Frequently Asked Questions
- Q.What is concrete?
Concrete is comprised of a combination of fine aggregate (sand), coarse aggregate (crushed stone or gravel), portland cement and water. Some concretes contain supplementary cementitious materials (SCM’s) that replace a portion of the portland cement. The two most common supplementary cementitious materials are slag cement and fly ash. The strength of concrete is principally controlled by the water-cementitious ratio. The water-cementitious ratio is the weight of water divided by the weight of cementitious material in a batch of concrete. Chemical admixtures are often used in concrete to reduce the water content (or increase the workability) or to control the set of the concrete. Air entraining admixtures are used for concrete flatwork that will be exposed to freeze/thaw environments while saturated. Air entraining admixtures produce very fine air bubbles in the concrete that help protect the concrete from freeze/thaw distress.
- Q.What is a petrographic examination?
A petrographic examination is simply an examination of a rock or concrete sample using various microscopic techniques. Petrographic examinations of concrete are typically performed due to some type of problem during a construction project or the premature failure of an existing concrete structure. The petrographer is retained to determine the cause of the problem and often to determine ultimately who is responsible. Petrographic examinations are also often performed as part of a condition study to determine if a concrete structure needs to be replaced or if it can be rehabilitated.
- Q.Is concrete scaling distress caused by deicing salts?
It is well known in the industry that deicing salts often play a role in the wearing surface distress of exposed exterior concrete. Deicing salts generally act to magnify the effects of freeze/thaw cycling by increasing the number of freeze/thaw cycles, increasing the saturation level of the concrete and increasing the duration that the concrete stays saturated. However, mature concrete that is properly finished and cured, has adequate strength, a sufficiently low water-cementitious ratio and a good entrained air void system typically should not scale as a result of exposure to deicing salts. Concrete that has scaled in relation to exposure to deicing salts typically has an underlying cause associated with a defect in the concrete.
It should be noted that magnesium based deicing salts can chemically attack the concrete. This chemical attack has the effect of softening the near surface cement paste. Over time, the chemical attack can completely deteriorate the near surface cement paste.
- Q.Is the contractor to blame for concrete scaling distress?
Possibly, we commonly see scaling distress related to improper finishing techniques or failure to properly cure the concrete.
When performing petrographic examinations on scaled concrete, we often determine that the concrete as delivered is of sufficient quality and the distress is related to a low quality cement paste layer comprising the upper few millimeters of the concrete. This low quality cement paste layer is often related to an elevated water-cementitious ratio caused by either adding water to the concrete surface during the finishing operation or by working the concrete bleed water into the near surface concrete.
The cement paste quality of the near surface concrete can also be adversely affected by improper curing. Curing is performed to retain moisture in the concrete, since water is needed to react with the portland cement. If the concrete is not properly cured, the surface can dry out and reduce the quality of the near surface concrete.
Over-finishing of the concrete surface can also cause freeze/thaw scaling distress. A proper entrained air void system is needed to protect concrete from freeze/thaw scaling distress. Over-finishing of the concrete surface can remove the near surface entrained air voids, leaving the concrete susceptible to scaling distress. To determine the exact cause of wearing surface distress, it is necessary to perform a petrographic examination on concrete cores removed from the structure.
- Q.Is the scaling distress going to continue to deepen over time?
This depends on the cause of the scaling distress. If the scaling distress is related to finishing or curing issues, then the distress will typically not proceed to depths beyond a few millimeters. In this case the distress is simply an aesthetic problem and will not affect the long term durability of the concrete pavement. If the scaling distress is related to a problem with the as-delivered concrete such as a high water-cementitious ratio or a poor quality air void system, the scaling distress can continue to deepen over the years with continued exposure to freeze/thaw conditions. In this situation replacement of the pavement may be warranted.
A petrographic examination on cores removed from the structure is necessary to determine the cause of the scaling distress.
- Q.What is popout distress?
A common wearing surface distress known as “popouts” is caused by the presence of substandard aggregates in the concrete. The American Concrete Institute’s “Guide for Conducting a Visual Inspection of Concrete in Service” (ACI 201.1)4 defines popouts as follows: The breaking away of small portions of a concrete surface due to localized internal pressure that leaves a shallow, typically conical, depression with a broken aggregate at the bottom. Popouts occur due to the freezing of near surface, low quality aggregate particles when they are in a saturated condition. When the water in the saturated aggregate particle freezes, it expands and exerts pressure within the particle. A popout occurs when this internal pressure exceeds the tensile strength of the rock and the rock particle fractures. The fracturing of the rock particle subsequently fractures the overlying mortar. The affected area of the overlying mortar is almost always significantly larger than the offending aggregate particle. The affected popout area is often affected by the depth of the particle, with larger areas occurring with deeper aggregate particles. The affected area is also dependent on the size of the aggregate particle.
Certain rock types are susceptible to freezing related popout distress. The most common rock type associated with popouts in concrete is porous chert. Other rock types commonly susceptible to popout distress are shale, mudstones and ironstones. Carbonate rock types (limestone and dolomitic limestone) are typically good quality concrete aggregates, however, some carbonate rock types are soft and highly absorptive and are susceptible to popout distress. Some types of siltstone and sandstone can also be susceptible to popout distress. One thing in common with all of these rocks is a relatively high absorption and a tight pore structure that does not allow the water to easily escape upon freezing.
Popouts can be related to both the fine and the coarse aggregate particles. Many of the natural sands in the Midwest can contain relatively high shale contents. These shale particles are highly susceptible to popout distress.
- Q.What is water-cementitous ratio?
Water-cementitious ratio (w/cm) is the weight of the water in concrete divided by the weight of the cementitious materials. The strength and the durabillity of concrete is directly related to the concrete w/cm. As the w/cm increases the strength decreases and the permeability increases.
Addition of water to the concrete once the ready mix truck is on-site is a very common practice. The concrete contractor needs to have an understnding on how these on-site water additions can affect the concrete. A one gallon addition of water per cubic yard of concrete will not have a significant effect on the concrete w/cm. However, oftentimes water additions exceed this amount and can significantly increase the w/cm, which can have an adverse effect on the strength or freeze/thaw durability of the concrete.
The American Concrete Institute’s “Guide to Durable Concrete” (ACI 201) recommends a maximum water-cementitious ratio of 0.45 for exterior concrete flatwork that will be exposed to severe freeze/thaw environments.
- Q.Why is concrete air entrained?
Concrete is air entrained to protect the concrete from freeze/thaw distress. All exterior concrete flatwork that will be exposed to moisture and freeze/thaw environments should be air entrained. An air entraining admixture is mixed into the concrete to produce very fine air voids uniformly distributed throughout the concrete. A proper air void system will protect good quality concrete from freeze/thaw scaling distress. Not only do you need a sufficient air void content (typically 4 to 8 %), but the air voids need to be the proper size and have the proper spacing. The optimum air void system has very fine air voids spaced closely together. CRT can perform an air void analysis on concrete cores removed from a structure to determine if the air void system is freeze/thaw durable.
- Q.Why is the strength of my concrete low?
There are numerous causes of low strength concrete. Some of the more common causes are high water-cementitious ratio, high air void content, poor coarse aggregate/cement paste bond and improper batching of materials. Weather conditions such as extremely hot weather or very cold temperatures may contribute to lower than expected concrete strengths. In some situations a combination of factors may be responsible for the low strength of concrete. It is also possible that the poor testing practices of concrete strength cylinders has led to a perceived low strength problem.
To determine the cause of low strength problems, a petrographic examination should be performed on concrete cores removed from the structure in question.