Concrete block

Concrete block (also known as cinder block or concrete masonry unit) is a pre-formed masonry building material created by mixing Portland cement and aggregate. These blocks usually measure 8 inches tall, 8 wide, and 16 long. The concrete blocks or concrete masonry units can be either solid or hollow with two or three cores and the endings can be flat or flanged. The composition and configuration will affect two characteristics: compressive strength and fire resistance.

History
Mass production of concrete blocks first appeared in the 19th century. In 1900, S. Palmer created a cast iron block machine. The first phase of the concrete block industry was molded by a Palmer-type hand operated machine that could create single blocks. Hollow Building Block Company was the first business to manufacture Palm’s machine, and it could produce roughly 400 machines a year. The market soon became overwhelmed with the Palmer-type machines. The popularity of the concrete blocks seemed to grow exponentially in the early part of the twentieth century thanks in part to a more reliable mixture of Portland cement and a booming availability level. Improvement of the look and quality was imperative and new machines, mixtures, and curing methods were becoming necessary. The Concrete Block Machine Manufacturers Association was founded in 1905, Concrete Producers Association in 1918, and the Concrete Block Manufacturers Association in 1919. Soon, a standard block size was developed as well as more rigorous testing to improve its reliability and durability. In 1919, roughly 50 million concrete blocks were made in the United States alone and 387 million just nine years later. While the number of blocks manufactured increased, the number of plants manufacturing them decreased. These companies were merging into large corporations and making distinct connections with ready-mix concrete companies. In 1917, a new type of aggregate was introduced to correct the weight problem. F.J. Straub soon patented the name “cinder blocks”. Straub was soon making more than 70 million blocks a year and marketed them as being lightweight and affordable that could be faced with traditional materials like brick. Cinder blocks were popular because they were easy to lay and have materials nailed into them. Cinder blocks found some competition with natural and manufactured products like expanded shale, pumice, and clay that were commonly the substitute for cinders and slag. One of the earliest expanded shale products was Haydite, which first appeared in 1919 and first used as building blocks in 1923. Pottsco was a blast furnace slag with a water treatment and came onto the market in 1930, while Waylite (slag expanded with steam) appeared just a few years later. Most of these aggregates had a lighter color and many found aesthetically pleasing.

Manufacturing Process
Most of the blocks created in the 19th century were created using a system like S. Palmer’s. The machines had a metal frame and mold box with a hand-release lever that allowed for the worker to remove the sides and the cores. Two machines were downface and sideface, named appropriately for the location of the face plate. A lightly moistened mixture of Portland cement, water, sand, and stone was shoveled into the machine and packed down to prevent voids. When the blocks were release, they were stored on a pallet or board to dry. A different way of making the blocks was the slush method. A wet mix was poured into metal molds that were set on open railcars, which were then rolled into a shed to cure. After the blocks dried, they were released and the mold was used again. Power tamping and stripper machines replaced hand-tamped forms in the 1920s. The stripper had inflexible sides that extruded the blocks from the molds. Two major changes in the 1930s were the introduction of the automatic vibrators and machines that could make multiple blocks. Louis Gelbman and the Stearns Manufacturing Company developed a machine that made nine standard blocks in one minute in 1934. The vibrating machines gave way for heavier mixtures because the machine could handle the mixture better than the tamp machine. By 1940 most of the process for creating these blocks became automated. The use of steam quickly became the ideal way to cure the bricks in the 1930s.

Uses and Installation
Concrete blocks were popular for several reasons. For the hollow concrete blocks, they were usually inexpensive and could be installed faster than traditional materials, like brick. They were also fireproof, easy to care for, and could be ornamental. Faceplates could be purchased that mocked the visual characteristics of cobblestone, brick, scrolls, wreaths, and even roping. Rockfaced was the most popular facing for concrete blocks and became standard on all machines from 1900 to 1930. This style was meant to look like quarried stone. Thousands used this rockfaced hollow concrete block in the early stages of this process. 75% of concrete block manufactured by 1915 were used as foundations and basement walls. Plainface was usually used in partition walls and could be covered in stucco for a popular look on exterior walls. As technology improved in the 1930s, the industry shifted into more production of the plainface blocks. Coloring pigments could be added if the blocks were destined to be an unfinished surface. Most of the blocks created after the 1915s were used as backup or cavity wall construction.

Conservation
Concrete block is known for its durability as a building material. As long as the blocks were properly manufactured, installed, and maintained, little to no trouble should come from them. New technologies make it possible to extend the life of concrete blocks. Improved patch mixes, corrosion resistant anchoring systems, and breathable coatings usually prolong the life of concrete blocks.

Deterioration
Concrete blocks are likely to experience shrinkage as they cure and change when exposed to thermal and moisture changes. When concrete blocks shrink or experience movement, they are likely to crack which can weaken their structural integrity. Also, concrete blocks are likely to experience a form of deterioration known as spalling. Spalling is a flaking or popping that is caused by water entering into the concrete blocks. Spalling is commonly caused by a flaw in the block mix. Efflorescence is caused by the accumulation of salts on the surface. Staining and dirt build up is another common problem but it unlikely to deteriorate the strength of the block. Symptoms of distress are easy to identify; looking for patterns of cracking, water movement, spalling, or other deterioration helps to determine the cause of the distress as well as the extent of damage. Differential settlement is usually the cause of step cracking. Water moving through a wall results in efflorescence while water entering from the ground could cause rising damp. Spotty water stains may indicate bad draining and flashing systems. Visually inspecting blocks during rainfall may help in determining where the water is coming into the blocks. Laboratory testing may be necessary to find the specific cause of deterioration. Petrographic and chemical analysis of samples is useful in identifying the cause of spalling. Mix problems are also to blame on occasion for cracking. Using sealers and coatings that trap water in the wall can be diagnosed through a combination of field survey and laboratory testing. If blocks appear to be structurally unsound, laboratory structural testing may be required.

Conservation Techniques
Typical maintenance is required for repairing joins and recoating the surface (if painted) or sealing it. It is important to address cracking or spalling, efflorescence, staining and dirt deposits, and water infiltration. The condition and finish of blocks, as well as the type of soiling, are to be considered when choosing a cleaning technique. Low pressure water and a mild detergent (non-ionic) can be used to clean unpainted blocks. If the soiling is severe, chemicals and detergents can be used in liquid or poultice form. It is imperative to use the least abrasive cleaner to begin with, and a sample piece is strongly suggested. Efflorescence can be limited by preventing moisture from entering the walls by installing flashings, using vapor barriers, and proper pointing. Moisture infiltration can also be prevented by the addition of paint or sealers on the masonry walls. When repairing a crack, finding the cause should be the first step. The wall will crack again if the cause of movement is not identified. Reinforcement of new blocks that are manufactured to have minimal shrinkage can prevent future cracking. When cracks extend through the joints between blocks, it should be repaired by repointing. Hairline cracks can be remedied with a surface coating. Wider cracks can be repaired by cutting the crack out to ½ inch wide and undercutting the crack to no more than ¾ inch wide at the back and ½ deep. Cleaning the crack and drying it is important before repairing the crack. Then, the crack should be dampened and filled with mortar. Control joints was be added into the construction as long as they are not visually intrusive. Spacing joints should be added to compliment the amount of expected movement, the wall’s resistance to horizontal tensile stress, and the number and location of wall openings. Changes in wall height, thickness or orientation, constructions joints and the sides of wall openings are the most common places to find these control joints. If concrete blocks are used as a backup material, the control joints should extend through the facing material and backup as well as the blocks. When installing the control joints is not possible, wider cracks may be repaired by cutting out the crack and installing a sealant. Patch mix similar to the makeup of the blocks may be used to fix spalling. These patches consist of dry masonry, cement, and a mixture that is meant to improve the bond. The sand in the patch should match the appearance of the preexisting blocks. Stainless steel pins, set in epoxy and covered by the patch surface, can be used to anchor large patches to the block. Pointing joints between blocks is important in keeping the walls watertight. Mortar should be complimentary to the existing blocks and walls. Samples of mortar can be taken to laboratories to develop a mortar than matches the composition and appearance. If the mix of water infiltration is involved, patching the wall may not be enough; the wall may require an overall repair. Incorporating flashing into the wall is an important way of preventing water penetration. Flashing should be used at all wall penetrations to direct the water away from the interior. Weep holes are also a remedy. Most concrete blocks do not require waterproofing; a coating may be applied for aesthetics or precautionary measures. Regular maintenance of a painted or coated wall is required infrequently as preventative measures. Oil based coatings, latex paints, rubber-based coatings, epoxy coatings, alkyd paints, urethanes, and silicones are the most common types of coatings for concrete blocks. The coating should be selected based on the conditions the block will be in and the coatings characteristics.

Replacement
Replacement of concrete blocks may be necessary is a severe crack extends across several blocks. If the blocks are of a specific shape, a mold can be created to reproduce an identical pattern. Custom blocks may have curbed or beveled edges. Molds can be used to form replacement blocks matching the texture and shape. Laboratory studies determine the composition can be useful in determining appropriate mixes for replacements blocks.