How to Minimize Risks for Portland-Limestone Cement Concrete Slabs

Best practices and strategies for reducing floor slab finishing and early-age, strength-critical challenges.

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While many contractors have successfully placed and finished concrete slabs constructed using Type IL cement (Portland-limestone cement), others have struggled with project delays and unacceptable finishes. To find the root causes of unsuccessful outcomes, jobsite data such as mixture proportions, weather conditions, construction practices and equipment, and finish requirements must be evaluated.

This article discusses early-age, strength-critical construction operations such as saw cutting, cold weather protection, post-tensioning and form removal, as well as best practices and strategies to minimize risks during floor slab finishing and early-age, strength-critical construction. This article also provides data collected on six mockups constructed with Type IL cement and one mockup constructed with Type I cement.

Concrete Finishing Challenges

The outcomes of concrete slab placements are highly dependent on the mixture constituents and the fresh concrete properties such as slump, air content, bleeding rate and setting time. The sensitivity of the fresh concrete to the environment impacts the finisher’s techniques and timing to produce a quality product.

One major factor that separates slabs requiring a trowel finish from slabs designed as paving is the length of time the fresh concrete slab is exposed to the environment. For slipform paving, T.S. Poole in the “Guide for Curing Portland Cement Concrete Pavements, Volume I,” indicated that final finishing is usually completed within a few minutes of placing the concrete, well before the time of initial setting and the end of the bleeding period. For slabs to receive a trowel finish, final finishing may occur three to eight hours after placement, with the longest delays occurring in cold weather with high relative humidity. This extended exposure time poses a substantial risk to contractors who are tasked with turning a sensitive, perishable product into a quality hardened product for the owner. Thus, slipform paving and parking lot examples are not comparable to slabs specified to receive a trowel finish.

PLC Concrete Survey Results

A recent joint ACI-ASCC survey on PLC concrete posed questions to elicit user experiences with finishing and performance of slabs requiring a trowel finish. The percentages reported in the following section represent the answers from 173 respondents.

As the survey shows, fresh concrete properties changed when the cement changed. Reported changes in fresh concrete properties associated with changing from Type I cement to Type IL cement include:

• Water demand—77% reported an increase while 7% reported a decrease.

• Bleed water—14% reported an increase while 39% reported a decrease.

• Setting time—51% reported an increase while 21% reported a decrease.

• Crusting—31% reported an increase while 1% reported a decrease.

• Changes in finishing—45% reported an increase while 3% reported a decrease.

• Need for evaporation reducer—38% reported an increase while 1% reported a decrease.

Reported PLC concrete performance characteristics (relative to concrete produced using Type I Portland cement) include:

• Plastic shrinkage cracking—43% reported an increase while 6% reported a decrease.

• Scaling—13% reported an increase while 1% reported a decrease.

• Dusting—13% reported an increase while 1% reported a decrease.

• Wear resistance—4% reported an increase while 19% reported a decrease.

• Delamination—17% reported an increase while 1% reported a decrease.

To read the rest of this technical article from the American Society of Concrete Contractors click here.

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