Winter is approaching and concrete repair work often takes place during cold weather which requires additional precautions. It is necessary to ensure that the concrete is not damaged by frost while it is setting and that it achieves the minimum compressive strength by the structural engineer.
It is vital to monitor the early strength gain of newly placed concrete forming suspended slab. This makes it possible to determine when it is safe to remove the soffit formwork. Regarding the garage floor and roof slabs, strength gain observations will provide imperative information when deciphering when it is safe to open them to traffic or to replace landscaping and paving materials.
There are a few ways to test and monitor newly poured concrete to safeguard against any possible inconsistencies.
- Field cured cylinders
- Lok test bolts
- Pull-out cylinders
Two other methods which are uncommonly used are maturity meters and thermocouples. Cores could also be removed from a slab and tested; however, this is expensive and is seldom done routinely for formwork removal purposes.
The soffit of the slab is normally heated with air below, often being maintained at relatively high temperatures while the temperature of the concrete forming the top surface of the slab often drops to around freezing point before trowelling of the slab surface is completed and the slab covered with insulated blankets. This results in a temperature gradient through the concrete which creates a variation in compressive strength during the period immediately following placing of the concrete with the soffit gaining strength ahead of the top.
Thermocouples and Maturity Meters are indirect test methods where the temperature of the concrete is measured and then used to calculate the compressive strength. Since these are non-destructive test methods, they have to be calibrated for each mix being monitored and even then there is a potential for the compressive strengths determined by these methods not to reflect the actual compressive strength of the concrete to which they relate.
Field Cure Cylinders are cured on site and regardless of where they are stored, they are usually in an environment which during cold weather often results in the concrete forming the test specimens curing at a different temperature to the slab concrete. This means that their compressive strength at the time of testing may not accurately reflect the compressive strength of the slab concrete to which they relate.
Lok test bolts are installed in the slab soffit and as a result, measure the compressive strength of the bottom 1.5 inches of the slab concrete. This creates a potential during cold weather for the test results not to accurately reflect the overall compressive strength of the slab, particularly if there is any delay in installing the thermal blankets.
It’s Beginning to Look a Lot Like Concrete
Pull-Out cylinders are installed in sleeves incorporated into the slab for this purpose with the air filled gap between the pull-out and the sleeve sealed to trap the air in the cavity. The top of the pull-out test specimen is protected against excessive heat loss by rigid insulation. Of the various monitoring methods in common use, pull-out cylinders are the procedure which, in our experience, provides compressive strength values most closely representative of the overall compressive strength of the slab concrete to which they relate.