QUESTION: After a cold and rainy period this fall, spots of mortar started popping off the surface of an exterior broom-finished slab we placed last spring. The spots have become holes that vary in diameter and depth from about 3/8 inch to just over 1 inch and occur fairly frequently over the entire project, which include sidewalks and a large pavilion area in a city park. After digging into several of the holes with a pocket knife, we found remnants of soft and sandy type rocks.
Obviously, we are disappointed and the city engineer wants us to repair the holes. What is causing these surface holes and will they get worse? Can this problem be avoided? Can these holes be repaired? If so, how should we repair them?
ANSWER: These types of surface defects are called popouts. Popouts occur when small portions of the surface mortar break away from the concrete flatwork. This leaves shallow and cone-shaped holes with broken, or as in your case, deteriorating material at the bottom of the holes.
Popouts are categorized as small, medium, and large. Holes up to about 7/16-inch in diameter and between 7/16 inches and 2 inches are considered small and medium popouts, respectively. Those exceeding about 2 inches in diameter are considered large.
Typically, popouts are caused by near-surface aggregate particles or contaminants such as chert, soft and fine-grain limestone, sandstone, shale, clay lumps, lignite, or coal. Any soft, lightweight and absorptive aggregate particles or contaminants can cause surface popouts.
These deleterious substances typically swell and create expansive pressures because of water absorption or freezing in a saturated condition. The expansive pressures commonly fracture or rubblize the aggregate particles or contaminants and pop off the overlaying surface mortar.
Popouts are a cosmetic detraction and typically do not affect the function or service life of the flatwork. Usually, popouts are tolerated or overlooked, especially if their size and frequency are not excessive.
Once popouts have occurred on exterior flatwork, it is unlikely that additional ones will occur. Additional surface damage should be limited to the erosion of incipient popouts and continual deterioration of the exposed aggregate particles or contaminants responsible for the popouts. Deleterious aggregate particles and contaminants below the near-surface region of the flatwork typically do not cause problems and are not a concern.
The occurrence of deleterious aggregates or contaminants in fresh concrete is outside the control of the flatwork finisher. Deleterious aggregates and contaminants inadvertently occur during the production and handling of aggregates and concrete. However, ASTM C 33, “Standard Specifications for Concrete Aggregates,” limits the amounts of deleterious substances in the fine and coarse aggregates.
ASTM C 33 limits clay lumps and friable (easily crumbled) particles in sand to 3 percent by mass of the total sample and for coal and lignite to 1 percent for all concrete and 0.5 percent where the surface appearance of the concrete is important.
For coarse aggregates, ASTM C 33 limits the sum of clay lumps, friable particles, and chert to 5 percent and 7 percent, depending on the weather exposure. Maximum allowable amount of coal and lignite are limited to 0.5 percent for coarse aggregates.
However, concrete containing as little as 0.2 percent or less of coal, lignite, or deleterious substances may be susceptible to popouts. Just because the fine and coarse aggregates satisfy the ASTM C 33 requirements does not ensure the concrete will be free of popouts.
Popout-free exterior flatwork may not be possible, especially if aggregate sources in your area contain deleterious substances. However, the following recommendations may help minimize their presence:
1. Discuss the popout potential with the concrete producer. If possible, use concrete made with aggregates having low percentages of deleterious substances and a good performance history.
2. Order 4500 psi concrete with a maximum water-cement ratio of 0.45 to reduce surface porosity and increase the strength of the concrete.
3. Use air-entrained concrete and pozzolanic admixtures such as fly ash.
4. Place the lowest-slump concrete possible to minimize the floating of lightweight particles to the surface.
5. Use proper finishing techniques and avoid steel troweling exterior slabs with air entrainment. Do not finish bleed water or added water into the surface.
6. Protect the surface from rapid moisture loss and properly cure to improve surface quality.
7. Consider sealing the surface with a breathable sealer such as silane or siloxane.
For small popouts, use a rotary-hammer drill and drill out the remaining deleterious substance and unsound concrete to create a hole with a minimum depth of ½ inch. For medium and large popouts, use a hammer and chisel, small chipping hammer, or core drill to remove all remnants of deleterious substances and unsound concrete. Create a hole with square edges (perpendicular to the surface) and a minimum depth of ½ inch. Do not feather edges. Remove all unsound concrete, dirt, and debris.
For site-mixed, dry-pack repair mortar, mix one-part portland cement to two-and-a-half parts sand. Use only enough water to produce a stiff mortar that will pack into a ball when molded by hand. If possible, use the same type of cement and sand used in the original concrete. To improve the color match of the repair mortar, substitute white cement for the gray cement. Determine the amount of white cement by trial mixes.
Before placing the dry-pack repair mortar, scrub the repair area with a thick and creamy bonding grout consisting of one part cement and one part fine sand. Immediately, tamp the repair mortar into place and finish or texture to match the surrounding surface. Do not place or smear the repair mortar over a larger area than necessary. Cure repaired areas using wet burlap and plastic sheeting or a spray-on curing compound.
Prepackaged concrete repair materials can be used but it will probably not match the color of the existing concrete. If there are too many popouts for individual repairs, consider using a thin-bonded concrete overlay.
Written by Kim Basham, PhD, PE, FACI, and president of KBE Engineering, who specializes in concrete construction, troubleshooting, nondestructive testing, forensics, and repair. E-mail email@example.com.