CONCRETE PROBLEMS

Concrete is the most widely used material in the construction industry and a great deal of attention has been paid to its proper use to perform its structural function and maintain appearance. The two basic types of problems with concrete can be categorized as structural strength and appearance. Appearance is mainly concerned with cracking of the hardened concrete. Other appearance items that may come into play from time to time are dusting, abrasion, honeycomb, or exposed re-bar.

 

Strength

Concrete must have certain specified strengths to do its job as a structural item and these strengths are specified by the design engineers. Strength of concrete is related to its mix design and specifically the water/cement ratio. This is a ratio of the pounds of water versus pounds of cement in a cubic yard mix. Other items of strength include aggregate maximum size, aggregate type and gradation. Strength is measured by making test cylinders out of fresh concrete, curing them for a specified amount of time and then crushing the cylinders in the laboratory. Typical strengths range from 2,500 psi up to 4,500 psi. Some special concretes can be produced to give much higher strengths and for some purposes extremely low strength concrete may be utilized. As a general rule the stronger the concrete the slimmer the slabs, beams or columns can be in a structure. Lower strength results from higher water/cement ratios, smaller aggregate, impurities (dirt) in aggregate or poor aggregate gradation. Freezing temperatures may result in damaged concrete and cold placement temperatures will greatly slow strength gain.

 

Cracking

Plastic Cracking is the result of excessive drying while the concrete is still in a plastic state. The cracking appears as small irregular patterns and has been called “craze cracking” and will lead to unsightly appearance and loss of surface integrity. This may occur when mixes are placed too wet and wind with low humidity prevails.

 

Drying Shrinkage Cracking is the most common form of concrete cracking. This is a result of the concrete losing water by evaporation as soon as it is placed and finished. As the water leaves the mix the volume reduces, and shrinkage takes place. As the concrete shrinks various restraints come into play, such as subgrade friction or structural restraint and tensile stresses in the green concrete increase to the point where the concrete cracks. It is easy to confuse drying shrinkage cracking with cracking caused by structural movement and both situations cause the concrete to have a poor appearance. The remedies for drying shrinkage cracking are to design mixes with the least amount of paste and least amount of cement, not use excessive water in the mix before placement, provide moist curing after finishing, and provide crack control joints. This type of cracking is most readily apparent in large flat slabs or pavements and careful attention to jointing and jointing details is important.

 

Thermal Cracking is the result of changes in temperature that affect the concrete after it has taken its initial set. This is especially a problem for pavements, since they are subjected to large extremes in temperature. Typically, concrete inside a building will have a controlled environment and not be subjected to variations in temperature to the extent that outdoor concrete, such as pavement is. However, freshly finished concrete in a building that is not yet enclosed or sealed off from the weather may experience a sudden drop in temperature, which would increase cracking by adding to the drying shrinkage cracking.

 

Cracking from Structural Movement is the result of foundation movement or other structural loads that may have exceeded the design criteria or the design was simply not strong enough for the forces imposed. When foundation movement affecting slabs or footings occurs a distortion may be introduced into the foundation or the floor slab. When distortion has sufficient severity the concrete is forced into a curvature, producing tensile effects on one face or the other of the concrete. This type of cracking is more significant than the ones previously discussed since it is an indication that the structure may be in trouble, from foundation movement or from other forces, such as wind, weak concrete or overload.

 

Cracking in Stained and Patterned Concrete is an appearance problem, but sometimes a very important problem. If the concrete, such as a floor slab is intended to be of architectural significance it may be stained or patterned and naturally the designer and structure owner do not want random cracking occurring across this surface. The best way to reduce cracking in this type of concrete is to supply sufficient steel, which would tend to hold the concrete together, in conjunction with properly spaced control joints.

 

Dusting

Dusting is a phenomenon that can occur if the surface of the concrete during finishing has had too much water added while trowelling and the cement is brought to the surface and does not completely set because of excess water. Upon drying this upper laitance can dust off and be difficult for the usability of the slab or cause problems with floor coverings to bond.

 

Abrasion

Abrasion can occur due to the use of a foundation element, usually a slab floor, in a manufacturing unit or warehouse where high pressure fork lift tires or skid loads are moved around on the surface. If the surface is not strong enough concrete will be abraded or spalled by the friction. These situations should be anticipated by the designer and for severe cases a surface hardener should have been applied. If the intensity of the abrasion forces is not major a good high strength concrete properly placed and finished may be sufficient.

 

Honeycomb

Honeycombing is an appearance problem and sometimes a structural problem and is the result of concrete being placed with insufficient fluidity and vibration to consolidate the concrete into the forms and properly surround the reinforcing steel. The honeycomb may be seen as voids when the forms are removed. It is generally seen on the sides of concrete slabs or beams. If concrete is placed with excess water it is possible for the coarse aggregates to settle out, collect at the bottom and then appear as “honeycomb” after hardening and the forms are removed.

 

Exposed Reinforcing Steel

Exposed steel is an appearance problem and may be a structural problem since it indicates that the reinforcing is not in a proper position and is not fully bonded to the concrete. Appearance problems result because exposed steel will rust, staining the outer surface of the concrete. This is a result of the reinforcing steel not properly placed and supported within the forms prior to the placing of concrete.

 

Repairing Openings Made in Finished Concrete

Occasionally after a concrete slab is placed it is necessary to cut openings for placement of plumbing or other adjustments. Sometimes openings are made well after the concrete has been in service because of the needs to access a plumbing pipe. Such openings should be repaired by replacing the subgrade soil with proper material that is well compacted, re-installing any vapor barrier membrane if it was originally used and placing adequate reinforcing steel in the patch area. The steel needs to be doweled into the concrete on each edge unless the previously existing steel was salvaged during opening sticking out of the concrete edges. It is usually best to make these openings using square or rectangle sided saw cut openings. After the steel has been established and tied to the old steel going into the remaining slab, concrete can be placed using adequate strength, mix and finished as usual. If a post-tensioning cable has been damaged, a special repair is needed.

 

If you would be interested in having MLAW present a one to two hour seminar on foundations, structures, drainage, pavements, earthwork, water penetration or construction practices, call (512) 835-7000. MLAW offers this no-charge service as our contribution to building civilization.