+ Dusting Concrete Surfaces

Symptoms: The presence of powder on the surface of the concrete.

Causes:

• A weak surface caused by finishing while bleed water is on the surface
• The presence of high amounts of carbon dioxide due to inadequate ventilation Premature finishing Inadequate curing Insufficient protection of surface from rainwater, snow or wind

Prevention:

• Do not try to finish concrete with bleed water on the surface.
• Use a moderate slump (5″), unless the mix design can handle a higher one without bleeding. This is especially important in cooler weather when setting time is lengthened creating greater opportunity for bleeding to occur.
• If excessive bleeding does occur, remove it by dragging a hose, rope or squeegee across the surface. Never sprinkle dry cement on the surface to absorb the excess water.
• Avoid direct placement of concrete on polyethylene or non-absorptive sub-grades, place 1 to 2 inches of sand between. For absorptive sub-grades, dampen the surface prior to placement.
• Protect concrete from the environment while curing by sealing or covering. In cold weather conditions, use warm concrete and accelerator to reduce setting time. Modifying mix proportions and using air-entrained concrete can also reduce bleeding. For more information, please talk to the dispatcher about these options when ordering your concrete.

+ Scaling Concrete Surfaces

Symptoms: Flaking and peeling from the surface of exterior concrete. Usually starts in small patches that may merge and expose large areas.

Causes:

• Concrete exposed to freezing and thawing in the presence of moisture and/or de-icing salts when it has been:
• Finished with bleed water on surface which causes a high water-cement ratio and a low strength surface.
• Over finished.
• Absent or insufficient amount of air entrainment in the concrete. Air entrainment is an essential component of exterior concrete.
• Applied with calcium or sodium chloride de-icing salts, or other salts that may cause a chemical attack on the surface of the concrete.
• Cured insufficiently.

Prevention:

• Use low slump, air-entrained concrete. Notify the High Grade Materials dispatcher when ordering that you are pouring exterior concrete and need air-entrained concrete.
• Avoid the use of de-icing salts the first winter.
• Assure proper curing by protecting concrete from the elements.
• Never finish with excess water on the surface.
• Avoid overworking the surface, a broom finish is recommended.

+ Crazing Concrete Surfaces

Symptoms: Large amounts of fine cracks on the surface of the concrete (also referred to as pattern or map cracking). These do not affect the structural integrity of the concrete, but can be aesthetically displeasing.

Causes:

• Rapid drying of the concrete due to high temperatures, low humidity or drying winds.
• The overuse of placing tools which brings excess mortar to the top.
• Premature finishing with a float and trowel brings excess moisture and paste to the surface. A rapid loss of this moisture could cause crazing.
• Use of dry cement to absorb excess water on surface.

Prevention:

• Use low slump, air-entrained concrete.
• Maintain the water-cement ratio at the surface by using evaporation retardants after screeding.
• Use fog spray or cover with wet burlap or canvas as soon as possible after strike off and darbying and maintain until concrete is nearly ready for finish.
• Do not overwork the surface pace.
• Begin curing immediately, and when possible continue mist curing to lower temperature by evaporation.
• Do not begin finishing with excess water on surface.

+ Cracking Concrete Surfaces

Symptoms: Unintentional breaks in the concrete.

Causes:

• Shrinkage
• Restraint
• Applied Loads

Prevention:

• Adjust the mix design by lowering the water-cement ratio and use the largest aggregate possible. Talk to the dispatcher about this when placing an order.
• Place control joints at proper distances. See the “How to Construct Joints in Concrete Slabs“ section.
• Use expansion joints where concrete meets other surfaces or slabs.
• Construct sub-grade properly and provide adequate drainage.

+ Plastic Shrinking Cracking

Symptoms: Small cracks that appear in the surface of the concrete soon after finishing. These usually do not intersect the perimeter of the slab and are parallel to each other.

Causes:

• Dry surface as a result of the evaporation rate exceeding the rate of rising bleed water to the surface. This high evaporation rate is due to a number of factors:
• High temperatures.
• Low relative humidity.
• High winds.
• Concrete temperatures.

Prevention:

• Be aware of relevant weather conditions and take proper precautions to protect concrete from the elements.
• Dampen the sub-grade and forms.
• Start curing as soon as possible.
• Prevent excess surface evaporation.
• The use of synthetic fiber may help. Talk to the dispatcher about this possibility when ordering your concrete.
• Minimize placing and finishing time as much as possible.

+ Concrete Blisters

Symptoms: Hollow, low-profile bumps on the concrete surface, typically from the size of a dime up to an inch.

Causes:

• Blisters form when the fresh concrete surface is sealed by troweling while the underlying concrete is plastic and bleeding. This can be caused by a number of factors:
• Premature finishing.
• High entrained air levels.
• The use of a dry shake, especially on air entrained concrete.
• Sticky concrete caused by higher cement content or excessive fine sand.
• A thick slab which may bleed for a longer period of time.
• A cool sub-grade will delay set in the bottom and make the top set first. Excessive use of a jitterbug or a vibrating screed, which works up a thick mortar layer on top.

Prevention:

• Do not seal the surface before air or bleed water from below have escaped.
• Avoid dry shakes on air entrained concrete.
• Use healed or accelerated concrete to promote even setting throughout the depth of the slab.
• Do not place slabs directly on polyethylene sheeting.

+ Construction Joints in Concrete Slabs

Joints are preplanned cracks that help prevent irregular cracking resulting from the natural expanding and shrinking of the concrete. Though these preplanned joints minimize cracking, it is important to note that concrete will almost always crack, no matter what precautions are taken, especially in high freeze/thaw areas.

There are numerous types of joints. Contraction joints are intended to regulate where cracks resulting from dimensional changes will occur. Isolation or expansion joints separate slabs from other parts of the structure, such as the driveway from the sidewalk. Construction joints are placed where two successive placements of concrete meet.

How to Construct Joints:

• Maximum joint spacing in feet should be two and a half times the thickness of the slab in inches and no wider than 15 feet no matter what the slab thickness.
• All panels should be as close to square as possible. The length should not exceed 1.5 times the width.
• For contraction joints, the joint groove should have a minimum depth of 1/4 the thickness of the slab, but not less than one inch.
• The timing of the joint creations depends on the method used and curing conditions.
• Raveling during saw cutting is due to immature concrete that has not yet reached proper strength. If raveling occurs, you are cutting too early and should allow for further strengthening of the slab.

+ Hot Weather Concreting

Hot weather affects fresh concrete by increasing the rate of evaporation which may cause shrinkage cracking or reduce the surface water needed for hydration. A rapid change in temperature may cause thermal cracking in slabs or walls.

• Use the appropriate mix designs and include enhancers such as water reducers or retarders.
• Have adequate manpower to quickly finish the concrete.
• Limit the addition of water at the job site. Do so only to adjust the slump and never add water to concrete that is more than 1.5 hours old. Do not place slabs on grade upon polyethylene sheeting. If a vapor barrier is required, then a bed of damp sand should be placed over it.
• Finish as soon as the sheen has left the surface; start curing immediately after and continue for at least three days. While curing, keep concrete properly covered to prevent evaporation or use a liquid membrane curing compound or cure slabs with water. Whatever approach taken, be sure to keep the surface constantly moist. The addition of white pigment to membrane curing compounds could also help by reflecting heat away from the concrete surface.
• Moisten the sub-grade, forms and reinforcement prior to placement but avoid standing water.
• Do not use accelerators.
• Consider early morning pouring.

+ Jobsite Addition of Water

This is the addition of water to concrete in the mixer at the request of the customer, either meeting the water-cement ratio or in excess of it. Additional water improves the pliability of the concrete, making it easier to move and place. However, the addition of water may also decrease the overall strength of the concrete and affect the quality of the concrete, by increasing the likelihood of cracking and other quality problems.

• The maximum allowable slump should be pre-determined and not exceeded.
• Prior to discharging concrete on the job, the actual slump of the concrete must be estimated or determined. If the slump is measured, it should be on a sample from the first quarter cubic yard of discharged concrete and the result used as an indicator of concrete consistency and not an acceptance test.
• At the jobsite, water should be added to the entire batch so that the volume of concrete being retempered is known. A rule of thumb that works reasonably well is: 1 gallon, or roughly 10 lb. of water per cubic yard for 1 inch increase in slump.
• All water added should be measured and recorded.
• ASTM C 94 (American Society for Testing and Materials) requires an additional 30 revolutions of the mixer drum at mixing speed after the addition of water.
• Do not exceed maximum slump or water-cement ratio.
• Upon obtaining the desired slump and/or maximum water-cement ratio, it is recommended that no further water be added on the job-site.