Concrete pipe is the only pipe that gets stronger as it ages. Increasing strength with age continues as long as any unhydrated cement is still present, provided the concrete remains moist, and the concrete temperature remains favorable. When the temperature of the concrete drops below freezing, hydration and strength gain virtually stops. Once temperatures rise above freezing, however, that hydration can resume. Considering that sanitary sewer pipe is generally wet and never subject to freezing temperatures, curing continues indefinitely. The principal factors affecting strength are water-cement ratio and age, or the extent to which hydration has progressed. The lower the water-cement ratio, the stronger the concrete.

The binding quality of portland cement paste is due to the chemical reaction between cement and water, called hydration. Cement paste is composed of water and extremely fine cement particles. Factors that influence the rate of hydration include fineness of cement grinding, admixtures, if any, water and temperature.

On occasion, where high bed loads and steep slopes exist, hydraulic structures may be subjected to abrasion. Results indicate that abrasion resistance is closely related to the compressive strength of concrete. High-strength concrete has more resistance to abrasion than does lower strength concrete. Since compressive strength depends on water-cement ratio and curing, low water-cement-ratio concrete pipe provides superior abrasion resistance.

Reinforced concrete pipe is designed to crack and then undergo a chemical reaction called autogenous healing. Cracking under load indicates that tensile stresses have been transferred to the reinforcing steel. A 0.01-in.-wide crack does not indicate structural distress and is not harmful. Cracks that are much wider than 0.01-in. may be sealed to ensure protection of the reinforcing steel. An exception occurs with pipe manufactured with greater than one inch cover over the reinforcing steel. In these cases, acceptable crack width should be increased.

Autogenous healing is common when buried pipe is in the presence of moisture, either on the soil side or inside the pipe itself. These cracks, when healed autogenously, are impermeable and can yield a structure stronger than the original. One of the reasons is that the concrete pipe seals the crack with calcium-carbonate crystals formed when the carbon dioxide in the surrounding soil, air and water carbonates the free calcium oxide in the cement and the calcium hydroxide liberated by the hydration of the tricalcium silicate of the cement. The formula for this reaction is: Ca(OH)₂ + CO₂ = CaCO₃ + H₂0

The first documented discovery of autogenous healing was by the French Academy of Science in 1836. Since then, there have been numerous examples where cracks in concrete have been repaired naturally by moisture reactivating the hydration process of the cement in the concrete.

In most environments, concrete pipe continues to cure at a slower rate than the rate set during production. Tests on old pipe discovered during reconstruction projects, or tested under circumstances decades later, repeatedly confirm that the strength of concrete pipe increases as time passes.

The American Society of Civil Engineers (ASCE) Pipeline Division's International Conference (with What's on the Horizon as its theme) on Pipeline Engineering and Construction was held in August 2004 in San Diego. A concrete pipe evaluation paper was given by Kienow Associates, Inc. that detailed the compressive and D-Load strength of reinforced concrete pipe that had been recovered and tested for strength. Following are the data presented at the conference.

CALIFORNIA

GETTING OLDER AND BETTER SINCE 1946

Spec. 0.01 -in. Crack D-Load	Test 0.01-in. Crack D-Load	Test Ultimate D-Load	Test Ultimate D-Load
1800	4489	2700	No Test
1800	4399	2700	4794
1850	3575	2775	4426
2000	4588	3000	4875
2000	5722	3000	No Test
2500	8055	3750	No Test
2500	8375	3750	8934
Los Angeles County sanitary 27-in. unlined reinforced concrete pipe sewer installed 1946, tested 2002 ±.

The 27-in. pipe was recovered from a landslide that took out 700 feet of 27-in. RCP belonging to L.A. County Sanitation Districts. The D-Load strength of the 27-in. tripled during 56 years in service.

ARIZONA

REINFORCED CONCRETE PIPE - 15 YEARS OLD
Concrete compressive strength more than doubled in 16 years

Core I.D.	Spec. ƒc psi	Test ƒc psi	Alkalinity A	Az Life Factor
A Flowline	6000	14,730	0.319	0.66
B Flowline	6000	14,560	0.309	0.62
E Crown	6000	11,920	0.330	0.62
E Crown	6000	12,720	0.308	0.54
Phoenix Salt River Outfall 69-in. RCP near 40th Street installed 1965, tested May 1980, 0.21 in. loss at crown of pipe.

The 69-in. unlined RCP was washed out by the Salt River Flood of 1980 near the east end of the Phoenix Sky Harbor runway.

REINFORCED CONCRETE PIPE AFTER 16 YEARS

Core #	Spec ƒc psi	Test ƒc psi	Steel CoverZ	Alkalinity A	Az (inches) CaCO3
C Crown	6000	8620	2.75 in. clear	0.967	2.66
D Crown	6000	9080	2.50 in. clear	0.975	2.44
Core tests on 48-in. RCP at Southern Avenue interceptor (corner of Priest Road and Southern Ave.), Tempe, Ariz., installed in 1964 and tested May 1980, minimum specification “A” of 0.80, limestone aggregates.

The Az or “Life Factor” is a direct measure of the corrosion resistance of the concrete that is protecting the steel from corrosion. The 48-in. Tempe RCP (produced with 100 percent limestone aggregates) has four times the corrosion resistance of the 69-in. RCP from 40th St. (produced with limestone sand and granitic rock). Tests were done in May 1980. The 48-in. RCP from Southern Ave./Priest Road was cored when the top 3-ft. × 4-ft. section was cut out of the pipe to add a manhole or structure in 1980. The top was trucked to Hydro Conduit's Phoenix plant and cored and tested by Engineers Testing Labs.

Viewing a videotape of the inside of the pipe gives little or no information regarding concrete cover over the steel, strength, corrosion resistance, or life expectancy of the pipe. Pipe inspectors must be aware of the attributes of the pipe material, and cores must be taken to understand fully what they are seeing on tape. Specifiers and pipeline designers must be aware of the performance and durability of reinforced concrete pipe to match service life of products and materials with the design life of projects.

Ken Kienow is President of Kienow Associates, Inc., Civil, Environmental, Forensic, Geotechnical and Pipeline Engineers, Big Bear Lake, Calif., 909/866-8636; report reprinted from American Concrete Pipe Association's Summer 2005 Concrete Pipe News

REFERENCES:

Steven H. Kosmatka and William C. Panarese, “Design and Control of Concrete Mixtures”, Thirteenth Edition, Portland Cement Association, 1988, pp. 3-5.