All concrete exposed to freeze and thaw action must be air entrained using chemical admixtures. Proper air entrainment involves the placement of very tiny air bubbles (≤ 1 mm in diameter) within the concrete at a very tight spacing. These
bubbles act to reduce the stresses created within the concrete when water expands during its transformation to ice during cold weather.
What Are Some Of The Key Things To Look For During Concrete Placement?
1.) Has the sub grade been properly prepared and compacted? While the minimum aggregate thicknesses vary depending on the application, you should see free draining aggregate that is properly compacted as a sub-base. If
the tires of the concrete truck are leaving large ruts in the sub-base then the material has not been properly compacted or it hasn’t been placed in sufficient thickness.
2.) Ensure that the area is properly drained and sloped to prevent water build-up. Flowing water and frost heaving are the most common reasons for early deterioration. Ensure that both the concrete and the sub-base
are properly drained.
3.) Ordering of the proper concrete. Ask to see the concrete delivery tickets from the ready mix truck to verify that the proper concrete has been ordered.
4.) Place the concrete within 120 minutes of batching. The standard “shelf life” of concrete is 120 minutes. This means that we have 2 hours from the time that we first mix the concrete until we must place the concrete. The time batched is always included on the concrete delivery ticket. Note: Chemical admixtures (set-retarders) can be used to extend this 120 minute placement time.
5.) Do not allow the addition of water to the concrete after the concrete placement has started. Retempering of concrete may be allowed once the concrete truck arrives on site to address minor slump variations but once the concrete placement proceeds past the 10% portion of the load, additional water should not be added. Water addition leads to higher concrete permeability and lower strengths. Slump enhancement with chemical admixture is acceptable after the 10% discharge point and will not adversely affect the concrete quality..
6.) Has the contractor prepared a control joint layout to prevent uncontrolled cracking? All concrete shrinks as it cures and proper control, isolation and construction joints must be used to prevent unsightly random concrete cracking.
7.) How will the concrete be properly cured? What curing method will be utilized to protect the concrete and how long will the protection be applied? Discuss this with the contractor prior to concrete placement and ensure that the proposed methods and times meet the Ontario Building Code requirements for your application.