3. Concrete Deterioration Possible causes of concrete deterioration: H2S Attack Hydrogen sulfide oxidation = H2SO4sulfuric acid Reacts with Ca(OH)2 (free lime)to become gypsum Gypsum reacts with aluminates to form ettringite Microbial Induced Corrosion Through a biological chain, H2S is converted into H2SO4 by Thiobicillus bacteria H2SO4 reacts with free lime in the concrete
6. Durability: Your Best Defense Durable concrete is the result of: A properly proportioned mix A low water-to-cementitious materials ratio The use of Admixtures and Secondary Cementitious Materials (SCM’s) Proper Curing Techniques Sealers / Densifiers / Coatings
11. Cement Hydration Cement hydration producesfour parts C-S-H (glue) plus one part Ca(OH)2(free lime) “Glue” “I’m Outta Here!”
12. Type I Type I Type I Proportioning Water WCR =Weight of Water toWeight of Cementitious Materials
13. Water/Cementitious Materials Ratio Low water/cementitious materials ratios are critical for increased concrete strength water tightness increased durability High water/cementitious materials ratios High water content will evaporate, leaving large capillary pores Increased porosity Decreased strength
14. Importance of a Low WCR A low water-to-cement ratio (below 0.45) will produce a denser concrete with a lower porosity than that of a high water-to-cement ratio. Dense concrete is stronger and more durable than porous concrete.
17. Curing Agents Cylinders 1-4 and 9 have a Cure and Seal applied. All cylinders were field cured after 24 hours. Photo is at 28 days of age.
18. Sealers and Densifiers Penetrating sealers can be used with old and new concrete. The chemicals react with the free lime present in the concrete and form a pore blocking crystalline structure. Some densifiers are added to the concrete in the mixer. Most are either Silicates, Silanes, or Siloxanes
19. Penetrating Sealers / Densifiers Sealed Unsealed Initial Water Placed on Sample Effects of Water After 120 Minutes Both samples of concrete are from the same mix, cured in the same manner.
20. Conclusion Remember: Not all concrete is the same!!! Durable concrete with low porosity provides better resistance to chemical attack Good production practices produce good concrete products: Well designed / proportioned mixes Low water-to-cement ratios Proper early age curing
Editor's Notes
Poor quality concrete is susceptible to deleterious attack from chemicals in a waste water environment. This distribution box is just one example of the effect that the gasses can have on concrete having a high water to cement ratio. This box was sent to Concrete Testing Laboratory in Skokie Illinois for petrographic analysis. The results of the test indicated that the water-to-cement ratio of the concrete is 0.58, far above the maximum specified by the National Precast Concrete Association’s best practices manual as well as ASTM C1227, the Specification for Concrete Septic Tanks.
The processes that occur which result in concrete deterioration vary. Most theories are similar, and the two primary causes of concrete corrosion are believed to be from either hydrogen sulfide attack or microbial induced corrosion. Sulfate in the water of the septic tank is converted to sulfide by a bacteria called a sulfate reducing bacteria. When released into the air space, it mixes with hydrogen and becomes hydrogen sulfide gas. It is believed that the H2S is oxidized by the aerobic environment and converted into H2SO4, or sulfuric acid. The sulfuric acid reacts with the calcium hydroxide, or free lime, to form gypsum which then reacts with the aluminates in the concrete to create ettringite, an expansive, deleterious substance.Another theory with merit is that a series of microbiological reactions occur beginning with the hydrogen sulfide gas with organisms oxidizing the H2S into sulfate. Thiobicillus, a bacteria, converts the sulfate into elemental sulfur, and then H2SO4. The H2SO4 reacts with the calcium hydroxide, or free lime, and to form gypsum which then reacts with the aluminates in the concrete to create ettringite, an expansive, deleterious substance.
Carbon dioxide can react with the calcium hydroxide to create carbonation, weakening the concrete. Chloride-ion attack is typically from deicer salts or sea water, and can react with concrete to cause severe deterioration. And another culprit of deterioration results from dry shrinkage cracking; small micro cracking caused by the rapid loss of moisture in the exposed concrete surface.Every one of these potential deleterious reactions can be reduced or completely prevented by using a high quality, durable concrete with a very low porosity.
Durable concrete does not happen by accident. And just because it looks like concrete, does not mean it is good concrete. High quality concrete that is durable with a low porosity is the result of good production practices. First of all, the concrete mix must be designed for the environment it will be used in. The mixture must be proportioned using a standardized method such as ACI 211 for volumetric proportioning. The water-to-cement ratio must be low, and never exceeding 0.45 for a wastewater environment. For environments where sulfate is more likely, a maximum water to cement ratio of 0.40 should be used. The free water in the aggregates must be measured and subtracted from the design water to maintain a correct water to cement ratio.Admixture are very important in producing high quality durable concrete and include air entrainers, water reducers, and set accelerators. In addition to admixtures, secondary cementitious materials such as fly ash, silica fume, slag, and metakalin can be used. This session will not go into depth on the use of concrete admixtures and SCM’s. Proper curing is essential. Concrete materials hydrate at various stages, some immediately, and others over time. Concrete needs moisture retention in order for the chemical reactions to occur at the proper times. Reducing the curing cycle will reduce the strength and durability of concrete, as well as increase its porosity.Sealers, densifiers, and coatings can be used to retain moisture, increase the hardness, and reduce the porosity of concrete.
Basic concrete is simply the blending of three materials: cement, aggregates, and water. The old method of making concrete was to scoop three shovels of rock, two shovels of sand, one shovel of cement, then and water until the mix was flowable. Today, we use computers, scales, and microwave moisture meters to accurately measure and blend these three components with chemical and mineral admixtures to obtain a high quality product with is consistent batch to batch.
Cement only needs a relatively small amount of water for hydration. The remaining water is for convenience. Note that a slump is not obtained until the water-to-cement ratio reaches about 0.40. Excess water evaporates leaving behind a series of pores and capillaries.
The hydration process produces Calcium Silicate Hydrate, or C-S-H. A by product of this chemical reaction is the release of calcium hydroxide, or free lime. If you recall, it is this free lime that is believed to be the first component attacked in the chemical corrosion process. Also, the free lime and increased porosity of the concrete increase the sorbency of water and contaminants that are absorbed into the concrete.Mineral admixtures, some chemical admixtures, and chemical sealers react with the free lime available to form additional Calcium Silicate Hydrate.
The effect of the water to cement ratio on permeability is not linear, and in fact increases significantly after a water to cement ratio of 0.50. As this chart shows, a water to cement ratio of 0.45 or less is necessary for freeze thaw protection, and a maximum water to cement ratio of 0.40 is best for corrosion protection. In addition, as the water to cement ratio is reduced, the concrete compressive strength increases, all other properties being the same.
Water is the key component in hydration. After placement, bleed water quickly evaporates, and the remaining water is necessary for complete hydration of the concrete. Optimally, concrete would be left in the form a minimum of 7 days to cure, and 28 days to reach maximum design strength. The concrete would be durable and impervious and resistant to deterioration. But in a precast operation, this is not practical. Forms need to be turned daily. As you see by this chart, the reduction in cure time can reduce the strength of the concrete by as much as 45%. The reduced strength is the result of increased pore space. Durability will be reduced and the porosity of the concrete will be increased.
Curing agents can significantly improve the durability of concrete by creating a membrane whereby the moisture in the concrete is retained. This is usually performed after initial set, but in precast, would also need to be applied after stripping.
