Race To Strength

On time-sensitive concrete repair projects, such as highways and airport runways, fast-setting concrete is a must, delivering high strengths in a matter

On time-sensitive concrete repair projects, such as highways and airport runways, fast-setting concrete is a must, delivering high strengths in a matter of hours and allowing contractors to avoid costly liquidation damages for failing to complete a project on time. But when used in conventional mixers, such concrete orders can sometimes cause more problems than they solve, requiring delicate scheduling balances to coordinate delivery. Even under the most detailed plans, concrete can sometimes set while in the mixer, creating catastrophic delays on time-restricted jobs.

But, California-based material supplier George L. Throop Co. has found a way to meet this challenge by producing fast-set, high-strength concrete mixes on site in its own volumetric mixers. Unlike a conventional barrel mixer, a volumetric unit carries the raw materials separately, mixing fresh concrete at or near point of placement. Throop’s volumetric mobile batch plants allow workers to place both small and large jobs, using any kind of aggregate or cement. However, Throop specializes in custom mixes of fast-setting concrete, using its volumetric mixers to keep costs down by producing only the amount of mix required at the job site.


Throop executives had heard stories for decades of unnecessary waste and loads of rapid-set concrete having been sent back, because of the way the product had been produced and delivered by barrel mixers. With volumetric mixers, we can’t compete on a volume commodity basis, but the conventional ready-mix industry isn’t set up to serve the custom mix niche, says Jeff Throop president of George L. Throop Co.

Research shows that fresh concrete is superior to concrete that has been hydrating and mixing for a time, Throop contends. When we get to a job, we can start mixing when the crew is ready. We also can stop and restart production at any time with no waste. And, we can use cements that conventional ready-mix producers can’t use, like [CTS Cement’s] Rapid Set.

Rapid Set specialty cement creates a workable, 5-in.-slump concrete that gains 2,500- to 3,000-psi strength just an hour and a half after placement. It materially reduces drying shrinkage, resulting in about 75 percent less shrinkage than the same slump portland cement concrete (PCC). It also reduces porosity for enhanced durability and freeze-thaw resistance.

Subjected to the ASTM 666 test, Rapid Set concrete specimens withstand 1,000 freeze-thaw cycles. Also, chloride ion penetration tests shows Rapid Set concrete to have low permeability. Such attributes make it ideal for work where speed and durability are key, including night-time repairs on urban routes.

For each project, the George L. Throop Co. submits a mix design to the owning agency for specification approval. The components of the mix design determine the calibration of the volumetric mixer.

We calibrate the cement first, and that drives the entire calibration process, Throop says. We know how many ÎcountsÌ will be required to produce a cubic yard of concrete, and then we calibrate the stone and sand by adjusting gate settings and sampling the sand first and then the stone. If you have a yard of cement, you’ll also get a yard of sand and rock.

Throop Co. also is able to heat water on its rigs to tailor the concrete mix for colder climates. We can produce in temperatures in the upper 30s or low 40s, Throop says. On interstate highways, it’s common to place concrete slabs at night, when the temperatures can be in the low 40s. By adding warm water to the mix, the concrete will hydrate faster. Colder mix temperatures slow down the opening strength gains, but can make the difference between opening on time or not. Penalties in some areas can be $1,000 per minute if there is not enough strength at scheduled opening.


Volumetric mixers are key to George L. Throop Co.’s success within the niche market. After some ups and downs with manufactured mixers, the company now makes its own volumetric mixers.

We bought our first volumetric mixer in 1987, Throop says. It was an all-electric unit and was a disaster. Frankly, the salesperson misrepresented what the unit could do. But, we worked through that and purchased trailer-mounted machines that have guided us to where we are today.

Throop Co. has the components for the bodies and trailers shipped for its staff to handle the final assembly. Over the years, the company has modified components on the machines as jobs have demanded. For example, a job in South Carolina, in which the firm performed the first closure of nuclear waste tanks in the country, led to the adoption of a software program that can monitor the mixers’ production and performance at any moment, building a database that can be consulted afterward.

It was the first time a job like this had been done, so there were a lot of concerns with quality, consistency and documentation of what was being produced, Jeff Throop says. Once the grout had gone into the tank, it was gone; there was no inspecting or replacing it because of the residual nuclear material inside the tank. We had to develop software that would print a log of what had been produced every five minutes and provide a live snapshot of what was going on.

As the result of that job, Throop Co. kept improving the consistency of the equipment and the components of the volumetric mixer. By having those requirements placed on us, we developed better ways of producing grouts and concrete, Throop says.

By hand-selecting components for specific tasks, the company optimizes its chances for success in the field. For example, air-operated vibrators on volumetric mixers are very loud, says Throop. They also have a limited life Û about a year with our utilization rate. So, we went to an adjustable-force, continuous-duty-rated, 12-volt vibrator. It was probably twice as expensive as the air-operated vibrator, but it’s a great benefit to go with the quieter, higher-quality, longer-lasting unit.

Other volumetric mixers have to go back to a cement silo or aggregate staging yard to reload, Throop says. We’ve had projects in which we can produce hundreds of yards of concrete from one machine. We bring the materials to the job and use a standard loader to charge the mixer while concrete comes out the back.

This article was adapted from information supplied by George L. Throop Co., Pasadena, Calif., www.throop.com