An essential component of almost any building project, concrete is used to make strong, durable structures. But to be effective, it needs to be treated properly. As it is a chemical compound, it can be affected by changes in temperature or humidity, which must be understood if the concrete is to work most effectively.
Concrete has a shelf life and does not last indefinitely. A bagged concrete mix can be kept for 3 months to a year, depending on how it has been stored. Because it hydrates when exposed to moisture and can suffer compaction in storage, concrete can lose integrity if not stored properly.
Concrete should always be used within the time period stipulated on the bag.
This article will examine the various factors that influence the sell-by date of concrete and give you guidance on how long you can store concrete and how to ensure it stays fresh for as long as possible.
Influence of Cement in Concrete Mix
Concrete mix consists of a mixture of cement and aggregates such as sand and stone. While the sand and stone are inert materials that do not, in any way, influence the aging of concrete mix, the cement is much less stable.
Concrete is usually made with Portland cement, which takes its name from its Portland grey color, and is derived from chalk and clay. The chemistry of Portland cement is triggered by a chemical reaction called hydration, whereby the cement paste hardens and gains strength.
Portland cement can last a long time if it is kept dry. Still, even the moisture present in the atmosphere is enough to cause the cement to harden, and it must be kept in an airtight environment. Once the moisture content exceeds five percent, cement cannot be used in construction (source).
Issues of compaction occur when Portland cement is bagged and stacked and the content is allowed to settle over a period of time. This is referred to as “warehouse pack,” resulting in the cement becoming hard, even though it has remained dry. Most of the time, the concrete can still be used after you roll the bag on the floor to loosen the lumps.
If a packaged concrete mix is less than six months old, has no lumps, and is a completely free-flowing powder, it should be okay to use for non-structural purposes. The concrete used in all structural jobs with high strength requirements must be made from a fresh concrete mix.
What Happens When Concrete Expires?
While many users may be tempted to use the concrete after its sell-by date, a few things have already happened to the concrete that may render it useless. Firstly, it may have formed lumps or hardened completely through hydration. Secondly, the strength of concrete reduces proportionally over time until it is ineffective.
Packaging that is soiled can be cleaned using a cleaning agent and disposed of in accordance with local, state, and federal regulations. For further information on other options, see “What Can I Do with a Hardened Bag of Concrete?”
How Do You Store Bags of Concrete?
There are a few practices that help maintain the freshness and strength of a concrete mix. While the product is stable in dry conditions, it must be transported from the factory to the customer. Once it leaves the factory, there are procedures to be followed to ensure optimum shelf life.
Bags of mixed concrete should be stored on raised surfaces in a dry, inside a dry building or shed away from rain, moisture, sunlight, and wind.
By following these general guidelines, the integrity of the product should be assured. Below are pointers for the ideal storage and transportation of bags of concrete mix.
Avoid Dampness and Moisture
The cement changes from a powder when water is added, becoming a paste. When the paste combines with the aggregate, it forms a binding mix. The effect of water on the cement is called hydration, which results in the cement paste hardening and strengthening.
As cement hardens when it comes into contact with any form of damp, so it must be protected from atmospheric moisture or water. Storage must be in dry conditions, in a closed structure, properly stacked and covered.
Even if the concrete mix has been kept in dry conditions, it still tends to react to humidity and moisture in the air — not just water and dampness — and, over time, it loses its strength.
Avoid storing concrete bags directly on concrete or wood floors. As moisture can permeate from the ground upwards, a bagged concrete mix should never be laid directly on a floor. Rather, it must be raised off the ground with a waterproof sheet underneath the stack.
Store indoors in a well-ventilated building to prevent moist air accumulation. The storage unit must be dry and safe from any possibility of exposure to water.
If bags need to be offloaded and are in transit to a storage area, this should be as brief a period as possible, secure from water exposure. To find out more about this, read “Can Bags of Concrete Be Stored Outside?”
How Concrete Packaging Affects Shelf Life
Concrete mix and cement are mostly packed in environmentally friendly, durable paper bags. Because these bags are not airtight, moisture can enter, even if unopened, and therefore they must be used within three months.
Trials of concrete mix or cement stored in airtight containers and controlled environments of constant temperature and humidity have successfully lengthened the shelf life of the concrete.
Other trials using plastic bags monitored the aging behavior of calcium aluminate cement in normal warehouse storage conditions for a period of 40 months. These trials revealed that plastic packaging protecting concrete from moisture greatly improved the product’s shelf life, and a one-year shelf life guarantee could be given (source).
Efficient warehouse packing seeks to limit any damage to the item once it has moved into the warehouse. The packaging is designed to protect the concrete mix inside but also to be efficient in terms of moving and stacking the material.
However, if the bags are piled too heavily, this could result in warehouse-pack and compaction of the lower layers. To prevent this, stacks should be kept to less than 12 bags in height.
Also, the first-in-first-out rotation method should be stringently applied to a concrete mix and cement products to ensure that bags in storage are rotated accordingly. A concrete mix should not be stored with any other products, as this could affect the strength and performance of the product.
Handle with Care
By following the handling procedures below, damage to the concrete mix will be minimized:
- Never use hooks to load concrete mix.
- Don’t throw or drop a bag as the packaging could split or weaken.
- Roll bags before lifting to help loosen the contents and reduce the risk of splitting.
- Carry the bag in your arms like a baby, supporting the underside.
- Lay cement bags flat when storing, broadside down.
Bags should be loaded with care, avoiding sharp protrusions that could pierce them. Ensure there is thorough waterproofing to prevent the bags from getting wet during transport.
How Long Will a Bag of Concrete Last Once Opened?
This depends on the conditions of humidity and temperature under which it has been stored. Frequently, partially used bags are leftover from a project, and the paper is torn, leaving the concrete mix exposed to the air.
This will result in product degradation. To avoid this, the contents should be re-bagged in tightly sealed impervious plastic bags and stored in a dry place. This may help it last a few more weeks.
While there may be a temptation to use an old concrete mix, you will ultimately regret it. The product will cause quality problems and will require repairing or replacement down the line, jeopardizing your relationship with your customer and damaging your reputation.
To avoid leftover concrete, calculators provided by manufacturers are great tools to help you calculate accurate quantities needed for the job at hand. If you are unsure, professionals can help too.
Either way, our advice is to try to avoid having leftover concrete mix, as you cannot be assured of retaining its integrity for any length of time.
The cement in a concrete mix displays hygroscopic tendencies. This means that any water it comes into contact with will change its character. This is good if you are going to use the concrete immediately as water acts as the binding agent to strengthen the concrete. However, as concrete mix ages, it becomes less effective until it reaches its sell-by date.
Typically, concrete has a sell-by date that is three months from its manufacture date, and this should be displayed on the packaging. If you store concrete mix responsibly, you could extend the life of the product, but it could also be shorter if stored badly. We hope this article has helped you understand the shelf life implications of concrete.