TIPS AND TROUBLESHOOTINg
FOR THE GREATEST RESULTS
The departure or variation of colour from that which is normal or desired.
Discolouration can take many forms in concrete. These forms include: gross colour changes over large areas of concrete typically caused by changes to the concrete mix; light patches of discolouration caused by efflorescence; dark blemishes or a mottled surface appearance.
Discolouration is more likely to occur when:
- There are significant changes to the mix design related to the raw materials (cementitious materials,aggregates, admixtures, )
- There are significant changes in the rate of hydration over the concrete slab typically due to inconsistentcuring periods or operations
- There are significant variations in the W/CM ratio of the concrete
- Chemical admixtures such as calcium chloride are used in the concrete
- The flatwork finisher improperly estimates the timing of the finishing operations resulting in a hard-troweled surface
How to avoid problems with Discolouration:
- Chloride is used
- Conduct finishing operations at the correct time. Early finishing tends to elevate the water/cementing materials ratio at the top surface and lighten the colour. Late finishing tends to lower the water/cementing materials ratio at the top surface and darken the colour
- Implement a uniform curing system for the necessary time Uneven curing directly affects the degree of hydration of the cement and the colour of the concrete
- Ensure that plastic sheeting, when used for curing purposes, does not come in direct contact with the Plastic sheeting tends to leave colour streaks on the concrete surface where it is in direct contact with theconcrete
- Utilize effective form release agents that prevent the non-uniform loss of moisture from the formed surface
- Ensure that the raw material sources are not varied during the concrete placement. Architectural concrete projects may require the stockpiling of raw materials at the concrete plant (at a considerable cost) for the entire project
Shrinkage cracks that develop before the cement paste, mortar, grout, or concrete sets.
Plastic shrinkage cracks are caused by the rapid loss of water from the surface of the concrete before the concrete has had time to set. These cracks form after concrete placement and consolidation, while the concrete is still plastic. They are relatively shallow in depth and tend to form in parallel to one another. The cracks are unappealing but rarely cause significant damage to the strength or durability of the floor due to their shallow depth.
Plastic Shrinkage Cracking is more likely tooccur when:
- There is a rapid loss of moisture from the surface of the concrete
- There is low relative humidity
- There is high ambient air or concrete temperatures
- High winds are blowing across the slab surface
- Limited bleed water is present in the concrete resulting in rapid surface moisture loss
How to avoid problems with Plastic Shrinkage Cracks:
- Consider erecting wind screens or sunshades to reduce the surface evaporation rate
- Utilize fog spraying equipment on the up-wind side of the slab to maintain the relative humidity of the slab at 100%
- Consider pre-dampening the sub-grade during hot dry weather to reduce the moisture loss from the concrete into the subgrade
- Consider the use of synthetic fibres to resist the tensile forces created during the plastic shrinkage period
- Consider placing the concrete during off-hours to avoid high temperature, high wind, high sun exposure periods
- Consider the use of evaporation retardant chemicals applied between the various finishing operations to prevent the rapid loss of moisture from the surface of the concrete
Cracking of a structure or member due to failure in tension caused by external or internal restraints as reduction in moisture content develops, carbonation occurs, or both.
Concrete both expands and contracts with changes in moisture and temperature and deflects depending on the element size, reinforcing, loading force and support conditions. All these factors can lead to uncontrolled cracking if the proper design and joint details are not addressed prior to the time of construction. In order to prevent uncontrolled shrinkage cracking, the designer and the contractor should effectively utilize the following concrete jointing systems:
- Isolation Joints – Joints that permit both horizontal and vertical movement between the slab and the adjacent concrete
- Contraction Joints – Joints that permit horizontal movement of the slab and induce controlled cracking at pre-selected locations
- Construction Joints – Joints that are stopping places in the process of construction
Uncontrolled Shrinkage Cracking is more likely to occur when:
- The designer or contractor has not developed a proper jointing system and layout prior to the start of concrete placement
- The sub-grade is not properly prepared and compacted
- There is job site addition of excessive amounts of water
- Improper finishing procedures are implemented or theinstallation of contraction joints is not completed in a timely fashion
- There is inadequate concrete curing
How to avoid problems with Uncontrolled Shrinkage Cracks:
- Ensure that the subgrade is adequately prepared based upon the exposure and loading conditions that the slab will be exposed to during its service This includes using the appropriate thickness of granular sub-base for the application and properly compacting the sub-base
- Ensuring positive drainage of both the sub-grade and the concrete surface to avoid the build-up of high moisture levels
- Utilizing the proper concrete for the application (See CSA 1minimum durability requirements)
- The designer and the contractor should review the concrete jointing details prior to the start of the Items to consider include concrete thickness, maximum joint spacing (4.5m max.), fibre reinforcement, structural reinforcement and early installation of contraction joints
- Implement curing procedures as soon as possible to ensure that the performance properties develop
The dislodging of small sections of concrete mortar directly over top of the coarse aggregate particles in exterior concrete exposed to freeze/thaw conditions.
Mortar flaking typically occurs in exterior flatwork that was not properly cured during the first 7 days. Mortar flaking is typically very shallow in depth and consists of distinct delamination (flakes) that occur directly over top of the coarse aggregate particles in the concrete. This is typically due to the fact that the concrete mortar directly over top of the coarse aggregate was allowed to dry out and was not properly cured. Any excess bleedwater that is present in the concrete is forced to travel around the coarse aggregate particles, as it rises to the surface and the mortar directly above fails to develop the necessary strength required to achieve proper durability and bond.
Mortar Flaking is more likely to occur when:
- The concrete is placed during periods of rapid surface drying
- When the concrete is not cured at all
- When curing compound is applied too late after concrete placement or not until the following day
How to avoid problems with Mortar Flaking:
- Ensure the proper curing procedures are followed at all times! Proper concrete curing should eliminate this problem
- During periods of rapid surface drying consider using initial curing procedures like evaporation retardants or placing plastic sheeting over the concrete surface between finishing operations
- Consider the use of wet curing methods during severe drying and hot weather conditions
- Consider the use of fog spraying systems to protect the surface of the concrete from drying out until wet curing can be initiated
Conical fractures of the concrete surface caused by the expansion of ice inside porous coarse aggregates located just below the concrete surface.
Popouts result when soft porous aggregates, found both in limestone and gravel sources, undergo freeze/thaw conditions in a saturated state. While air entrainment provides protection for the cement paste from the expansive forces generated during winter freeze/thaw cycles, it does not provide any protection for the coarse aggregate particles. If the local aggregate sources contain even a small percentage of soft, porous particles (like chert), the coarse aggregate can absorb significant quantities of moisture and fractures during freeze/thaw cycles. The resulting expansion fractures both the aggregate and the surrounding concrete. Popouts are distinguished from mortar flaking by the presence of fractured aggregate in the popout (as shown in the sample photo).
Popouts are more likely to form when:
- The local aggregate sources contain soft, porous material
- The concrete is in a fully saturated condition
- The concrete hasn’t achieved its ultimate durability potential due to inadequate curing
How to avoid problems with Popouts:
- Consider the use of alternative aggregate sources. This may require the import of aggregates at a significant additional expense
- Consider changes to the aggregate processing process to further reduce the quantity of soft porous material (the complete elimination of all soft material is not normally possible)
- Ensure that the overall design minimizes the build-up of excess moisture in the This can be achieved through proper surface grading and draining of the surrounding subgrade
- Ensure that proper concrete curing and protection practices are followed to achieve the highest possible concrete durability
Consider the use of sealers to limit the inflow moisture into the concrete