Concrete Surface Defects
Defects that appear on the surface of concrete during construction or within a relatively short time after completion, are usually caused by poor quality materials, improper mix design, lack of proper placing and curing procedures, or poor workmanship. The repair of surface defects is both difficult and costly. The best repair work will not be as good as an original properly finished surface. Every effort should be made both prior to and during construction to avoid the use of materials or construction practices that can cause surface defects.
The major causes of surface defects and their prevention and repair will be discussed.
Honeycomb surfaces are caused by the use of a dry mix that was not properly consolidated. The concrete mix should be designed to provide a workable mix for the type of consolidation that will be used on the job. When honeycombing occurs, don’t just add water to the mix to correct the trouble. That will decrease the strength and durability of the concrete. The mix should be redesigned to provide improved workability or the procedure for consolidating the concrete should be improved. When concrete is consolidated by hand the puddling sticks should be pushed through the entire layer of freshly placed concrete. Concrete along the forms should be thoroughly spaded. The use of vibrators will consolidate a stiffer mix than can be consolidated by hand. The entire depth of a new layer of concrete should be vibrated. The systematic spacing of the points of vibration should be such that no part of the concrete is missed.
Removing the defective concrete and replacing it with new concrete is the only effective method for correcting a honeycomb surface. If the area to be repaired is large in relation to its depth, it may be filled with pneumatically placed mortar or concrete. For this method of repair the surface should be sloped outward. When hand placed mortar or concrete is used, the edges should be sharp and straight and all portions of the area should be at least one inch deep. The surface of the old concrete should be thoroughly scrubbed to remove dust or dirt, and should be damp but not overly wet when the new concrete is placed to secure a good bond. The new concrete should have a color matching that of the older adjoining concrete. The new concrete should be adequately moist cured.
AIR POCKETS AND BOLT HOLES
The occurrence of air pockets on the formed surfaces can be prevented by the proper use of form oil, the use of a well-designed mix and proper placing procedures. The use of excessive amounts of form oil will cause the air bubbles to stick to the surface more tenaciously. The use of an over-sanded mix makes it more difficult for the air bubbles to escape upward through the mortar. Placing the concrete in successive layers with a maximum depth of about three feet with adequate consolidation of each layer and with proper spading along the forms should remove the air pockets. The air pockets are less likely to occur when the concrete is consolidated by vibration than when it is consolidated by hand.
Some engineers are of the opinion that the use of air-entrained concrete increases the number and size of air bubbles on the formed surfaces of concrete. There is much evidence to the contrary, except when the concrete is placed under a sloping form.
Air pockets and bolt holes can be filled with mortar carefully packed into place in small amounts. The mortar should be mixed as dry as possible so that it will be lightly compacted when forced into place. Tie rod holes extending through the concrete can be filled with mortar by using a pressure gun similar to the guns used for greasing automobiles.
ROCK POCKETS OR GRAVEL STREAKS
The loss of cement or mortar through cracks or knot holes in the forms will cause rock pockets or gravel streaks that impair the appearance of the surface. This is most likely to occur when the concrete is consolidated by vibration. Such a surface defect is very difficult to repair, but it can easily be prevented by the use of tight forms.
The use of wet mixes that bleed excessively is the major cause of sand streaking on the formed surfaces of concrete. Such a surface defect is very difficult to repair, but it can easily be prevented by correcting the cause. The use of a stiffer mix will prevent sand streaking. The use of stiff mixes consolidated by vibration will seldom develop sand streaking. Air-entrained concrete is more cohesive and bleeds less than non-air-entrained concrete. The use of air-entrained concrete will greatly reduce sand streaking.
Laitance is a soupy mixture of cement, fine sand and water that accumulates on the surface when wet concrete mixes that bleed excessively are used. In structural concrete it leaves a light colored streak of poor concrete between lifts. In hydraulic structures it has a high permeability and is a source of water leakage. The laitance layer is particularly vulnerable to deterioration by freezing and thawing.
The use of a stiff mix of low bleeding capacity will prevent the accumulation of laitance. The use of air-entrained concrete, because of its cohesive nature and low bleeding capacity, reduces the occurrence of laitance.
When laitance occurs on the top of a concrete lift, it should be completely removed before the next lift is placed. The surface of the concrete should be wet sandblasted and washed thoroughly just prior to a placement of fresh concrete. That is the most effective method for removing the laitance and it provides a good surface for obtaining bond with the new concrete.
Repairs are seldom made until the laitance layer becomes badly deteriorated by weathering. Then the defective concrete is removed and replaced with new concrete. All of the unsound or damaged concrete must be removed. This is a step that is often neglected or not completely performed. The underlying concrete should have a good ring when struck with a hammer. The procedure for placing the new concrete should be the same as that described for repairing honeycomb surfaces.
COLORED SURFACES FROM USE OF PLYWOOD FORMS
The use of plywood forms has sometimes caused a red to pink color on the surface of concrete. Apparently the resin used in the production of the plywood reacts with the concrete to produce the color in a relatively thin surface film. The color will appear with the first use of the plywood, but it is not likely to appear after the forms have been used a few times.
This type of surface color is hard to remove. It usually disappears with time. In some cases it disappears within a few days, sometimes it remains for several weeks. It does not occur with all plywood forms and it does not always occur with plywood from any one producer. Apparently the color is related in some way to the formulation or processing of the resin used. When the color occurs it should be called to the attention of the producer of the plywood in the hope that corrections can be made to avoid the trouble in future construction.
Rust stains are caused by reinforcing steel and tramp that are exposed on the surface or inadequately covered with concrete. Rusting of the metal or oxidation of iron containing substances in the aggregates will cause a red or reddish brown stain on the surface of the concrete.
Reinforcing bars and stirrups should be firmly fixed in place according to drawings and specifications and adequately covered with concrete. Precaution should be observed in placing and consolidating the concrete so as not to displace the reinforcing. There is nothing that can be done about a reinforcing bar that appears exposed when the forms are removed.
In the process of welding the reinforcing steel in place in the forms in the production of precast elements, pieces of tramp metal are likely to drop to the bottom of the form. All such metal should be removed before the concrete is placed.
Rust stains can be removed by washing the surface of the concrete with a solution of oxalic acid. However, as long as the underlying cause remains the stain is likely to reappear. Every precaution should be used to avoid the cause of rust stains prior to and during construction.
There are several different causes for dusting on concrete surfaces. Such surfaces can be avoided by the use of good materials and good construction practices. The presence of an excess of clay or silt in the concrete will cause dusting. The clay or silt comes to the surface during finishing operations to produce a soft surface that will wear rapidly under traffic. Clean, well-graded coarse and fine aggregates should be used to obtain a hard dust-free surface.
Premature floating and troweling will bring excessive fine material and water to the surface to produce a soft cement paste. Floating and troweling should be delayed until the free water has disappeared from the surface and the concrete has attained a slight initial set.
Carbon dioxide from salamanders, power buggies or mixer engines will react with plastic cement paste to impair normal hydration and produce a soft surface. Proper ventilation must be provided to remove the carbon dioxide.
Spreading dry cement on the concrete surface to dry it for finishing operations will cause both dusting and craze cracking. These practices should not be used.
Inadequate moist curing is a major cause of soft, dusty surfaces. Cement hydration and hardening stops when the concrete dries out. Proper moist curing is essential to provide a hard dust-free surface. Delayed moist curing will partially harden concrete surfaces that did not receive adequate initial moist curing.
Surface treatments with a solutions of magnesium fluosilicate, zinc fluosilicate or sodium silicate will harden the surface of concrete and reduce dusting.
Surface scaling can be of two types. One is a relatively thin scaling caused by improper finishing and curing operations. The other is the scaling of non-air-entrained concrete caused by freeze-thaw damage and the application of salts for snow and ice removal.
The materials and finishing and curing operations that cause dusting of concrete surfaces also cause thin surface scaling. The procedures used to prevent dusting will prevent this type of surface scaling.
Scaled concrete surfaces can be repaired by applying a thin resurfacing of concrete properly bonded to the underlying old concrete. All defective concrete must be removed from the surface, by scarifying or scrubbing with hydrochloric acid, before applying the new concrete. A thin layer of neat cement paste should be brushed into the damp surface of old concrete just before the new concrete is placed to secure a good bond. The new concrete is placed, finished, and cured by normal procedures. Any relief joints present in the old concrete should be carried through the new resurfacing.
Thanks to Kaiser Cement who supplied information for this document.