Microbial growth occurs when there's excessive moisture in the concrete and high levels of humidity in the air. Microbial growth can corrode and weaken concrete. Signs of microbiologically induced deterioration include cracking and flaking on the surface.
Cracking. If the moisture contained in the substrate has nowhere to go, it will make its way up to the surface. With no escape, it can cause your concrete to crack, show up as chips and hairline faults in your concrete coatings. If you are seeing cracks in your concrete, you need to correct the issue before it worsens.
How Moisture Affects Concrete Strength. Increased space between cement grains: Higher water-to-cement ratios result in greater spacing between the aggregates in cement, which affects compaction. Similarly, increased moisture levels reduce the concrete's compressive strength and durability.
Adding more water to the concrete increases workability but more water also increases the potential for segregation (settling of coarse aggregate particles), increased bleeding, drying shrinkage and cracking in addition to decreasing the strength and durability.
“Moisture that rises from the substrate will be absorbed into and rise through the concrete slab, filling the pores and capillaries.” Hydration will usually occur — eventually.
Adding one gallon of water per cubic yard increases the slump by one inch, decreases compressive strength 150 to 200 psi, wastes about ¼ bag of cement, and increases shrinkage by 10%. Measure and record all water added on the jobsite.
Concrete is one of most durable manmade materials, but even this old industry workhorse has its weaknesses. Exposure to harsh weather, reactions with common elements, and poor construction can all lead to concrete failure.
Strength Reduction
The excess water will not participate in the hydration process and retains in concrete even after hardening. This water will evaporate when exposed to atmosphere and form voids in the concrete. These voids formed are therefore responsible for the reduction of compressive strength of concrete.
Modern concrete—used in everything from roads to buildings to bridges—can break down in as few as 50 years. But more than a thousand years after the western Roman Empire crumbled to dust, its concrete structures are still standing.
When hydration – a chemical reaction between cement and water – takes place, concrete hardens and therefore becomes stronger. If too much rain falls into the concrete mix – or if it is laid onto wet surfaces or trenches – this will affect the mix, resulting in weak concrete.
Water plays a critical role, particularly the amount used. The strength of concrete increases when less water is used to make concrete. The hydration reaction itself consumes a specific amount of water. Concrete is actually mixed with more water than is needed for the hydration reactions.
What are the most common causes of concrete deterioration? Chemical attack, overloading and impact, carbonation, dry and wet cycling, and fire are major causes of concrete damages.
For large scale projects like buildings, concrete should last up to 100 years if it's properly cared for. Concrete projects that experience more wear-and-tear like sidewalks and driveways have an expected lifespan of about half that—50 years.
Concrete is typically believed to last forever. While it may have ancient durability, its life span doesn't usually exceed 100 years. Architects recognize concrete as a stone-like, homogeneous material, a mix of limestone and other rock.
Besides its quantity, the quality of mixing water used in concrete has important effects on fresh concrete properties, such as setting time and workability; it also has important effects on the strength and durability of hardened concrete.
The two most common causes of failure are carbonation and chloride contamination of the concrete. Both of these lead to corrosion of the embedded steel reinforcement and as the steel corrodes it expands and exerts pressure on the concrete so that, eventually, the concrete cracks and spalls.
Unsound concrete can be identified through various signs such as visible cracks, micro-cracks, spalling, scaling, blisters, and delamination. Multiple factors lead to different types of damages, and as a result, a specific repair strategy may be required for each type of concrete damage.
Water damage in a foundation is often gradual, starting with small cracks allowing in small amounts of water. This weakens the foundation and lets in more water. This process can take months or even years to destroy your foundation.
A watery mix actively reduces the compressive strength of the dried concrete. Usually, every additional inch of slump in the concrete reduces the compressive strength of the final product by roughly 500 psi.
According to the American Concrete Institute, concrete gains 90% of its strength within the first 28 days of curing. However, the concrete continues to gain strength over time, with some concrete structures becoming stronger even after 50-100 years.
Over a century, the carbonation depth may be on the order of several inches depending on the quality of the concrete. If reinforcing bars are present within the carbonated concrete, the protective oxide film normally present in concrete is absent, leaving the surface of the steel potentially active for corrosion.
Concrete typically takes 24 to 48 hours to dry enough for you to walk or drive on it. However, concrete drying is a continuous and fluid event, and usually reaches its full effective strength after about 28 days.
Assessing potential exposure before construction or repair can prevent premature deterioration. Specific cement types, water-repellent sealers or chemically-resistant barrier coatings are all common preventative measures to protect concrete against chemical attack.
Tensile strength
Concrete is very weak in tension. The tensile strength of ordinary concrete ranges from about 7 to 10 percent of the compressive strength.
Concrete has tremendous compressive strength but very low tensile strength. This makes it great for building the support pillars of a bridge, for example, but not the connecting elements between the pillars. Gravity would stress the spans to the breaking point, causing the concrete to crack.