Then, after a period of heavy rainfall late last month, a massive crack was exposed. According to the local news outlet, Daily Nation, it measures 50 feet deep and 65 feet wide in some spots. Geologist David Adede, who spoke with the paper, said the crack was likely filled previously with volcanic ash from nearby Mt. Longonot. This means the space was only exposed when rainwater washed the ash away. Reuters reports that the opening formed rapidly. One resident named Eliud Njoroge Mbugua saw the crack run through his home. He was only narrowly able to collect some of his belongings before his house collapsed. So what caused the break in the first place
The name is often used to refer to a cultural region from the Middle East to Mozambique, but is not actually connected to the same unit. Rather it's made of multiple rifts all running through the same system. (Read more about the social rifts felt in this region.) A rift valley refers to a lowland region where tectonic plates rift, or move apart. The large crack that recently exposed itself in Kenya is from the East African Rift. In the 3,700 mile-long East African Rift, there are two smaller systems called the Gregory Rift and the Western Rift, and each is speckled with volcanoes. The rifts are growing larger as two tectonic plates, the Somali plate in the east and the Nubian plate in the west, move away from each other.
Some of these factors can lead to an unfortunately common outcome: cracked or split fruit. What was once a semi-round, leathery globe is suddenly a mess of exposed, decaying arils, still attached to the limb.
Most slabs-on-ground are unreinforced or nominally reinforced for crack-width control. When positioned in the upper or top portion of the slab thickness, steel reinforcement limits the widths of random cracks that may occur because of concrete shrinkage and temperature restraints, subbase settlement, applied loads or other issues.
Shrinkage and temperature reinforcement is different than structural reinforcement. Structural reinforcement is typically placed in the bottom portion of the slab thickness to increase the slab's load capacity. Most structural slabs-on-ground have both top and bottom layers of reinforcement for controlling crack-widths and increasing load capacities. Because of constructability issues and costs associated with two layers of reinforcement, structural slabs-on-ground are not as common as nonstructural slabs.
Steel reinforcing bars and welded wire reinforcement will not prevent cracking. Reinforcement is basically dormant until the concrete cracks. After cracking, it becomes active and controls crack widths by restricting crack growth.
If slabs are placed on high quality subbases with uniform support and consist of low shrinkage concrete with joints properly installed with spacings of 15 feet or less, reinforcement is generally unnecessary. Most likely, there will be few random or out-of-joint cracking. If random cracks do occur, they should remain fairly tight because of the limited joint spacing and low concrete shrinkage thereby limiting future serviceability or maintenance issues.
When slabs are placed on problematic subbases with risks of non-uniform support or consist of moderate to high shrinkage concrete or joint spacings exceed 15 feet, then reinforcement is necessary to limit widths of cracks should they occur. As crack widths grow and approach about 35 mils (0.035 inches), the efficiency of load transfer through aggregate interlock diminishes and differential vertical movements across cracks or slab \"rocking\" can occur. When this happens, crack edges become exposed and edge spalling will likely occur, especially if the slab is exposed to wheeled traffic and especially hard-wheeled lift trucks. Once spalling starts, crack widths at the surface become wider and slab deterioration along cracks increase significantly.
When contraction joints are unacceptable and not installed, shrinkage and temperature reinforcement is required. This design approach is sometimes referred to as continuously reinforced or joint-less slabs and allows numerous, closely spaced (3 to 6 feet), fine cracks to occur throughout the slab.
In general, there are two options for controlling cracks in slabs-on-ground: 1) control the location of cracking by installing contraction joints (does not control crack widths) or 2) control crack widths by installing reinforcement (does not control crack location).
With Option 1, we tell the slab where to crack and widths of contraction joints or cracks in the joints are largely controlled by the joint spacing and concrete shrinkage. As joint spacings and concrete shrinkage increases, joint widths increase. Similar to cracks, if joint widths approach about 35 mils, the efficiency of the aggregate interlock to transfer loads and prevent differential vertical movements across joints can be significantly reduced. For this reason, many designers use load-transfer devices including steel dowels, plates or continuous reinforcement through contraction joints to ensure positive load transfer and to restrict differential vertical movements across joints.
With Option 2, we allow slabs to crack randomly but control crack widths with steel reinforcing bars or welded wire reinforcement. Typically, contraction joints are not installed with this option. Instead, cracking occurs randomly forming numerous, tightly held together cracks. Because of appearance, this crack control option should always be discussed with the owner.
Use caution when using both crack control options in the same slab. If too much reinforcement passes through contraction joints, joints become too stiff and may not crack and open as designed. When contraction joints fail to activate (i.e., crack and open) because of reinforcement, out-of-joint or random cracking typically occurs. If both options are used, it is necessary to limit the amount of reinforcement passing through joints to ensure proper activation.
Some designers specify to cut all the reinforcement at contraction joints while others may specify to cut every other bar or wire. By cutting every other bar or wire, the remaining reinforcement will help provide load-transfer and minimize differential panel movements but not restrict joints from activating. If the specifications and construction drawings do not indicate what to do with temperature and shrinkage reinforcement at joints, contractors should submit a request for information. Many times contractors are inappropriately blamed for out-of-joint cracking associated with this design issue.
Steel reinforcing bars and welded wire reinforcement should be positioned in the upper third of the slab thickness because shrinkage and temperature cracks originate at the surface of the slab. Cracks are wider at the surface and narrow with depth. So, crack-control reinforcement should never be positioned below the slab's mid-depth. Reinforcement should also be placed low enough so saw cutting does not cut the reinforcement. For welded wire reinforcement, the Wire Reinforcement Institute recommends steel placement 2 inches below the surface or within the upper third of the slab thickness, whichever is closer to the surface. Designers typically specify the reinforcement position by specifying concrete cover (1 1/2 to 2 inches) for the reinforcement.
Positioning a single layer of reinforcement in the center or at mid-depth of the slab is not recommended (except for 4-inch-thick slabs). This is an all-purpose location where the designer hopes to increase the load capacity of the slab in addition to provide crack-width control. However, positioning reinforcement in the middle of the slab will not effectively accomplish either objective.
Reinforcement partially buried in the subbase does not provide crack width control. Without supporting chairs or pre-cast concrete blocks, reinforcement typically ends up in the bottom of the slab or buried in the subbase.
When a hole for the service pipe line is drilled through the concrete, the coring process can cause damage to the concrete structure by creating cracks. The core hole can often provide a direct path for water to pass through the concrete structure.
Control joints are pre-planned and installed to prevent concrete cracking due to shrinkage during curing. A control joint is saw-cut into the curing concrete when the concrete is just hard enough, usually within 6-12 hours after the concrete has been poured. The timing depends on the concrete mix and the surrounding environment. The cuts should be made at a predetermined spacing, depth and pattern to meet structural engineering specifications and only after the concrete has obtained sufficient strength, but before internal cracking begins.
An expansion joint is used in concrete to allow the concrete to absorb predicted movement by expanding or contracting with daily temperature variations. Lack of expansion joints may lead to uncontrolled cracking.
Cracks in concrete are of common occurrence and they develop when stresses in the concrete exceed its strength. Cracks are often caused by normal shrinkage of the concrete when hardening and drying. Concrete cracks can range from being a non-structural and unsightly, to being detrimental to the structural integrity and safety of a building.
Structural cracks may endanger the safety and durability of a building. They can form due to incorrect design, faulty construction and/or overloading. Non-structural cracks are mostly formed due to internally induced stress in building materials and do not result in weakening of the structure. However, if left untreated, a non-structural crack may facilitate ingress of moisture and other destructive environmental substances which may lead to corrosion of reinforcement, making the concrete structure unsafe.
Cracks that are identified as small and fine (less than 0.3 mm in width), are generally deemed acceptable as part of minor settlement depending on the purpose