Engineers have used maturity calculations to help contractors complete jobs faster since the late 1980s. If they can know when a placement will achieve minimum strength without conducting time-consuming break tests on field-cured cylinders, contractors can strip forms, post-tension, remove shoring, end cold-weather protection, and move on to the next step that much sooner.
A plethora of companies offer solutions for gathering and sharing temperature data used to predict set time and strength. The first solutions were external: A sensor at the end of a wire buried in concrete attached to a processing unit outside the placement and someone came along at various intervals to note the readings. Then came sensors that gather and read data but still require connecting to a processing unit to download data. The smartphone ushered in the latest era: sensors with wireless transmitters that gather and send data through the concrete into the phone via Bluetooth or cellular connection and an app uploads it to a cloud-based website where anyone authorized to access the data can view it.
The numerous case studies promoting these solutions emphasize how much time the contractor saved. However, they often fail to mention that maturity curves must be developed for each mix before placement to ensure readings reflect what’s going to happen in the field – a time-consuming process in and of itself. And until the industry fully adopts the maturity method, owners and contractors are still going to conduct break tests for peace of mind or compliance with contract documents.
Given these two factors, are sensors worth the investment?
In January 2015, the company began work on a project related to the Louisiana DOT’s program to upgrade intersections to federal interstate standards: converting the at-grade intersection of U.S. Route 90 and state Route 318 in Jeanerette, La., to two 1,900-foot overpasses (one eastbound and one westbound with two lanes of traffic each) with concrete substructures and superstructures.
The state requires elements to be placed start-to-finish within 4 hours. The decks were a standard 8 inches thick, but placing the extremely wide sections between construction joints “took every bit of that time – and then some,” says Project Manager J.J. Hickey, P.E. When noncompliance reports prompted the state to hold back payment, Gilchrist began using the maturity method to prove the concrete hadn’t begun setting before completion of deck sections.
Louisiana doesn’t allow the maturity method on design-bid-build projects, but this was a design-build contract. Gilchrist installed 170 SmartRock sensors, a product developed by the Canadian firm Giatec Scientific Inc., with two sensors in each footing, column, and cap. But first, the contractor and ready-mix producer Baldwin Redi-Mix in New Iberia had to conduct initial maturity calibrations. Because this was Gilchrist’s first experience with the product, they prepared 28 cylinders for three or four mixes to develop a strength-maturity curve for each.
One temperature sensor is placed in each cylinder. The sensors record temperatures every half hour for a specified period – 24 hours, 72 hours, etc. Maturity is calculated every 30 minutes and recorded at the end of the period. The cylinders are then broken to determine compressive strength. From there, a maturity-strength curve can be developed so sensors placed in the bridge components can provide maturity (estimated strength) at any given time.
Working Toward Industrywide Adoption
Other Articles on the Maturity Method
Gilchrist plans to continue using temperature sensors when allowed. Giatec is trying to minimize how much time contractors must spend developing maturity-strength curves by partnering with ready-mix producers throughout North America to precalibrate mixes. Baldwin is one such partner.
The smartphone app was helpful, too. “This was a state project, but the state wasn’t present on the field,” says former Project Manager Frank Maury. “I was able to send data reports to the 10 other engineers on the project right from my phone.”