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Ideally, you should pour concrete when the conditions are perfect, not only the weather but also the subgrade, subbase, base, etc. But what if there is a rain forecast for the day or you live in a typically rainy place? Therefore, you may wonder if you can pour concrete in the rain.
You cannot pour concrete in the rain unless the site is shielded from rainwater, runoff, and splashes. You may pour concrete before it rains, but only if you have a fast-setting mix with an accelerator and water reducer. Else, the rain will damage fresh and wet concrete.
You cannot pour concrete immediately after the rain, either, unless it is a drizzle and there is almost no adverse impact on the site. The rest of this article will explain why you shouldn’t pour concrete in the rain and the few ways you can. Keep reading to learn more.
What Happens if You Pour Concrete While It’s Raining
Rain can contaminate a fresh and wet concrete mix. If you pour concrete while it’s raining, the exposure to and interaction with rainwater will cause various problems. The number of problems and the severity of each issue depends on many factors, including but not limited to the following:
- The composition and type of concrete
- The duration and severity of rainfall
- The exposure of concrete to rainwater
- The protections you put in place, if any
- The reinforcements in concrete, if any
There is no way to generalize what happens if you pour concrete while it’s raining because the variable factors have a significant influence. However, one fact is undisputed. Rain will have some adverse effects on fresh and wet concrete and ruin the mix or slab in the worst cases.
Thus, not only can you not pour concrete while it’s raining, even a freshly poured mix that is still wet must be protected from rainwater. The concrete slab won’t cure and dry properly otherwise.
Let me explain everything that will probably happen if you pour concrete while it’s raining.
Rain Will Increase the Water Content in Fresh or Wet Concrete
Fresh and wet concrete mixes comprise the following materials:
Generally, the water-to-cement ratio is between 0.3 and 0.6 by weight, not volume.
Reinforced concrete or mixes using heavyweight aggregates, such as iron ore, and fillers tend to have the lowest water-to-cement ratios of ~0.33. Most concrete mixes used for residential construction have more water content.
The Departments of Transportation of most states in the US use concrete with 0.4 to 0.5 water-to-cement ratios. Suppose the concrete mix you have has a water-to-cement ratio of 0.55. This concrete composition is not the strongest variety, to begin with.
If you pour this concrete in the rain, the exposure to water will skew the ratio. Unchecked water exposure can easily spike the water-to-cement ratio to 0.8 and higher, which is the red line for concrete’s strength. Therefore, you will have significantly weakened and probably ruined concrete.
Even limited or moderate exposure to rainwater can cause the following problems if you pour concrete in the rain:
- Bleeding and laitance
- Concrete efflorescence
- Shrinkage and cracking
Bleeding is the first effect of excess water in concrete, whether due to the composition of a mix or exposure to moisture, such as rain. This bleeding causes concrete laitance, which isn’t the same as efflorescence. Concrete laitance is caused by the following:
- Excess water, i.e., in mixing, rain, etc.
- Improper finishing and site traffic
- Improper or poor curing and drying
- Overworking the concrete surface
And all these causes apply to pouring concrete while it’s raining. Rain will naturally overwork or over-manipulate the fresh and wet concrete mix. Excess water is inevitable if you don’t shield the site from rain. And curing and even drying will obviously be a nonstarter as long as it rains.
Raindrops hitting the concrete are akin to site traffic, and finishing is impossible if you pour a mix in the rain. Therefore, you won’t be able to prevent concrete bleeding and subsequent laitance. The same moisture problem is relevant to concrete efflorescence, too.
Concrete efflorescence looks like white deposits, which are basically mineral salts that dissolve in excess water and eventually turn into powdery residue on the surface. Such stains don’t even need rainwater. High hydrostatic pressure inside poured concrete can cause efflorescence.
So, excess moisture in the form of humidity or vapor may also cause or facilitate efflorescence. Most concrete mixes have mineral salts in the cement. Rain will dissolve these substances, and the water will eventually evaporate, leaving behind visible white deposits on the surface.
Excess water weakens all types of concrete mixes, irrespective of the type of aggregate, filler, or reinforcement. However, if you use standard concrete, the water content is likely on the higher side already. So, not pouring concrete in the rain or protecting it from rainwater is critical.
Rain May Wash Away Some Cement in Freshly Poured Concrete
Standard concrete mixes typically have the main ingredients in the following proportions:
- 10% cement
- 20% air and water
- 30% sand
- 40% gravel
The composition varies based on the concrete type. Also, the choice of aggregate influences the exact ratios, such as crushed stone, iron ore, etc. But the proportion of cement is almost always between 10% and 15%. So, cement has the smallest proportion by volume.
As you know, cement is the primary binder in concrete, and the component is already used in modest proportions for most mixes. If rain washes away some of that cement or interferes with the even spreading of the concrete mix, your slab is unlikely to bind and strengthen properly.
While excess water will prevent the cement from binding adequately during the drying and later in the curing phase, you will experience many problems even while pouring the concrete. As you can see, a mix won’t have the desired consistency if it is exposed to rainwater, and it will fail the slump test.
The Entrained Air in Concrete Might Turn Into Entrapped Air
Broadly, concrete is either air-entrained or not. An air-entrained concrete mix has microscopic air cells to allow some room for water to expand if it freezes. But these air pockets cannot be too large or more per cubic foot than necessary. Both issues will lead to entrapped air.
Air-entrained concrete mixes have slightly lower water-to-cement ratios than those that don’t have any entrained air. Regardless of this difference, excess water exposure during rain can enlarge the microscopic air pockets and even create new cells by displacing other materials.
This trapped water in the concrete mix will evaporate eventually, leaving behind the enlarged cells and probably new and unnecessary pockets with entrapped air. These entrapped air cells make concrete vulnerable to shrinkage. Significant shrinkage will inevitably lead to cracking.
Air-entrained concrete has its advantages, especially how the pockets can relieve the internal pressure of a concrete slab. But such concrete mixes also require deft handling, whether you use air-entrained portland cement or introduce agents in the concrete mix at the site.
The thumb rule is 4% to 7% of entrained air cells by volume for a concrete mix, albeit there are variations. Pouring concrete in the rain without impeccable protection from rainwater makes it impossible to ensure that you don’t have too large and excess air pockets by volume per unit.
Even if you use non-air-entrained concrete, direct exposure to rainwater will make your mix an air entrained version. Excess water mixing with wet concrete or penetrating the surface of a slab that has just been poured will create entrapped air pockets eventually.
Thus, both air and water will lead to shrinkage when the latter evaporates, and the concrete slab is likely to crack sooner or later. The severity of any such shrinkage and cracking is, of course, subject to the extent of excess exposure to water and the resulting entrapped air pockets.
Torrential Rain May Displace Some Gravel and Sand in Concrete
Excess water contaminates all concrete mixes, but torrential rain may compromise the integrity of the composition. Concrete has two types of components:
- Aggregate: crushed stone, gravel, sand, etc., and specialty mixes have fillers.
- Paste: a combination of cement and water, and also air for entrained mixes.
The structural integrity of the paste component is vulnerable to rain regardless of the severity of the downpour. But the aggregate may not have much of a threat from a drizzle or light rain. Only a torrential downpour can dislodge any aggregate material, especially heavyweight ones.
Heavy rain can cause extensive pitting on and into the fresh and wet concrete that can dislodge some or much of the aggregate, thereby causing one or more of the following problems:
- Bad finish: bumps, craters, dents, depressions, etc.
- Poor curing: structural deformation and unevenness.
- Wasted concrete, if the material damage is immense.
Besides, excess water adversely affects the binding properties of cement, so the paste won’t do its job to facilitate the aggregate to bond, cure, and dry as it should. Weak binding will make the aggregate more vulnerable to displacement if you pour concrete in torrential rain.
A Drizzle or Light Rain Affects and Prolongs Curing and Drying
Suppose you aren’t expecting a heavy downpour. A drizzle or light rain for a while is unlikely to damage concrete to the extent of displacing any aggregate or washing away the cement. But a fresh and wet concrete mix is unlikely to dry as quickly as it should if you pour it in the rain.
The delayed drying will also prolong the curing process. Besides, both curing and drying phases have specific requirements, depending on the concrete type and other factors. Poured concrete won’t turn out as expected otherwise. Consider the following facts:
- Standard concrete mixes require 24 to 48 hours to be sufficiently dry, a state when you can walk on it without any adverse effects.
- Concrete typically requires 28 days from pouring to cure completely, which won’t happen if it cannot dry in time or has improper conditions.
- Concrete curing is a long process requiring the right levels of moisture and temperature, both of which are usually a problem when it rains.
- Reinforced or specialty concrete mixes for heavy-duty constructions may have specific pouring, drying, and curing requirements.
Rain Alters the Mechanical and Physical Properties of Fresh Concrete
If you pour concrete while it’s raining, excess water and other related factors will definitely alter the mechanical and physical properties. Even a light but sustained drizzle can wreak havoc on the key attributes of concrete, including but not limited to the following:
- Durability, form, surface finish, etc.
- Creep, shrinkage, structural integrity
- Poisson’s ratio and elastic modulus
- Strength: characteristic, compressive, tensile
Apart from the severity of rainfall, the extent of any impact on these attributes also depends on the concrete mix, especially the type of aggregate, cement, and fillers. For instance, 10 types of portland cement are used in different grades of concrete mixes:
- Type I: mostly residential construction.
- Type IA: I with air-entrainment.
- Type II: for mild sulfate resistance.
- Type IIA: air-entrained variant of II.
- Type II(MH): moderate heat of hydration.
- Type II(MH)A: air-entrained variety of (MH).
- Type III: high early strength.
- Type IIIA: air-entrained III.
- Type IV: low heat of hydration.
- Type V: high sulfate resistance.
These grades react differently to rainwater, but concrete mixes comprising any of the 10 types of cement are not immune to rain. Likewise, the rain will alter the properties of admixtures or fillers in concrete, albeit standard variants for residential uses may not have such materials, like:
- Fly ash (micronized)
- Silica fume
How To Pour Concrete in the Rain
The only way you can pour concrete in the rain is by covering and protecting everything at the site, including the mixer. You may use some or all the following materials to waterproof the site to pour concrete in the rain:
- Plastic sheets: above and on all sides.
- Posts: bamboo, plywood, timber, etc.
- Tarp: to form a canopy over the site.
If the site isn’t elevated ground or a raised platform, you need to protect the concrete mix from potential rainwater runoff. You could dig a trench to direct water away from the site or rely on the existing drainage system if it is sufficient. Still, curing and drying will be tricky if the rain persists.
How To Pour Concrete Before the Rain
You may pour concrete before the rain, but only if you use fast-setting concrete. The fast-setting concrete mixes available nowadays tend to harden in under 30 minutes up to one hour. You should also consider using the following materials if you expect rain immediately or soon after pouring:
- Accelerator: calcium chloride flake or liquid, non-chloride variant, etc.
- Additives or admixtures: superplasticizer, water reducer or retarder, etc.
- Reinforcement: the likes of fiber mesh, fiber-reinforced polymers, etc.
Plus, you should always use a vapor barrier at the base. Here’s a helpful YouTube video you may want to check out if you wish to pour concrete just before the rain:
Pouring concrete before the rain isn’t a fail-safe approach. If you suspect any adverse effects, a simple scratch test may reveal the dryness and hardiness of poured concrete.
Note: Laying bricks in the rain can also be challenging. For more information, please read my article on this subject.
Pouring concrete in the rain is a nonstarter unless you have an impeccably waterproofed site or the project is only a patch job. You may pour concrete before the rain, but the process has to be expedited and expertly executed, which isn’t easy without professional tools and sufficient help.