Designing Concrete for Special Uses
Special concretes are sometimes required for improved structural design, durability, or for other reasons such as sulfate resistance, acid resistance, water tightness, controlled heat of hydration, radioactive shielding or reduced weight. The production of special concretes for such usage involves the selection of the proper type of cement and aggregates for high quality concrete.
The durability of concrete exposed to alkaline sulfate soils or water containing alkaline sulfates has been a problem of long standing in many localities. The areas where alkaline sulfate soils and ground waters occur are particularly prevalent in the western part of the United States. Poor quality concrete made with cements of high tricalcium aluminate, C3A, content will deteriorate rapidly when exposed to sulfate soils or sulfate waters.
Both laboratory tests and field experience have shown that good quality concrete made with sulfate resistant cement, low C3A, has excellent resistance to sulfates. Such concrete will have excellent durability when exposed to sulfate soils and sulfate waters.
It is both the quality of the concrete and composition of the cement that provides sulfate resistance. A poor quality concrete with a high W/C ratio will not have good sulfate resistance even when made with the best sulfate resistant cement.
Type II portland cement should be used in all concrete construction exposed to moderate sulfate action. Type V cement should be used when high sulfate resistance is required.
A rich concrete mix with a maximum W/C of 5.0 gallons of water per sack of cement should be used. With the lower W/C and lower porosity that accompany the use of richer mixes, high resistance to sulfate attack has been attained with the use of sulfate resistant cements.
Sea water contains a moderate quantity of sulfate and about 3.0 percent of sodium chloride. The sulfate can attack poor quality concrete, while the chloride can cause rusting of the reinforcement. Rusting of the reinforcement is accompanied by expansion which is likely to cause spalling of concrete over reinforcing bars.
Experience has shown that good quality concrete made with either Type II or Type V portland cement will resist the sulfate attack by sea water. It will also prevent the rusting of the reinforcement when adequate coverage is provided.
A rich concrete mix with a maximum W/C of 5.0 gallons of water per sack of cement should be used for all concrete construction exposed to sea water. The reinforcing steel should have a minimum cover of three inches of concrete. The low permeability attained with the low W/C will prevent rusting of the steel with such coverage; and the low W/C together with the use of sulfate resistant cement will prevent sulfate attack of the concrete. The use of pozzolan can help reduce the W/C ratio and develop a less porous surface.
RESISTANCE TO ACIDS
Hardened portland cement paste is an alkaline material that is not normally resistant to acids. Protective coatings should be applied to the surface of concrete that is likely to be exposed to acid solutions having a pH of 5.5 or lower.
The action of sulfuric acid on concrete is a special case. Concrete in sewer lines and in chimneys where fuels containing sulfur are used, will sometimes be exposed to sulfuric acid. Both laboratory tests and field experience have shown that the use of sulfate resistant concrete will improve its performance when exposed to sulfuric acid. The sulfate resistant concrete should be of the same quality as that previously described.
The growth of slimes in sewer lines is often accompanied by biological action that generates hydrogen sulfide in the sewage. The hydrogen sulfide is then oxidized to sulfuric acid on the crown of the sewer pipe. Sewer lines should be constructed to provide a free flow of the sewage or the raw sewage should be properly chemically treated. Either process will inhibit the growth of slimes and thereby prevent the generation of hydrogen sulfide and its oxidation to sulfuric acid. Such construction practice or sewage treatment will extend the life of a concrete pipe and also prevent the escape of obnoxious hydrogen sulfide from manholes.
Sulfur trioxide is a combustion product of fuels containing sulfur. The sulfur trioxide and moisture sometimes condense as sulfuric acid on the cool surface near the top of a chimney.
The action of sulfuric acid on concrete occurs in two stages:
- The acid reacts with calcium in the cement paste to form calcium sulfate.
- The calcium sulfate diffuses into the concrete to form calcium sulfoaluminate which can cause expansion and cracking.
The first reaction causes a softening of the surface of the concrete. The use of sulfate resistant concrete will not prevent this surface reaction. It will prevent the more disruptive expansion and cracking of the second stage. For that reason the use of sulfate resistant concrete will greatly improve the performance of concrete exposed to sulfuric acid.
Watertight concrete has been used in hydraulic structures, water tanks and other structures. Correct methods of construction must be followed, and close attention must be given to a number of details. Sound aggregates of low porosity and suitable grading should be used.
The W/C has a pronounced effect on the permeability of concrete. The W/C should not exceed six gallons per sack of cement. The permeability of concrete increases rapidly with higher water-cement ratios.
A plastic, workable mix is necessary so that the concrete can be thoroughly compacted without separation of the materials. Every precaution should be taken to prevent segregation of materials while the concrete is being transported and placed.
The use of air-entrained concrete reduces permeability. Its use provides a workable mix with a lower W/C. It reduces segregation, bleeding, and laitance.
Vibration should be used to consolidate the concrete. It provides complete consolidation of a stiffer mix of lower W/C ratio.
Placing should be continuous whenever possible. When interruptions cannot be avoided, every precaution is necessary to obtain a good joint. Waterstops should be placed across construction joints.
Adequate curing is very important. Moist curing should be extended as long as practicable. Tanks and reservoirs should not be filled until they have been adequately cured.
The stresses and cracking which occur as a result of temperature changes constitute one of the major problems confronting the designers of massive concrete structures. During the process of hydration and hardening, cement liberates heat which raises the temperature of the concrete. In small structures this is generally of little consequence since the heat is rapidly dissipated. In massive structures the heat of hydration may cause a temperature rise of as much as 50 to 60°F.
As the exterior surface cools and contracts, while the interior temperature remains high, tensile stresses are developed. When these stresses exceed the tensile strength of the concrete, cracks will appear in the structure.
Very significant progress has been made in solving this problem during the last 30 years. This progress has been accomplished through the development and use of cements of moderate or low heat hydration, through the use of concrete of low cement content, through precooling of materials to lower the temperature of the concrete as placed, and by improved construction procedures.
Both the type of cement used and the cement content of the mix have a significant effect on the temperature rise in mass concrete. Table 2 shows the effect of cement type and cement content on the temperature rise of concrete cured under adiabatic conditions, that is, without any gain or loss of heat from external sources. These data show that, for the same cement content, Type I cement produces a considerably higher temperature rise than Type II cement and with Type II the temperature rise is higher than with Type IV cement. The data also show the effect of the cement content of the mix on the temperature rise. A mix containing 5 sk. of Type II cement develops less temperature rise than a mix containing 4 sk. of Type I cement; and a mix containing 5 sk. of Type IV cement has about the same temperature rise as one containing 3 sk. of Type I cement.
Precooling and postcooling are further methods for controlling the range in temperature in the concrete. Precooling is the cooling of materials to lower the temperature of the concrete as placed. Postcooling is accomplished by circulating cold water through pipes embedded in the concrete.
Lightweight structural concrete can be used in practically all types of concrete construction. It has a unit weight of 90 to 110 pounds per cubic foot. In comparison with 145 to 155 pounds per cubic foot of concrete made with natural aggregates. Its lighter weight reduces the dead load in structures and provides for easier handing and transportation of precast structural elements. The production and use of structural lightweight concrete has increased rapidly in recent years. Lightweight concrete can reduce the dead weight ratio as much as 30 to 35 percent and still provide structural strength.
The lightweight aggregates used in the production of lightweight concrete are expanded clay, shale, and slate and expanded blast furnace slag.
Lightweight structural concrete mixes can be designed to meet any desired strength. Compressive strengths as high as 10,000 psi at 28 days have been obtained. The concretes show about the same strength gain with age as natural aggregate concretes under similar curing conditions. Laboratory tests and the performance of structures have shown that the concretes have excellent resistance to weathering.
Lightweight aggregates usually have a high absorption. For that reason it is difficult to use net water cement ratio accurately as a basis of mix proportioning. Lightweight aggregate concrete mixes should be established by a series of trial mixes proportioned on a cement content basis at the required consistency. Table 3 gives an approximate relationship between cement content and compressive strength. These values may serve as a guide in proportioning the first trial mix.
Lightweight aggregates should be damp at the time of mixing to reduce the amount of water absorbed from the mix and thus reduce the rate of loss of slump. If dry aggregates are used, it is desirable to mix the aggregate with about two-thirds of the mixing water prior to introducing the cement.
Table 1 Attack on Concrete by Soils and Waters Containing Various Sulfate Concentrations
-Taken from the Bureau of Reclamation Concrete Manual, provides a guide on the effect of sulfate concentrations on concrete and the type of cement to be used.
- Use Type II cement
- Use Type V cement
Effect of Cement Type and Cement Content on the Temperature Rise of Concrete Cured Under Adiabatic Conditions
Approximate Relationship Between Cement Content and Strength of Lightweight Aggregate Concrete
The air content of lightweight concrete cannot be determined accurately by the pressure method because of the high porosity of the aggregate. The volumetric method described in ASTM C173 gives the most reliable results and the use of that method is recommended.
Table 3 - Approximate Relationship Between Cement Content and Strength of Lightweight Aggregate Concrete
|Compressive Strength at 28 Days, psi||Cement Content sk. per cubic yard|
|2000||4 to 7|
|3000||5 to 8|
|4000||6 to 9|
|5000||7 to 10|
|Cement Type||Cement Content (sk. per cu. yd.)||emperature Rise in Concrete at Age Indicated (°F)|
Table 1 - Attack on Concrete by Soils and Waters Containing Various Sulfate Concentrations
|Relative Degree of Sulfate Attack||Percent Water-Soluble Sulfate (as SO4) in Soil Samples||P.p.m. Sulfate (as SO4) in Water Samples|
|Negligible||0.00 to 0.10||0 to 150|
|Positive*1||0.10 to 0.20||150 to 1000|
|Considerable*2||0.20 to 0.50||1000 to 2000|
|Severe||Over 0.50||Over 2000|