My husband Patrick comes down with the home-improvement bug a few times a year. And he’s had it bad lately. He’s tired of the tiny bits of gravel being tracked into the house from the decomposed granite path that runs between our back deck and the side gate. He’s decided he wants to pour a new concrete sidewalk. When he told me his plan, I smiled like a good wife, but inside I was grimacing with the memory of his last concrete project, a simple landing at the foot of a porch. He filled our car with 60-pound bags of concrete, started mixing them up one by one in the wheelbarrow only to come up ten bags short. Cursing his lack of foresight, he raced to Home Depot for more bags. When he got back, the concrete he’d already poured had begun to set and didn’t mix well with the new concrete. So I was glad when he told me that this time he would not be dragging home bags of concrete. “I want to try some cart-away concrete,” he stated. “It’s concrete in a trailer that you pick up from a rental yard. Could you call around and get some information on it?”

“Sure,” I fired back, with no idea what he was talking about.

I got some answers from Jim Westmoreland, operations manager at United Rentals in Chula Vista (619-422-1105). “You need a full-size pickup with a frame-mounted hitch to haul the concrete trailer,” Westmoreland explained. “You can’t use just a bumper hitch. It’s a weight issue. A fully loaded trailer carries a yard of concrete.” A yard weighs about 3000 pounds and covers an eight-by-ten-foot area four inches thick.

United Rentals has 15 rotary mixing trailers, which look and work like mini cement trucks. United Rentals’ computer-operated system “mixes the sand, concrete powder, and gravel, and it shoots it out through a belt into the trailer dry. Then we add water to it, and you mix it up as you drive off. That is why it rotates like it does, so it’s mixing constantly as you drive. When you get to your drop site, the concrete is mixed up, and you don’t have to worry about the rocks settling.”

Some rental-yard cart-away concrete trailers are basically a big tub on wheels. Westmoreland says that type is not as versatile as the roto-mix trailers into which “you can add water to it and drive further if you need to, and you can put additives like hardeners or fiber that reinforces it, or chemicals that make it set up real quick or slow down the process of setting. With the roto-mix trailers, you can mix those right in at the job site.”

With an older tub-style concrete trailer, “It would be very hard to mix those things in. And another problem with them is the rocks settle down to the bottom, so when you go to pour it out, the rocks aren’t mixed in with the concrete.”

Westmoreland says you can travel farther with rotary mixing trailers than with the older tub style. “We had a customer go all the way to Tecate with one because he was able to add enough water to it. You couldn’t do that with a tub concrete trailer.”

Westmoreland says the mix concrete offered at United Rentals is as good as the trailers you haul it in. “Our mix is computerized,” he says, “and we provide a pretty rich mix; we kind of fattened ours up a little bit.”

And what does fattening a mix of concrete mean?

“There are different strengths of mix. It depends on what they call the sack number. The sack refers to how much cement powder is in that yard of concrete. The higher the sack number, the stronger the mix, meaning you can put more weight on it; it is harder. There are four-sack, five-sack, six-sack, and seven-sack. The stronger the mix, the more sack it has in it. So, if I give you a six-sack mix, that is strong enough for an average driveway. The four-sack is fine for a sidewalk, good for a wall, filling up the blocks. The five-sack is for a sidewalk; the six-sack would be for a driveway; seven-sack is for an industrial driveway, where you would be expecting heavy trucks.

“The other advantage to having a stronger mix is the concrete is more creamy; it is easier to finish. The higher concrete mix makes it stronger and also makes it finish nicer.”

Can you return unused concrete?

“Yes, but we charge you quite a bit. It is a $120 minimum cleaning charge because we pay dearly to get rid of it. So, it’s important to know how much concrete you need and make a plan that will work.”

United Rentals rents trailers for $15 and sells concrete by the quarter yard, half yard, three-quarter yard, or yard. “A four-sack mix is $84 for a quarter yard, $102 for a half yard, $120 for three-quarters of a yard, and $135 for the yard. A five-sack mix runs $89 for a quarter yard, $107 for a half yard, $125 for three-quarters of a yard, and $140 for one yard. A six-sack mix costs $92 for a quarter yard, $110 a half yard, $128 for three-quarters of a yard, and $145 for one yard. The seven-sack mix is $96 for a quarter yard, $114 a half yard, $132 for three-quarters of a yard, and $150 for a yard. “We prorate it, so if you came in and got a yard-and-a-half of a five-sack mix, it would be $140 for the first yard and only $70 for the second half yard that you buy.”

United Rentals does not rent concrete tools, though they sell all the necessary tools for the job. Westmoreland was hesitant to recommend a trowel. “There are many different trowels: wooden ones, aluminum ones, magnesium ones. But $20 will buy you a pretty decent trowel.”