concrete. Even though additional admixture will increase slump to maintain workability, hydration is occurring and changes are ongoing in the concrete mixture.

Many ready-mix producers have transported concrete more that 60 to 90 minutes on their own accord. I have investigated situations where this has occurred and did not find evidence of poor quality issues, although I do not recommend exceeding the time constraints without studying the effects first. With proper investigation, producers have transported concrete in excess of three hours. Others have stopped hydration completely until reactivated with additional admixture.

Some state DOTs allow longer times between batching and discharge, but they often require additional evidence that there are no negative effects. Typically, the most common concern is with the air void system. Studies have shown that with longer mixing times, the air void system diminishes, with a decrease in total air content and increase in spacing factor. This may not be standard with today's air entrainers. New synthetic air entrainers are more stable and maintain the air void system under much longer mixing cycles.

Many specifications are allowing longer transit times if you can show that air entraining is not affected. Often the producer must batch concrete and measure air content at several intervals over the desired transit time for a project. We have been involved with several of these projects where pressure meter tests are taken and cylinders are cast for hardened air content and strength measurements every 15 or 30 minutes after initial batching.

We have seen extensions in transit time up to 120 minutes with this approach.

Air content is important, but do not to focus only on this one property. Other issues may develop regarding strength gain, finishing time, or placement issues. The concrete must be designed correctly for longer transit times.

Using fly ash or slag in a mix can extend the working time and help with longer transit times, although it is important to understand how they affect the finishing window if the surface requires a steel trowel. Fly ash or slag usually extends working time, but extending the finishing window too much may cause finishing issues such as blisters or dusting.

The effects of retarders are typically easy to predict and measure when wanting to slow a set time. Depending on the placement, retarders may cause the opposite effects from fly ash or slag. Retarders will delay the set time and finishing window, but they often make the finishing window much smaller.

This can catch a concrete crew off guard and miss the opportunity to put a nice tight surface on a slab. If the placement is not a steel-troweled slab, these affects may be irrelevant.

Remember that slump loss does not equal set time. Set time may be delayed but the slump may still decrease over time, causing placement issues. This will stump people many times, as they attempt to deal with long transit times. Often, someone on the jobsite will indicate the concrete is setting up too fast when it's really loosing slump. Additional retarder may not provide the desired effect in correcting the issues.

Almost every admixture supplier offers slump retention admixtures. These are designed to maintain a constant slump over time. Typically, water reducers—high-range, mid-range, or low-range—provide an initial slump gain and then gradually lose slump over the next 45 minutes. Slump retention admixtures are designed to maintain constant slump over time.

The downside is that after their design life expires, there is a rapid slump loss. These admixtures can be applied to longer transit times to help maintain slump and be redosed in transit for very long hauls.

So we've talked about long transit times, but there is still the problem of getting the specifying agency to accept longer mixing time periods. Historical documents and national standards all limit batching-to-discharge to 90 minutes. Most specifiers rely on these standards.

It is difficult, but not impossible, to get a specifier to change from a national standard. You must do your homework, and in the case of a DOT or other public agency, you will likely have to engage them in a research study. As mentioned, some states have already allowed longer time periods, mostly due to partnering with the industry in studying the concerns, issues, and concrete quality.

Contributed by Braun Intertec. For more,